DOI:
10.1039/C5RA27069C
(Review Article)
RSC Adv., 2016,
6, 37784-37922
Brønsted acid-promoted synthesis of common heterocycles and related bio-active and functional molecules
Received
17th December 2015
, Accepted 14th March 2016
First published on 22nd March 2016
Abstract
Various heterocyclic systems based on natural and unnatural biologically active products were synthesized by Brønsted acid-promoted cyclization. We have made an attempt to summarize the progress made in this area during the period 2005 to 2015. This article describes the synthetic routes of these heterocycles that involve a Brønsted acid-promoted cyclization as a crucial step or Brønsted acids as co-catalysts with other transition metals.
 Sudipta Ponra | Sudipta Ponra received his B.Sc. in 2006 and M.Sc. in 2008 from Vidyasagar University. After qualifying CSIR-NET fellowship he joined the research group of Prof. K. C. Majumdar at the University of Kalyani in 2008. He obtained his Ph.D. in December 2013, working on synthesis, reactivity and biological activity of novel heterocycles. In 2014, in order to complete his scientific experience, he joined research group of Prof. B. C. B. Bezuidenhoudt at Department of Chemistry, University of the Free State and worked on aluminium triflate catalyzed substitution reactions of propargylic alcohols and related cyclization reactions. Later on he moved to ENSCP-Chimie ParisTech as a result of successful application to the prestigious research grant of “Fondation Pierre-Gilles de Gennes Pour la recherché”. There he worked under the supervision of Dr Maxime R. Vitale, Dr Véronique Michelet, and Dr Virginie Ratovelomanana-Vidal on new catalytic chemical processes based on the synergistic use of two different catalysts. Now he is working jointly in ENSCP-Chimie ParisTech and Ecole Normale Supérieure (ENS) on ‘Green Asymmetric Electro Organocatalysis’ under the joint supervision of Dr Maxime R. Vitale and Dr Laurence Grimaud. |
 K. C. Majumdar | Krishna C. Majumdar received his B.Sc. (1966) and M.Sc. (1968) degrees from the University of Calcutta and Ph.D. (1972) from the University of Idaho under the direction of Professor B. S. Thyagarajan and continued in the same University as a research associate till mid 1974. He also carried out postdoctoral work (University of Alberta) with Professor J. W. Lown till mid 1977. He was with the University of Kalyani, lecturer (1977), reader (1984), Professor (1995–2010), and UGC Emeritus Fellow at the same University. He had also served North-Eastern Hill University as a visiting Professor (1996), Indian Institute of Technology (Kharagpur) as Assoc. Professor during 1990–1991, and Tezpur University (2011) as professor of Eminence. His research interests center around synthetic organic chemistry with over 400 publications including review articles and book chapters. He has also edited a Wiley-VCH publication “Heterocycles in natural product synthesis”. His recent research interests include design and synthesis of liquid crystals. He is a fellow of the West Bengal Academy of Science and Technology, and recipient of the Chemical Research Society of India medal (2004) and Indian Chemical Society award (2006). |
1. Introduction
There were approximately 20 million chemical compounds identified by the end of the second millennium, among them more than two-thirds are fully or partially aromatic and more or less half of them are heterocyclic compounds. Undeniably, heterocycles are the largest of the classical divisions of organic chemistry and have a gigantic importance in biology and industry. Papaverine, theobromine, quinine, emetine, theophylline, atropine, procaine, codeine, reserpine and morphine are some examples of natural drugs1–4 containing heterocycles. Diazepam, chlorpromazine, isoniazid, metronidazole, azidothymidine, barbiturates, antipyrine, captopril and methotrexate are various synthetic drugs containing heterocycles. The mainstream pharmaceutical products that impersonate natural products with biological activity are heterocycles. Hence, researchers are keenly engaged in searching for improved pharmaceuticals, pesticides, insecticides, rodenticides, and weed killers by following natural models. Some dyes (e.g. mauveine), luminophores (e.g. acridine orange), pesticides (e.g. diazinon) and herbicides (e.g. paraquat) are also heterocyclic in nature. Since, all the biological processes are chemical in nature, these natural and synthetic heterocyclic compounds take part in chemical reactions in the human body. Heterocycles are also chemically used as antibacterial, antifungal, antimycobacterial, trypanocidal, anti-HIV active and antileishmanial agents as well as genotoxic, antitubercular, antimalarial, herbicidal, analgesic, antiinflammatory, muscle relaxants anticonvulsant, anticancer and lipid peroxidation inhibitor, hypnotics, antidepressant, antitumoral, anthelmintic and insecticidal agents.5–11 Essential amino acids (proline, histidine and tryptophan),12 photosynthesizing pigment (chlorophyll), oxygen transporting pigment (haemoglobin),13 hormones (kinetin, heteroauxin, cytokinins),14 neurotransmitter (serotonin, histamine) are also contain heterocyclic moieties (Fig. 1). Additionally, aromatic heterocycles are extensively used for synthesis of dyes and polymeric materials of high importance.15 There are several reports of application of aromatic heterocycles as intermediates in organic synthesis.16 Heterocycles with five- or six-membered rings, containing oxygen (O), nitrogen (N) and sulfur (S) atoms are the most general. Therefore, development of new approaches for their syntheses, employing efficient and atom economical method, is currently an important research area.
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| | Fig. 1 Example of some drug containing heterocyclic core. | |
For more than 300 years, the substances that behave similar to vinegar have been classified as acids. The name “acid” is derived from the Latin ‘acidus’ which means “sour” and refers to the sharp odor and sour taste. Lewis-acid catalysts are extensively engaged as catalysts for carbon–carbon bond-forming reactions and received significant awareness due to its non-toxic, recyclable and readily available nature for a variety of organic transformations, affording the analogous products in excellent yields with high selectivity. On the other hand, Brønsted acids have been employed mostly as catalysts for hydrolysis and/or formation of esters, acetals, etc. The synthetic efficacy of a Brønsted acid as a catalyst for carbon–carbon bond-forming reactions has been relatively limited until recently. Brønsted acid-promoted reactions are useful because it is cheap, readily available and easy to handle. In the recent years numerous reviews dealing with transition metal catalyzed synthesis of heterocyclic compounds have appeared.17–38 In modern synthetic organic chemistry levels of molecular complexity and better functional group compatibilities in a convergent and atom economical fashions from readily available starting materials and under mild reaction conditions, is one of the main research endeavors. Brønsted acid-mediated transformations, which often help to meet the above criteria, are among the most attractive synthetic tools. Numerous reviews are found on the synthesis of heterocycles using phosphoric acid or chiral Brønsted acid or using organocatalyst as an effective catalyst.39 Therefore, only the recent representative examples of simple Brønsted acid-promoted cyclization leading to the formation of heterocyclic compounds and relevant mechanistic aspects have been discussed. We have tried to include many recent examples in this review. Although a discussion of all Brønsted acid-mediated cyclizations reports is desirable, it is not possible to explain all papers in this text. Therefore, we have included only the most essential reactions here. This review covers literature up to 2015, and any omissions on this wide topic are unintentional. Only the reactions in which a heterocyclic ring is essentially generated are described. The reactions that are included those involving the formation of heterocyclic compound having biological importance. Special attention has been paid to the recently published acid-promoted formation of five- and six-membered heterocycles.
2. Heterocycles with one hetero-atom
2.1. Nitrogen heterocycles
2.1.1. Three membered aziridine. Aziridine moiety is important heterocycle and found in many biologically active natural products and alkaloids, such as mitomycin, albomitomycin A and C, ficellomycin, porfiromycin, azicemicin A and B, azinomycin A, maduropeptin, madurastatin A1 and B1, and miraziridine A.40–52 Due to high strain and high reactivity aziridines are used as versatile synthetic intermediates to synthesize various functionalized α- and β-amino acids, β-amino alcohols, β-lactams, alkaloids, and other heterocyclic compounds, via ring-opening and ring-expansion reactions.15,53–66 Aminolysis of epoxides, aziridination by carbene transfer to imines and the nitrene or nitrene equivalents transfer to olefins, aza-Darzens reactions, and ring-closure of amino alcohols are useful methods for the preparation of aziridines.67–75In 2004, Johnston et al. developed Brønsted acid-catalyzed direct aza-Darzens synthesis of N-alkyl cis-aziridines 3. This Brønsted acid-catalyzed reaction is unusually mild and extends the scope of Lewis acid-catalyzed [2 + 1] annulation, and demonstrates for the first time that a diazo compound can be used as an acetate enolate synthon without decomposition during a Brønsted acid-catalyzed carbon–carbon bond-forming reaction.76 Significantly, no products resulting from acid-promoted aziridine ring-opening was observed. This TfOH-catalyzed method is competitive with its metal and Lewis acid-catalyzed counterparts in terms of selectivity and turnover yet is considerably economic and environmentally benign (Scheme 1).
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| | Scheme 1 Protic acid catalyzed aza-Darzens reaction. | |
The same group also synthesized aziridines 6 by the application of triflic acid-promoted addition of azides 4 to activated olefins 5 under relatively mild and non-redox conditions.77 The cycloaddition reactions of electron-rich azides 4 to electron-deficient olefins 5 were promoted by Brønsted acids because the presence of a protic acid favor an intermediate aminodiazonium ion generation over the concerted [3 + 2] cycloaddition to produce triazoline. The catalysis is achieved by either providing access to TS or promoting the formation of the aminodiazonium ion intermediate by either direct conjugate addition or triazoline fragmentation. Electron-rich alkyl azides are generally effective donors. Benzyl azide engages the olefin to construct the N-benzyl-protected terminal aziridine 6a in 79% isolated yield. Alternatively, the aziridine 6a was isolated in slightly higher yield 92% by direct crystallization of the triflic acid salt. Hindered azides are also effective, including diphenylmethyl azide and adamantyl azide. Electron-rich azides derived from aniline produced the desired aziridine but competitively forms what appears to be the azide dimer or trimer. Azides such as tert-butyl glycinyl azide also smoothly converted to the aziridine 6e without identifiable ester hydrolysis. A variety of allylic azides formed their derived aziridines 6f and 6g in 68–76% yield (Table 1).
Table 1 Brønsted acid-promoted aziridinations
When the acrylate derivative of N-benzyl methyl carbamate (7) was subjected to the standard aziridination conditions it gave exclusively the oxazolidine dione 8a as a single regioisomer (>20![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:1, 1H NMR). Use of α-substituted acrylates led to generally high yields of the expected oxazolidine diones, 8b–f (Table 2, entries 2–6). Among various substrates, the α-benzyl-substituted acrylate was noticeably more sluggish and required warming to achieve complete conversion (Table 2, entry 5). Series of‚ β-substituted imides in the aminohydroxylation reaction conditions gave similar results.
Table 2 Regiospecific, stereoselective formal anti-aminohydroxylation of activated olefins
In 2008, Li et al. described a combined theoretical and experimental approach to systematically study the Brønsted acid-promoted aziridination of electron-deficient olefins and showed that Brønsted acid-promoted aziridination of electron-deficient olefins proceeded through the attack of the internal nitrogen of the azide 4 to the terminal carbon of the protonated olefin 7, which afforded an acyclic adduct that subsequently discharged N2 to produce the aziridine ring (Scheme 2).78 The basicity of the electron-deficient olefins 7 is an important parameter to determine the efficiency of Brønsted acid-promoted aziridination. More basic carbonyl compounds including vinyl ketones and acrylamides were readily activated by Brønsted acid such as TfOH, whereas less basic carbonyl compounds predicted to be poor substrates. A systematic evaluation of TfOH-promoted aziridination of acrylamides produced a number of aziridine-2-carboxamides by a new, single-step method. TfOH-promoted reactions of various organic azides 4 with different acryl-amides 7 affords aziridine-2-carboxamides 6 as the main products and it was found that both benzylic and normal alkyl azides 4 could smoothly react with the unsubstituted acrylamides 7 to give the desired products 6 as white solid in 50–70% isolated yields. Substituted acrylamides 7 carrying substituents on the nitrogen atom also participated in the reaction and slightly lower yields 30–60% were obtained. Furthermore, 2- or 3-substituted acrylamides 7 did not afford the desired aziridine-2-carboxamides 6 under the TfOH-catalyzed reaction condition. The TfOH-promoted aziridination of 3-butenones were mostly reactive and gives around 70% yield, higher than the yield for the aziridination of acrylamides because acrylamides are more reactive than 3-butenone toward some side reactions under the TfOH conditions, as significant amounts of highly polar by product mixtures in the aziridination of acrylamides were obtained.
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| | Scheme 2 Brønsted acid-promoted olefin aziridination of acrylamides. | |
2.1.2. Four membered azetidine. Acetic acid can be used as additive to synthesize azetidine, pyrrolidine, and indoline compounds via palladium-catalyzed intramolecular amination of C–H bonds at the γ and δ positions of picolinamide (PA) protected amine substrates.79 Low catalyst loading, use of inexpensive reagents, convenient operating conditions and predictable selectivities are features of this reaction. The notable features are the use of unactivated C–H bond, especially the C(sp3)–H bond of methyl groups, as functional group. Use of 2 equiv. of AcOH along with 2.5 mol% Pd(OAc)2 and 2.5 equiv. PhI(OAc)2 the reaction to provide in 85% isolated yield in 24 h. Under this reaction condition picolinamide substrates bearing primary γ-C(sp3)–H bonds (Table 3) the OAc-protected valinol substrate 9 gave a similar cyclization and gives 10 in 82% yield with higher diastereoselectivity. Similarly substrates 12, 17, and 20, bearing both α and β substituents, afforded 70–90% yields. The substrate bearing no β substituent, 14 gave only 25% of the cyclized product 15 along with 70% of the acetoxylated product 16 (entry 3). β-Substituted aliphatic substrate 22 gives 68% yield (entry 6). The reaction proceeds through the PdIV intermediate formation via the PhI(OAc)2 oxidation of the palladacycle intermediate and the subsequent C–N and C–O reductive elimination which lead to the formation of the cyclized and acetoxylated products. The rate of the reductive elimination processes depends on the associated OAc ligand and the steric effect of the substrates. Trace amounts of acetoxylated products (<2%) were observed for substrates 12 and 20 bearing R2 substituents. In contrast, when R2 = H acetoxylated product 16 is predominated. One plausible interpretation for the observed selectivities might be the torsional strain imposed by the R2 substituent on the β position and the γ-C–H bonds (and possibly R1 group on the α position) during the bond reorganization process for the out-of-palladacycle-plane C–O formation. Such torsional strain might kinetically favor the in-palladacycle-plane C–N cyclization despite the ring strain in the resulting azetidine.80 It was also noteworthy that no β-H elimination product was detected under the above-mentioned reaction conditions. The torsional strain was smaller in substrate 14 (R2 = H), and the azetidine's ring strain dictated the out-of-plane C–O formation (Fig. 2).
Table 3 Syntheses of azetidines via intramolecular amination of γ-C(sp3)–H bonds
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| | Fig. 2 PdIV intermediate. | |
Along with four-membered azetidine product, formation of a five-membered pyrrolidine product from a six-membered palladacycle intermediate by this reaction condition also possible. Observing various γ-selectivity in previous examples the possibility of the δ-C(sp3)–H activation under these oxidative conditions was also tested in view of the unique driving force for C–N reductive elimination from a PdIV center. The cyclization of the leucine substrate 25 bearing both primary δ-C(sp3)–H bonds and a sterically less accessible γ-C(sp3)–H bond proceeded smoothly to give the pyrrolidine product 26 under the same azetidine formation conditions, as a mixture of two diastereomers (dr ∼ 7/1), in 82% yield.
The reaction in presence of 10 equiv. of AcOH suppresses the formation of the undesired acetoxylated product 27 (entry 1, Table 4). Substrates bearing both primary δ-C(sp3)–H bonds and γ-substituents, also gave satisfactory yields. High diastereoselectivity was observed for substrate 32. Here also substrate bearing no γ-substituents 30 gave only 17% yield of the pyrrolidine product 31 along with trace amount of the acetoxylated side product but no azetidine product. Moreover >70% of unreacted 30 was recovered (entry 3). Similarly, no pyrrolidine products were formed in the cyclization reactions of substrates 17 and 22 bearing methyl groups at both γ and δ positions in the previous examples. ortho-tert-Butylaniline substrate 34 also cyclized to give the indoline product 35 in 61% yield (entry 5). ortho-Methylbenzylamine substrate 36, cyclized to isoindoline product 37 in 56% yield (entry 6).
Table 4 Syntheses of pyllidines via intramolecular amination of δ-C(sp3)–H bonds
2.1.3. Five membered.
2.1.3.1. Pyrrolidine. Kaitocephalin, bioxalomycin β1, enalapril, captopril, ramipril, radicamine A, nakadomarin A, manzamine A, (+)-castanospermine, and (+)-casuarine, with a wide range of biological activities are examples of natural and non-natural products containing pyrrolidines as important heterocycles.81 Pyrrolidines have also found application in pharmaceuticals, as organocatalysts, as chiral ligands in asymmetric synthesis, and building blocks in organic synthesis.82 Most useful approaches for the pyrrolidine synthesis are [3 + 2]-cycloaddition, Pd-catalyzed carboamination, intramolecular cyclization of epoxy and halogenated sulfones under basic condition, intra-molecular carbolithiation of homoallylic amines,83–89 Brønsted acid-catalyzed intramolecular hydroamination of alkenylamines, and ring-closing metathesis of bis-allylic amines.90
Brønsted acid as catalyst.
In 2006, M. C. Elliot and co-workers reported a convenient two-step preparation of alkylidenepyrrolidines 40 in 44–86% by TFA-mediated enolate addition followed by cyclo-dehydration reaction of 39 (Scheme 3).91
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| | Scheme 3 TFA-catalyzed synthesis of alkylidenepyrrolidines. | |
Reddy and Rao successfully achieved the synthesis of the antibiotic (−)-codonopsinine 44, from D-alanine 41 as a chiral template. The key steps of the strategy are the asymmetric dihydroxylation of the allylic double bond via a modified Sharpless reaction and a highly stereoselective intramolecular TFA-catalyzed amidocyclization by the nucleophilic displacement of the acetate with carbamate (Scheme 4).92a The same group also described a short and stereoselective total synthesis of (−)-codonopsinol 47 and its C-2 epimer 48 from commercially available starting material D-1,5-gluconolactone 45, using TFA-mediated amidocyclization as the crucial step92b (Scheme 5).
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| | Scheme 4 Total synthesis of (−)-codonopsinine. | |
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| | Scheme 5 Stereoselective total synthesis of (−)-codonopsinol and its C-2 epimer. | |
Rabasso and Fadel synthesized β-aminopyrrolidinephosphonates (51) in 80–99% yield via 1,3-dipolar cycloaddition of amine 49 with vinylphosphonates 50 in the presence of 0.4 equiv. trifluoroacetic acid (TFA) in toluene at rt for 2 h (Scheme 6).93 Similar reactions of 50 with the Z-isomer of β-(aminomethyl)vinylphosphonate 52 afforded the cis-γ-aminopyrrolidinephosphonate (53), whereas the E-isomer 54 led to the trans-product (53) (Scheme 7) under similar reaction condition.
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| | Scheme 6 β-Aminophosphonates by 1,3-dipolar cycloaddition. | |
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| | Scheme 7 Synthesis of cis- and trans-γ-aminophosphonates. | |
Various five-membered heterocyclic compounds were synthesized from hydroxylated enynes. Hydroxylated enynes 55 in the presence of F3CSO3H (0.1 mol%) could be selectively transformed into five-membered heterocyclic compounds 56, with an allene moiety at the 3-position (Scheme 8).94 When R1, R2 = Ph, diphenylvinyl-2,3-dihydro-1H-pyrrole (56a) was obtained (Scheme 9). In the presence of HSbF6 (5 mol%) as the catalyst, polycyclic skeletons 57 and 58 with adjacent stereocenters were obtained (Scheme 10). When R1 = H and R2 = styryl, 1,3-dienyl-2,5-dihydro-1H-pyrrole (59) was formed (Scheme 11). This Brønsted acid catalyzed domino process involves the formation of an allene carbocation intermediate, which can be readily trapped by olefins to give various novel five-membered heterocyclic skeletons. Furthermore, the method is the simplest, and least expensive with a low catalyst loading. Brønsted acid shows excellent catalytic activity in the reaction.
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| | Scheme 8 F3CSO3H-catalyzed synthesis of heterocyclic allenes 56 from hydroxylated enynes 55. | |
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| | Scheme 9 Synthesis of diphenylvinyl-2,3-dihydro-1H-pyrrole. | |
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| | Scheme 10 HSbF6-catalyzed synthesis of polycyclic skeletons 57 and 58 from hydroxylated enynes 55a. | |
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| | Scheme 11 Synthesis of 59. | |
The mechanism of the reaction has been rationalized as follows (Scheme 12). In the presence of trace amounts of H+, the hydroxyl group of 55 is lost to give the putative allene carbocation intermediate B,95 which may be readily trapped by the olefin nucleophile to form the carbocation intermediate C. The intermediate C may lose a proton selectively to give the heterocyclic allene 56 (path I). Alternatively, water can attack the carbocation intermediate C as the nucleophile to form the intermediate D. The intermediate D may loses one water molecule in the presence of trace amounts of H+ to give the heterocyclic allene 56 (path II). When R1 and R2 are cyclohexylidene substituents, water attacks the carbocation intermediate C as the nucleophile to form the intermediates D1 and D2. The intermediate allene alcohol D1 may undergo domino carbon–carbon bond formation pathway in the presence of H+ and generate the intermediate cation E1. The hydroxyl group of intermediate cation E1 may attack the allyl carbocation to afford H1, which releases a proton to afford the polycyclic skeleton 58. Next in the presence of H+ the intermediate allene alcohol D2 may also undergo the domino carbon–carbon bond formation pathway to produce the intermediate cation E2. Due to steric factor, the hydroxyl group of the intermediate cation E2 cannot attack the allyl carbocation. In the presence of H+ the intermediate cation E2 may be in equilibrium with the carbocation G2 via intermediate F2. Thus, the hydroxyl group of intermediate cation G2 may attack the allyl cation to give H2, which by losing a proton may give the polycyclic skeleton 57.
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| | Scheme 12 Proposed mechanism. | |
T. Rovis group used co-operative catalysis by an N-heterocyclic carbene and a Brønsted acid for the efficient enantioselective synthesis of trans-γ-lactams 62 in up to 99% yield, 93% ee, and >20:1 dr using unactivated imines 60 (Scheme 13).96 The electron deficient cyclohexyl-substituted azolium and o-chlorobenzoic acid are most suitable for this transformation. In literature electron-rich carbenes have been used to catalyze homoenolate chemistry but here electron-deficient carbene are used to catalyze homoenolates. On investigation of the electronic nature of the imine (Table 5) it was found that when an electron-neutral group (–Me) or an electron-deficient group (p-CF3, p-Br, p-Cl, m-Cl, m-MeO) was present on the phenyl group of the aldimine, the cyclization was slightly affected. The desired products 62a and 62e–g, respectively, were obtained in good yields, good dr, and high enantioselectivity (86–92% ee). But, when a methoxy group was present at the para or meta position of the phenyl group the reaction was more efficient in CH2Cl2 62b giving 87% yield, 81% ee and 4:1 (dr) while the yield is 35% in acrylonitrile with 86% ee and 6:1 (dr). The aldimines derived from cinnamaldehyde, p-nitrocinnamaldehyde, and 3-(2-furyl)acrylaldehyde also gave the corresponding products 62h–j in excellent yields and good enantioselectivities (92, 93, and 89% ee, respectively). As a result of decomposition of the imine, the imine from (E)-4-methylpent-2-enal gave cyclized product 62k in low yield in acrylonitrile, but, the same reaction proceeded well in CH2Cl2 to yield cyclized product 62k in 73% yield and 88% ee. However, in situ generated imine from enal and aniline gave 62l in 85% yield, ee (92%), and dr (8:1).
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| | Scheme 13 Synthesis of γ-lactams by NHC/Brønsted acid cooperative catalysis. | |
Table 5 Evaluation of the imine substrate scopea
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| Conditions: 61, 0.2 mmol; imine 60, 0.1 mmol; o-chlorobenzoic acid, 20 mol%; NHC, 20 mol%; solvent, 0.8 mL. All reactions were carried out under Ar in the presence of 4 Å MS at 0 °C for 15 h. The trans/cis ratios were determined by 1H NMR analysis prior to purification. The ee's were determined by chiral HPLC. CH2Cl2 was used as the solvent instead of acrylonitrile. The starting imine was formed in situ. |
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Various enals were also used as suitable substrates (Table 6). First, using a ketone as a substituent on the enal gave the cyclized lactam 62m in 62% yield and >15/1 dr but only 66% ee. However, using less nucleophilic p-nitrocinnamaldehyde as enal resulted in full conversion to 62n in 93% ee and 14/1 dr. But, much less nucleophilic p-bromocinnamaldehyde and cinnamaldehyde as enal less efficiently cyclized to give 62o and 62p in 56 and 48% yields, respectively, with good ee's and dr's. This suggests that hydrogen bonding exists using p-nitrocinnamaldehyde as enal and indicates that it is possible to develop an approach with achiral carbenes and chiral acids to deliver products with high enantioselectivity.
Table 6 Exploration of various enalsa
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| Conditions: aldehyde 61, 0.2 mmol; 60, 0.1 mmol; o-chlorobenzoic acid, 20 mol%; NHC; 20 mol%; solvent, 0.8 mL. All reactions were carried out under Ar in the presence of 4 Å MS at 0 °C. The trans/cis ratios were determined by 1H NMR analysis prior to purification. The ee's were determined by chiral HPLC. 15 h in acrylonitrile. 36 h in CH2Cl2. 66 h in CH2Cl2. |
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Baeza, and Nájera demonstrated that different Lewis as well as Brønsted acids like TfOH, can act as catalysts in the intramolecular hydroamination of conjugated acyclic aminodienes 63.97 Similar to the Lewis acid-catalyzed reaction TfOH-catalyzed reaction also proceeded in a 1,4-addition manner by a 5-exo–trig cyclization affording the corresponding 2-propenyl-substituted pyrrolidines 64 as a mixture of E/Z diastereomers (Scheme 14). Among various catalysts examined for this particular reaction, TfOH was found to be most efficient and as efficient as [(PhO)3P]AuOTf complex. This Brønsted acid-catalyzed reaction furnished excellent yields 89–95% at room temperature and only in the presence of 5 mol% of catalyst loading.
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| | Scheme 14 TfOH-catalyzed synthesis of 2-propenyl-substituted pyrrolidines. | |
G.-Q. Liu et al. used trifluoroacetic acid (10 mol%) in intramolecular hydroamination of unfunctionalized olefins 63 bearing electron-rich amino groups for the efficient synthesis of 2-methylpyrrolidines 64 in good isolated yields 16–94% (Scheme 15).98 Thorpe–Ingold effect was observed during the reaction. The amino group and the C![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
C double must close to each other, acidity of a Brønsted acid and olefins bearing electron rich nitrogen sources are essential for promoting the intramolecular hydroamination. Substrates bearing different N-substituents did not significantly influence the reaction outcomes. Various substituents like methyl, isopropyl, methoxy, fluoro, chloro, bromo, nitro, cyano or ester on the aromatic ring were well tolerated. The electronic effect of the benzylamines had little impact on the course of the reaction. Alkyl substituents, at the nitrogen atom also showed little effect on the course of the reaction. Heteroatom containing substrates also underwent the cyclization efficiently under the optimal condition.
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| | Scheme 15 Synthesis of 2-methylpyrrolidines. | |
In 2012, Moriyama et al. achieved heavy metal-free condition for the synthesis of substituted pyrollidine derivatives 66 in high yields by an intramolecular aminohydroxylation of N-alkenylsulfonamides 65 (Scheme 16).99 Oxone activated by catalytic Brønsted acid TsOH·H2O worked well as an electrophilic oxidant for this reaction to produce N-sulfonyl prolinol derivatives (66). Moreover, 66 can be transformed into N-sulfonyl proline derivatives by oxidation using a DIB/TEMPO system as well as N-sulfonyl proline derivatives can also be converted into proline ethyl ester by desulfonylation under mild condition. N-Alkenylsulfonamides 65 bearing other sulfonyl groups, such as 4-fluorobenzenesulfonyl, 4-nitrobenzenesulfonyl, n-butanesulfonyl, and (S)-camphorsulfonyl, gave the corresponding pyrrolidine derivatives 66 in high yields 78–97%. Similarly, monoalkyl- and dialkyl-substituted alkenylsulfonamides 65 when treated with oxone (1.5 or 2.0 equiv.), cyclization products were obtained in excellent yields 91–98%. π-Electron-rich disubstituted internal alkenes and disubstituted terminal alkene were also efficiently converted into prolinol derivatives bearing a secondary alcohol group and a quaternary carbon center, respectively, in high yields 78–91%. N-Alkenylsulfonamide 65 bearing a hydroxy group also provided 4-hydroxyprolinol derivative 66 in 91% yield. However, the reaction of diastereotopic N-alkenyl sulfonamides gave moderate to low diastereoselectivities (dr = 77:23–54:46).
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| | Scheme 16 Intramolecular aminohydroxylation of N-alkenylsulfonamides. | |
In this particular reaction catalytic Brønsted acid (TsOH or KSO4H) activates oxone as an electrophilic oxidant to form activated peroxymonosulfate intermediate (A).100 Intermediate (A) may promote the intramolecular aminohydroxylation of N-alkenyl-sulfonamides, particularly electron-poor monosubstituted olefins and also proceed through a tandem reaction via the epoxidation of olefins, followed by the exo-selective intramolecular amination of epoxides (Scheme 17).
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| | Scheme 17 Plausible reaction mechanism for intramolecular aminohydroxylation of N-alkenylsulfonamides. | |
In the same year Coldham et al. showed that chiral, enantiomerically enriched 2-arylpyrrolidines 68 could be prepared by simple asymmetric deprotonation-electrophilic quenching followed by cationic cyclization, either with a Brønsted acid or with iodine (Scheme 18).101 Amine 67 on heating with TFA (1.0 M) in CH2Cl2 gave the expected pyrrolidine product 68 in 96% yield.
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| | Scheme 18 TFA-catalyzed synthesis of pyrrolidine 68. | |
Very recently Pei et al. reported a facile synthesis of 3-hydroxy-3-(trifluoromethyl)-1H-pyrrol-2(3H)-ones 71 by trifluoroacetic acid-catalyzed condensation-cyclization of β-enamino esters 69 and ethyl trifluoropyruvate 70.102 A series of α-trifluoromethylated γ-lactams 71 in good to excellent yields 68–98% were obtained under this mild reaction condition (Scheme 19). Chiral BINOL-derived phosphoric acid as enantioselective catalyst can also catalyze this reaction and showed promising enantioselectivity of the desired product.
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| | Scheme 19 Trifluoroacetic acid-catalyzed synthesis of 3-hydroxy-3-(trifluoromethyl)-1H-pyrrol-2(3H)-ones. | |
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| | Scheme 20 Heterocoupling of two in situ generated electron-rich alkenes. | |
 |
| | Scheme 21 Pyrrolidine derivatives obtained from the reaction of amine derivatives and alkenes or alkynes. | |
 |
| | Scheme 22 Pyrrolidine derivatives obtained from the reaction of amine derivatives and allyltrimethylsilane. | |
 |
| | Scheme 23 Pyrrolidine derivatives obtained from the reaction of amine derivatives and trimethylpropargylsilane. | |
Shimizu and co-workers synthesized 2,3,5-trisubstituted pyrroles 91 using double nucleophilic addition of α,α-dialkoxy ketene silyl acetals 87 and ketene sily thioacetals or trimethylsilyl cyanide 88 to α,β-unsaturated imines 60 followed by acid-promoted cyclization (H2SO4:H2O 3:1) and oxidation with DDQ.106 All the 1,4- and 1,2-addition products 89 obtained using double nucleophilic addition were converted readily into the corresponding multisubstituted pyrroles 91 via cyclization with acids such as H2SO4, methanesulfonic acid, and TFA into dihydropyrrole 90 followed by dehydrogenation with DDQ (Table 7). Both steps proceeded to give the products in high yield. Imidazole glycerol phosphate dehydratase inhibitor (IGPDI) possessing a monopyrrole aldehyde moiety was also synthesized by using this methodology.
Table 7 Synthesis of various dihydropyrroles and pyrroles
Viso et al. developed a useful synthetic route for the preparation of functionalized 2-sulfinyl allylic sulfinamides 92 from readily available chiral sulfinimines and α-metalated vinyl and dienyl sulfoxides.107 Treatment of sulfinamide 92 with m-CPBA in toluene resulting in a fast oxidation of both sulfinyl groups followed by slow epoxidation at the distal double bond and producing a mixture of diastereomeric epoxides. In situ cyclization of these epoxides with a catalytic amount of CSA afforded a 75:25 mixture of sulfonyl dihydropyrroles 93 and 94 in 54% and 19% yields, respectively, where 2,5-cis sulfonyl dihydropyrroles 93 is predominant (Scheme 24).
 |
| | Scheme 24 Synthesis of enantiopure 2,5-cis-disubstituted dihydropyrroles. | |
Kim et al. improved the reactivity of Grubbs catalyst by synthesizing imidazole and pyridine containing novel ligands for the Ru-catalyst.108 These modified catalysts 97 and 98 were treated with a range of acids and the formed acid salts were used as activated catalysts for ring-closing metathesis (RCM) reactions. As a result, reactions employing the acid-modified catalysts showed considerable reactivity enhancement in RCM. Results of various acid-activated catalysts for the synthesis of 96 from 95 are described in Table 8.
Table 8 Results of RCM with various acid-activated catalystsa
|
|
| Entry |
Catalyst |
Acid |
Conversionb (%) |
| Catalyst activation with acids was per formed through stirring the catalyst with an acid for 30 min prior to RCM reaction. Conversion were determined from GC analysis. A 4 N solution in 1,4-dioxane was used. |
| 1 |
97 |
None |
45 |
 |
| 2 |
HClc |
99 |
| 3 |
TFA |
95 |
| 4 |
PTSA |
89 |
| 5 |
Perfluoropropanoic acid |
77 |
| 6 |
Triflic acid |
60 |
| 7 |
CSA |
55 |
| 8 |
98a |
None |
48 |
| 9 |
HClc |
97 |
| 10 |
TFA |
99 |
| 11 |
98b |
None |
99 |
| 12 |
HClc |
96 |
| 13 |
TFA |
24 |
Tu and co-workers developed a three-component domino reaction method for divergent synthesis of fused pyrroles with different substituted patterns by varying N-substituted enaminone substrate starting from arylglyoxal monohydrate 100, enaminone 99 and aromatic amines 101.109 The direct C(sp3)–N bond formation proceeded through domino [3 + 2] heterocyclization and afforded fused pyrroles 103 in good yields 65–87%. This synthetic route via allylic amination allows the formation of several building blocks of pyrrole derivatives with a wide diversity of substituents. In another pattern, different substituted fused pyrrole frameworks 104 were obtained by intermolecular N-arylation in 63–89% yields (Scheme 25). This Brønsted acid-catalyzed reaction involved mild reaction condition, convenient one-pot operation, short reaction time of 15–32 min, and excellent regio- and chemoselectivities.
 |
| | Scheme 25 Synthesis of fused pyrroles with different substituted patterns. | |
Zheng et al. synthesized pyrrole 107, which is an intermediate for the synthesis of BMS-690514 in 78% overall yield. This multistep synthesis was initiated by alkylation of sodium diethyl oxalacetate 106 with bromohydrazone 105 in toluene in the presence of AcOH at 20 °C for 5–6 h, followed by one-pot cyclodehydration by the addition of p-TsOH under heating at 40–45 °C for 4–6 h, to give 107 (Scheme 26).110
 |
| | Scheme 26 Synthesis of pyrrole 107. | |
In 2013, Tu group also described Brønsted acid promoted indolation and thiolation-based three-component domino reactions for the synthesis of polyfunctionalized, fully substituted pyrroles in good to excellent yields (Scheme 27).111 The reactions was further expanded by using an aromatic amine as a monodonor component to prepare multi-functionalized 4,5-dihydro-1H-pyrroles 110, 112 or 114 with high stereoselectivity. The reaction is easy to perform by simply mixing three common reactants in acetic acid under microwave heating and proceeds with such a faster rate that the reaction was completes within 26 minutes.
 |
| | Scheme 27 Synthesis of fully substituted pyrroles 110, 112 and 114. | |
N-Acylpyrrole 118 can be synthesized by a two step method involving condensation of carboxylic acids 115 with 2,4,4-trimethoxybutan-1-amine 116, followed by acid-mediated cyclization to form the pyrrole ring 118 (Scheme 28).112 This CSA-catalyzed mild reaction condition is highly tolerant of a variety of functional groups. N-Acylpyrroles 118 could readily be converted to other carbonyl functionalities such as aldehydes, ketones, esters, and amides and provides an easy access to carbonyl functionalities in the synthesis of complex molecules.
 |
| | Scheme 28 Synthesis of N-acylpyrrole. | |
Frederich et al. synthesized substituted 2,2′-bipyrroles 124 and pyrrolylfurans 121 via intermediate isoxazolylpyrroles 122 or 119.113 In this reaction isoxazole ring serves as a β-diketone equivalent and a directing group for palladium catalyzed chlorination of the attached pyrrole. Reduction of N–O bond and CSA-promoted cyclization afforded roseophilin segment in five steps and 19% overall yield. This methodology was useful for the synthesis of 3-chloro-(4′-alkoxy)-2,2′-pyrrolylfurans 121 and 4-alkoxy-2,2′-bipyrroles 124 (Scheme 29).
 |
| | Scheme 29 Synthesis of substituted 2,2′-bipyrroles and pyrrolylfurans. | |
 |
| | Scheme 30 Fischer indolisation protocol for synthesis of 2,3-disubstituted indoles. (a) 1 eq. phenylhydrazine, 0.5 mol% Rh(acac)(CO)2, 1 eq. PTSA, 50 bar CO, 20 bar H2, 100 °C, 1 d. (b) 1 eq. phenylhydrazine, 0.5 mol% Rh(acac)(CO)2, 1 eq. PTSA, 50 bar CO, 20 bar H2, 100 °C, 2 d. (c) 1 eq. phenylhydrazine, 0.5 mol% Rh(acac)(CO)2, 1 eq. PTSA, 50 bar CO, 20 bar H2, 100 °C, 3 d. | |
Butin et al. reported 1-R-3-(2-indolyl)-1-propanones 148 by applying Reissert indole synthesis and recyclization of 2-(2-aminobenzyl)furan derivatives 144.121 Here furan ring was used as the source of carbonyl function. This reaction tolerated various substituents on the aromatic ring (Scheme 31). Refluxing benzylfurans in ethanolic solution saturated with hydrogen chloride gave indole derivatives 148. The recyclization reaction starts with protonation of furan ring and subsequent nucleophilic attack of the nitrogen atom lone pair onto the furyl cation may generate the intermediate 146 which may undergo reorganization to give 148. Reactions generally required 20 to 40 min to complete, except for benzylfurans.
 |
| | Scheme 31 Synthesis of 1-R-3-(2-indolyl)-1-propanones from 2-(2-aminobenzyl)furan. | |
Seven years later Gevorgyan et al. modified the aforesaid methodology to a one-pot procedure.122 Various substituted indoles 149 have been synthesized from o-aminobenzyl alcohols 142 and furans 143 in the presence of 10 mol% TfOH (Scheme 32). This one-pot procedure expectedly operates via in situ formation of aminobenzylfuran, followed by its recyclization into the indole core. Further the synthesized indoles 149 could easily be transformed into various scaffolds, including 2,3- and 1,2-fused indoles, and indoles possessing an α,β-unsaturated ketone moiety at the C-2 position. Sterically bulky 2-arylfurans 143 are also competent reactants in this reaction providing the corresponding indoles 149 in good yields 48–87%. Furans containing an ester group as a side chain, recyclized uneventfully producing indoles in 61% and 68% yields, respectively. Similarly, 2,3-dimethylfuran and 4,5,6,7-tetrahydrobenzofuran produced the corresponding indoles in 87% and 70% yields, respectively. Benzylphenoloindole derivative was obtained in moderate yield (45%) when benzofuran was used as a starting material. However, when C-4 substituted menthofuran was used, the reaction failed. Various R1 substituents at the α-position of aminobenzyl alcohol 142 such as methyl, isopropyl, tert-butyl, and cyclohexyl groups did not affect too much. Thus, the reported catalytic one-pot conditions are more convenient experimentally and exhibit wider substrate scope than the previous one.
 |
| | Scheme 32 One-pot synthesis of 1-R-3-(2-indolyl)-1-propanones. | |
D. Yin et al. developed an efficient synthesis of 2,3-unsubstituted nitro containing indoles 151 via TFA acid-catalyzed intramolecular electrophilic cyclization starting from commercially available 2,4-difluoro-1,5-dinitrobenzene or 2,4-difluoronitrobenzene and 2-aminoacetaldehyde dimethylacetal 150 (or its N-substituted derivatives).123 Subsequent reduction of nitro groups allowed the construction of some indole fused heterocycles and indole quinine diimines (Table 9). This is an efficient method for the preparation of biologically and medicinally interesting heterocyclic molecules. The nitro groups of resulting indoles can further be reduced and can be transformed to other indole fused structures or indole quinone diimines. These are also useful for the construction of biologically interesting molecules.
Table 9 Synthesis of various substituted indolesa,d
Readily accessible α-keto-N-arylacetamides 152 bearing alkyl side chain residues readily undergoes TFA-mediated Friedel–Crafts reaction for the synthesis of diversely functionalized 3,3′-disubstituted oxindoles 153, 155 and spirooxindoles 158.124 In TFA, α-ketoanilides 152 cyclized readily at room temperature to afford the 3-hydroxy-3-alkyl(aryl)-2-oxindoles 153 in 50–89% yields (Scheme 33). Double cyclization took place for appropriately functionalized substrate 156, and gave spirooxindoles 158 with the creation of an all carbon chiral quaternary center (Scheme 34). While in situ prepared α-iminocarboxamides 160, from α-ketoamides, cyclized under similar conditions to 3-amino-3-alkyl-2-oxindoles 161 in good to excellent 59–90% yields (Scheme 35).
 |
| | Scheme 33 Synthesis of 3-hydroxy-2-oxindoles. | |
 |
| | Scheme 34 Synthesis of spirooxindoles 158 by double intramolecular arylation of α-ketoanilides. | |
 |
| | Scheme 35 Synthesis of 3-amino-2-oxindoles. | |
Acetic acid was used as an effective catalyst in three component domino reaction between arylglyoxal monohydrate 100, diverse enaminones 99 and indoles 109 for the regioselective synthesis of 3,2′- and 3,3′-bis-indoles 162 under microwave irradiation.125 3,2′-Bis-indole skeleton was formed from 2-unsubstituted indoles. If indoles contain methyl or phenyl groups at C-2 led to 3,3′-bis-indoles with simultaneous formation of three sigma-bonds (Scheme 36). This acid-catalyzed reaction proceeded through [3 + 2] heterocyclization and gives bis-indoles 162 in good yields 60–88%. The reaction may involve the ring closure cascade process which included initial protonation (100 to A), nucleophilic substitution (A to B) and subsequent second protonation (B to C), second nucleophilic substitution (C to D and E), and intramolecular cyclization followed by dehydration (E to 162). This regioselectivity was attributed to intramolecular hydrogen bond of intermediate B (Scheme 37) (Fig. 3).
 |
| | Scheme 36 Three-component domino reaction for regioselective synthesis of bis-indole derivatives. | |
 |
| | Scheme 37 Possible mechanism of product 162. | |
 |
| | Fig. 3 Diversity of substrates for synthesis of indole 162. | |
In the same year the same group reported the synthesis of tetrahydroindole derivatives 164 by p-TsOH promoted three component domino reaction under microwave irradiation.126,127 The one-pot reaction was faster and completed in just 30 minutes and afforded C3-alkoxylated indole framework 164 in good yields 65–87% (Scheme 38).
 |
| | Scheme 38 Three-component domino synthesis of tetrahydroindoles derivatives. | |
Similarly, Zhao et al. used TFA-promoted bicyclization between 2,2-dihydroxyindene-1,3-dione 165 and cyclic enaminones 99 for the efficient synthesis of isochromeno[4,3-b]indoles 166 under microwave irradiation.128 2.0 equivalent of TFA was sufficient for this method and installs C–N, C–O and C–C bond in one operation in 76–86% yield (Scheme 39). This protocol tolerates a wide range of substituents in cyclic enaminone 99.
 |
| | Scheme 39 Domino formation of isochromeno[4,3-b]indoles. | |
Very recently, Uchuskin et al. reported two opposite reactivity modes of the α-carbon of the furan ring in one process and synthesized various 2-(2-acetylvinyl)indoles derivatives 168 by domino reaction of N-tosylfurfurylamine 167 with 2-tosylaminobenzyl alcohols 142 (Scheme 40).129 The α-carbon of the furan behaves unusually, which reacts initially as a nucleophile in the Friedel–Crafts alkylation and then as an electrophile in the Piancatelli-like rearrangement, followed by aromatization of the rearranged product. When the tosylamino group in the furan side chain of 167 was substituted by the phthalimide moiety chemoselectivity of the reaction changes and exclusively gave 2-(4-phthalimido-3-oxobutyl)-indole 168. The α-carbon of furan demonstrates the same ambiphilic behavior as that of N-furfurylbenzamides which represent the intermediate case producing both types of products. Control experiments suggested two independent pathways for the formation of these indole derivatives.
 |
| | Scheme 40 Synthesis of indoles by domino reaction of 2-(tosylamino)benzyl alcohols with furfurylamines. | |
A. Srivastava et al. used commercially available inexpensive (±)-CSA (30 mol%) as a Brønsted acid for domino dehydration/condensation/cyclization sequence reaction for the synthesis of substituted indole derivatives.130 The reaction of cyclic enaminones 99 with 3-hydroxy-3-ethoxycarbonylisoindolin-1-one derivatives 169 in toluene at 90 °C in presence of catalytic amount (±)-CSA (30 mol%) provided various 1-aryl/alkyl-substituted 6,7-dihydrospiro[indole-3,1-isoindoline]-2,3,4(1H,5H)-trione derivatives 170 in good to excellent 74–87% yields (Scheme 41).
 |
| | Scheme 41 Synthesis of (6,7-dihydrospiro[indole-3,1-isoindoline]-2,3,4(1H,5H)-trione derivatives. | |
Structurally diverse types of indole derivatives such as thieno- and furo-indoles, spiro-indolethiones, spiro-oxindoles, and 3-alkylidene-oxindoles derivatives were synthesized via triflic acid-promoted cycloisomerization with anionotropic rearrangement of a substituent or hydrogen in R of 2-(alkyn-1-yl)phenyl isothiocyanates and isocyanates. Although, as anticipated, the substituents R of 2-(alkyn-1-yl)phenyl isothiocyanates and isocyanates are limited to those having rich migrating ability (Scheme 43).131 Isothiocyanate 171 (R = t-Bu) in presence of 3 equivalents of triflic acid gave good yield (78%) of the expected product 172 (Scheme 42). The reaction of 171 containing a noncyclic secondary substituent of R = i-Pr under triflic acid catalyzed-conditions produced 2,3-dimethyl-8H-thieno[2,3-b]indole (173) in 45% yield (Scheme 44). Isothiocyanates 174 having a cyclic secondary substituent [R = c-Hex (n = 2) and c-Pent (n = 1)] produced the ring-expanded cycloalkane-fused 8H-thieno[2,3-b]indoles 175 in 34% and 37% yields, respectively (Scheme 45). When isocyanate 176 (R = t-Bu) was similarly treated with triflic acid (3.0 equiv.) at 0 °C for 10 min in dichloromethane, 2,2,3-trimethylfuro[2,3-b]indole (177) was obtained in 85% yield together with 3-(prop-3-en-2-ylidene)oxindole (178) in 9% yield (Scheme 46). The same reaction in one pot from 176 for 25 h gave furo[2,3-b]indole 177 in 99% yield. When the isolated oxindole 178 was treated with triflic acid (3.0 equiv.) under the conditions of 0 °C to rt for 25 h in dichloromethane, the furo[2,3-b]indole 177 was obtained quantitatively.
 |
| | Scheme 42 Triflic acid-promoted cycloisomerization of 171 to produce thieno-indole 172. | |
 |
| | Scheme 43 Possible reaction pathway for formation of thieno-indole 172. | |
 |
| | Scheme 44 Triflic acid-promoted cycloisomerization of 171 to produce thieno-indole 173. | |
 |
| | Scheme 45 Triflic acid-promoted cycloisomerization of 174 to produce thieno-indole 175. | |
 |
| | Scheme 46 Triflic acid-promoted cycloisomerization of 176 to produce indole 177 & 178. | |
 |
| | Scheme 47 Synthesis of indole-2-carboxylates via Fischer indolization. | |
 |
| | Scheme 48 Synthesis of benzo[b]carbazoles. | |
 |
| | Scheme 49 Synthesis of 6-fluoro-4-oxopyrido[2,3-a]carbazole-3-carboxylic acids. | |
2.1.4. Six membered.
2.1.4.1. Piperidine. Pharmaceuticals, such as paroxetine, alvimopan, and morphine and biologically active natural products and drugs, such as manzamine A, haliclonacyclamine F, corydendramine B, incarvillatiene, reserpine, pseudodistomin C, adalinine, (S)-coniine, (S)-anabasine, adaline all contains piperidine ring motif.136–141
Brønsted acid as catalyst.
Reddy et al. demonstrated an efficient and practical protocol for the synthesis of 4-amido-2-aryl or 2-alkyl piperidine 190 using triflic acid catalyzed aza-Prins cyclization between allyl amines 188 and aldehydes 189 (Scheme 50).142 1.2 equivalent of triflic acid was sufficient for this transformation and gives piperidine derivatives 189 in 68–85% yield. No aza-Prins cyclization occurs in the absence of triflic acid even in refluxing in CH3CN and aliphatic, simple aromatic or moderately activated aldehydes gave better result than the strongly activated or deactivated aldehydes.
 |
| | Scheme 50 Synthesis of 4-amido-2-aryl or 2-alkyl piperidine. | |
Lin and co-workers reported a trifluoromethanesulfonic acid-catalyzed tandem semi-pinacol rearrangement/alkyne–aldehyde metathesis reaction for the efficient synthesis of 9-benzoyl-7-tosyl-7-azaspiro[4.5]dec-9-en-1-ols 192 (Scheme 51).143 This metathesis reaction afforded spiropiperidines 192 14–91% yields in the presence of 10% TfOH in DCE at 50 °C which could be further converted to 193. Electron-neutral and electron-rich arenes at the alkyne terminus were found to be good substrates, as the yields of desired spiropiperidines 192 as a mixture of diastereomers ranged from 66% to 91%. However, substrates with a bromine atom at the phenyl ring, or an electron-withdrawing group, such as an ester or nitro group at the phenyl ring, were less effective and required prolonged reaction time to provide spiropiperidines 192 in 14% to 56% isolated yields.
 |
| | Scheme 51 Synthesis of spiropiperidines. | |
A concise synthesis of the tetracyclic core (ABCE rings) of daphenylline using 13 steps144 was achieved in 7.5% overall yield. Among the total number of steps the most important step was Brønsted acid promoted intramolecular Friedel–Crafts type Michael addition reaction of a δ-benzyl α,β-unsaturated lactam 194, which efficiently constructed the benzomorphan (benzobicyclo[3.3.1]lactam) backbone 195 (Scheme 52). The desired benzobicyclo[3.3.1] lactam 195 was obtained in excellent yield (90%) using 2 equiv. of triflic acid at 50 °C in 8 h, with 1,2-dichloroethane (DCE). The limitation of the reported reaction is that methoxy group located at the meta-position to the reaction site significantly lowered the activity and gave no desired product. 4-Methoxy and 2,5-dimethoxy substitution failed to give any desired cyclization product but 3,5-dimethoxy and 3,4,5-trimethoxy substituted lactam 194 proceeded smoothly under the acid-catalyzed reaction condition.
 |
| | Scheme 52 Synthesis of benzobicyclo[3.3.1]lactam. | |
Pigge and co-workers showed that aldehyde and ketone electrophiles incorporated into the side chains of 2- and 4-alkylpyridines 196 participate in the intramolecular aldol-like condensation with pyridine benzylic carbons in the presence of Brønsted acid catalysts. 10 mol% TfOH at 120 °C in dioxane was sufficient for pyridines featuring β-ketoamide side chains 196 to undergo cyclization to afford pyridyl-substituted hydroxy lactams 197 in 59–98% yields (Table 10).145 In contrast, under the same reaction condition of TfOH-catalyzed cyclization of pyridine tethered to aliphatic aldehydes with amine linkers gave pyridyl-substituted dehydro-piperidine products 197. Similarly, intramolecular condensation of salicylaldehyde and salicylketone-substituted pyridines afforded pyridyl-substituted benzofurans 199 (Scheme 53).
Table 10 TfOH catalyzed benzylic cyclization of alkylpyridines
 |
| | Scheme 53 Synthesis of pyridyl-substituted benzofurans. | |
Table 11 Endo selective aminotrifluoroacetoxylations of alkenes
| Entry |
Alkene |
Product |
Conditiona |
Yieldb |
rsc |
|
|
| Condition A: alkene (0.1 mmol), PhI(OAc)2 (0.12 mmol), TFA (0.2 mmol), CH2Cl2 (1 mL), rt, 12 h. Condition B: alkene (0.1 mmol), PhI(OAc)2 (0.12 mmol), TFA (1.2 mmol), CH2Cl2 (1 mL), rt, 24 h. Condition C: alkene (0.1 mmol), PhI(OTFA)2 (0.12 mmol), TFA (1.2 mmol), CH2Cl2 (1 mL), rt, 24 h. Isolated yield of aminoalcohol(s) following saponification, K2CO3/MeOH, rt, 5 min. rs = regioselectivity, determined by 1H NMR spectroscopy. |
| 1 |
R = Ph, PG = Ts |
|
A |
99% |
(>20:1) |
| 2 |
R = C5H10, PG = Ts |
|
A |
99% |
(>20:1) |
| 3 |
R = H, PG = 2-Ns |
|
C |
95% |
(14:1) |
| 4 |
 |
 |
B |
81% |
(15:1) |
| 5 |
 |
 |
B |
82% |
|
 |
| | Scheme 54 Oxyamination reaction. | |
6-endo Aminotrifluoroacetoxylation products in excellent yields 82–94% were also obtained in the presence of various sulfonamide protecting groups such as 2-nitrobenzene-sulfonyl (2-Ns), 4-nitrobenzenesulfonyl (4-Ns), and trimethylsilylethanesulfonyl (SES). When TsOH was used in place of TFA, acid incorporation also occurred and afforded tosylate 202 in good yield (Table 12).
Table 12 Endo-cyclization protecting group assay
|
| Entry |
PG |
Oxidant |
Acid |
% yieldb |
| Reaction conditions: 1 (0.1 mmol), CH2Cl2 (1 mL), oxidant (0.12 mmol), acid (1.2 mmol), rt, 24 h. Isolated yield of product following saponification, K2CO3/MeOH, rt, 5 min. TsOH·H2O (5 equiv.). Yield of tosylate (R = Ts). |
| 1 |
Ts |
PhI(OAc)2 |
TFA |
88 |
| 2 |
Ts |
PhI(OAc)2 |
TsOHc |
82d |
| 3 |
Ts |
PhI(OTFA)2 |
TFA |
92 |
| 4 |
2-Ns |
PhI(OTFA)2 |
TFA |
92 |
| 5 |
4-Ns |
PhI(OTFA)2 |
TFA |
90 |
| 6 |
SES |
PhI(OTFA)2 |
TFA |
94 |
Various 1,1- and 1,2-disubstituted alkenes underwent cyclization under standard conditions to give aminotrifluoroacetoxylation products in good yields 49–92% (Fig. 4). Unlike the mono-substituted pentenyl substrates, styrenyl substrates provided the 5-exo cyclization products. Whereas cyclohexenyl substrate also selectively underwent 5-exo cyclization to form the 5,6 fused bicycle rather than a 6,6 bridged bicycle. 1,2-Disubstituted alkenes with alkyl substituents preferred endo cyclization, with the Z alkene exhibiting significantly better regioselectivity than the E isomer. The trifluoroacetate is preferentially affixed to the carbon where it is suitable to stabilize a carbocation, providing a reliable predictor of regioselectivity. Substituents on the tether had little effect on this preference. Substitution alpha to the sulfonamide resulted in good selectivity for the trans product, whereas substitution in the beta position gave mostly cis stereochemistry.
 |
| | Fig. 4 Various piperidine and pyrrolodine derivatives. | |
The reaction proceeds through (Scheme 55) the oxidation of alkene to generate an iodonium ion, A (pathway a). The iodonium ion may then be intramolecularly attacked by the sulfonamide to form the kinetically preferred intermediate B. For the more reactive styrenyl substrates or for strained structures, intermediate B is then rapidly trapped by the trifluoroacetate counterion to form the 5-exo products. For less strained and reactive substrates the nitrogen may eventually displace the iodine to generate an aziridinium ion, C. Subsequent nucleophilic attack onto the more substituted carbon of C would then generate the endo cyclized products with anti selectivity. Alternatively, the sulfonamide may be directly oxidized prior to formation of the aziridinium ion (pathway b). Either oxidation pathway would converge to aziridinium ion, C, leading to an endo cyclization. The observation that there is no apparent difference in the rate of reaction of the Ns and Ts substituted substrates would appear to favor an alkene oxidation mechanism (pathway a) over a sulfonamide oxidation mechanism (pathway b).
 |
| | Scheme 55 Proposed reaction mechanism. | |
 |
| | Scheme 56 Various general routes to synthesis of substituted pyridines. | |
 |
| | Scheme 57 Synthesis of 2,3,5-substituted or annulated-6-(methylthio)pyridines. | |
Jadidi and co-workers reported the formation of spiro-pyridodipyrimidine-oxindole derivatives161 by p-TsOH-catalyzed cyclocondensation of isatins with two molar equivalents of 2,6-diaminopyrimidine(3H)-4-one.
Chuang et al. synthesized a series of disubstituted pyridine derivatives 210 or trisubstituted pyridine derivatives 211 from the corresponding acryloyl azides 209 by the acetic acid-promoted cycloaddition (Scheme 58).162 The yield of the acid-promoted cycloaddition depended on the substituent on the double bond and the solvent used. The reactivity of the acid-promoted cycloaddition increases when the corresponding acryloyl azides 209 contain aryl groups such as phenyl and pyridinyl.
 |
| | Scheme 58 Thermal cycloaddition of acryloyl azide to give pyridine. | |
Donohoe group used olefin cross-metathesis reaction to provide a rapid and efficient method for the synthesis of α,β-unsaturated 1,5-dicarbonyl derivatives 214 which then served as effective precursors to mono-, di- and tri-substituted pyridines 215.163 Tetra-substituted pyridines could be synthesized via further manipulation of the key 1,5-dicarbonyl intermediate. High levels of regiocontrol, short reaction sequence, and facile substituent variation are all notable aspects of this methodology (Scheme 59).
 |
| | Scheme 59 Synthesis of pyridine derivative. | |
The catalytic activity of p-TsOH has been used for the novel synthesis of benzonaphthyridines 218 from the simple synthons aminocarbazole 216 and 2-chloro-3-formylquinolines 217 by Prasad et al. (Scheme 60).164 The catalytic activity of p-TsOH is superior to other reported catalysts with respect to enhanced yields and reduced reaction time.
 |
| | Scheme 60 Synthesis of benzonaphthyridine derivative. | |
Nechayev et al. synthesized substituted pyrrolo[2,3-c]pyridine-7-ones by the application of TFA-promoted intramolecular cyclization of 2-pyrrolecarboxylic acid amidoacetals as the key step.165
An efficient and new type of polysubstituted spiro[dihydropyridine-oxindole] 221 have been developed by a one-pot three-component reaction of arylamine 104, isatin 219 and cyclopentane-1,3-dione 220 in acetic acid at room temperature.166 On the other hand the condensation of isatin 219 with two equivalents of cyclopentane-1,3-dione 220 gave 3,3-bis(2-hydroxy-5-oxo-cyclopent-1-enyl)oxindole 222 in high yields (Scheme 61).
 |
| | Scheme 61 Synthesis of spiro[dihydropyridine-oxindole] derivative. | |
Wang et al. used p-toluenesulfonic acid (20 mol%) under refluxing condition in CH3OH for 24 h to achieve the multi-component cascade reaction of aldehydes 189, ketones 205 and ethane-1,2-diamine 223 for the preparation of trisubstituted hexahydroimidazo[1,2-a]pyridines 224.167 Under this acid catalyzed transformation all the reactants being efficiently utilized and two new cycles and five new bonds were constructed (Scheme 62).
 |
| | Scheme 62 Synthesis of hexahydroimidazo[1,2-a]pyridines. | |
 |
| | Scheme 63 Synthesis of substituted 1,2-dihydropyridines. | |
The reaction proceeded via first Au-catalyzed cyclization-induced rearrangement169,170 to generate enamine A in a classical condensation reaction. A is tautomerized to azatriene C by protonation of A through formation of achiral iminium ion B. Subsequent C is converted to 1,2-dihydropyridine 226 by 6π-electrocyclization (Scheme 64).
 |
| | Scheme 64 Mechanistic proposal. | |
 |
| | Scheme 65 [Hmim]TFA mediated synthesis of substituted quinoline derivatives. | |
 |
| | Scheme 66 Friedländer synthesis of quinolines catalyzed by PEG-bound sulfonic acid. | |
Brønsted acid organocatalyst o-benzenedisulfonimide 227 can also be used for the synthesis of quinoline derivatives 231 via Friedländer annulation.176 Yields of the target products are good in the presence of catalytic amount of o-benzenedisulfonimide 230 (Scheme 67). Chitosan-SO3H is also an efficient catalyst for Friedländer condensation/annulation reaction.177
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| | Scheme 67 o-Benzenedisulfonimided-mediated quinoline synthesis. | |
Very recently, Xu Zhang et al. used HOTf-catalyzed three component aza-Diels–Alder reaction for the construction of quinolines from readily available aldehydes 189, amines 101, and alkynes 79/alkenes 80.178 Amine (1.0 mmol), aldehyde (1.0 mmol), and alkyne 79 (1.1 mmol) in the presence of HOTf (5 mol%) in toluene at 100 °C for 4 h gave 54–95% yields of quinoline derivatives 234. In case of alkene 80 (1.1 mmol) under the same reaction condition in 8 h gave the products 234 in 42–78% yields. Both electron-donating and electron-withdrawing substituents on the aldehydes, amines, and alkynes/alkenes were suitable in this acid-catalyzed transformation (Scheme 68).
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| | Scheme 68 HOTf-catalyzed three-component reaction. | |
TfOH is a superior promoter of the tandem Friedel–Crafts alkenylation–cyclization reaction of 2-alkynylphenyl isothiocyanates for the synthesis of a variety of 4-arylquinoline-2-thiones 236 and 3-arylthieno[2,3-b]indoles 239 in high yields.179 The reaction between 171 and arenes 235 furnished 4-aryl-3-unsubstituted quinoline-2-thiones 236 at 0 °C (Scheme 69). On the other hand, the reaction of 237 produced indoline-2-thione 238, which were readily converted to the corresponding thienoindoles 239 via the dehydrogenative cyclization (Scheme 70).
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| | Scheme 69 TfOH-mediated synthesis of quinoline-2-thiones. | |
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| | Scheme 70 TfOH-mediated synthesis of indoline-2-thiones 238 and thienoindoles 239. | |
Kim et al. synthesized a series of 3,4-disubstituted 2(1H)-quinolinones 243 via a series of reactions, the most important step was the H2SO4-assisted intramolecular Friedel–Crafts cyclization.180 Substituted quinolines 243 were synthesized starting from the Baylis–Hillman adducts via hydrolysis of the Baylis–Hillman adduct to acid 240 followed by coupling with anilines gave the enamine 241. Next H2SO4-assisted intramolecular Friedel–Crafts cyclization gave 242. DBU-mediated isomerization of 242 afforded the products 243 in 80–99% yields (Scheme 71).
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| | Scheme 71 Synthesized of 3,4-discubstituted 2(1H)-quinolinones. | |
A variety of polycyclic quinolines was synthesized in two steps from the substrates 244 or 245. The compounds 244 and 245 were separately treated with imidazol-2-carboxaldehyde 246 under Baylis–Hillman condition to give the adduct 247. The substrate 247 underwent second intramolecular cyclization with HCl in THF–H2O (1:1, v/v) at room temperature to give the quinoline derivatives 249 in 35% yields. Reductive cyclization of 247 with Fe and AcOH at 120 °C produced the tetracyclic heterocycles 248 in 47–78% yields (Scheme 72).181
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| | Scheme 72 Synthesis of fused polycyclic quinolines. | |
Peng et al. developed a p-TSOH-promoted convenient and single-step procedure for the 4-alkylquinolines 251 from readily available 2-alkynylanilines 250 and activated ketones 205.182 The reported method tolerated a wide range of functionalities on both substrates providing complementary access to the Friedländer reaction, resulting in the formation of a variety of 4-alkylquinolines 251 (Scheme 73). Starting from symmetric 2-alkynylanilines 250 dimeric quinolines 252 with potential biological attention could be synthesized efficiently (Scheme 74).
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| | Scheme 73 p-TSOH-promoted annulation of 2-alkylanilines and ethyl acetoacetate in ethanol. | |
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| | Scheme 74 p-TSOH-promoted annulation of 2-alkylanilanilines with activated ketone in ethanol. | |
Baba et al. used HCl (5 mol%) in DMSO for synthesis of quinolin-4-carboxylate 254 in 43% yield by the reaction of methyl pyruvate with benzylideneaniline (1.2 equiv.) under an air atmosphere at 80 °C for 6 h (Scheme 75).183
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| | Scheme 75 Synthesis of methyl quinolin-4-carboxylate. | |
Chen et al. reported the reversal Skraup–Doebner–Von Miller quinoline synthesis for an efficient synthesis of 4-arylquinoline-2-carboxylates 256 in 42–83% yields. By refluxing a mixture of substituted aniline 101 and γ-aryl-β,γ-unsaturated α-ketoester 255 (2 equiv.) in TFA for 8–18 h, first condensation of aniline with ketone moiety of γ-aryl-β,γ-unsaturated α-ketoester occurred to give the corresponding imines, which underwent intramolecular cyclization followed by oxidative aromatization leading to the formation of quinoline-2-carboxylates 256 (Scheme 76).184
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| | Scheme 76 Synthesis of 4-arylquinoline-2-carboxylates via reversal Skraup–Doebner–Von Miller reactions. | |
Chaskar and co-workers described a simple, efficient, and ecofriendly procedure for the synthesis of quinoline derivatives 257 79–97% in a one-pot reaction of aniline 101 with crotonaldehyde or methyl vinyl ketone 61 using phosphomolybdic acid in toluene in miceller media at 80 °C for 50 min (Scheme 77).185 The acid catalyst could be recycled and reused.
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| | Scheme 77 Synthesized of quinoline derivatives by using phosphomolbdic acid. | |
TfOH is a commonly used super acid (H0 = −14.1) for effective catalysis of many transformations,186 its use is preferable to that of other acids with similar acid strengths (e.g., H2SO4, ClSO3H, FSO3H) and since, it does not promote oxidative side reactions,186g,187 TfOH was chosen as the acid of choice for tandem ring-closure-aryl-migration reaction of 2′-amino chalcone epoxide 258 for the synthesis of 3-aryl-4(1H)-quinolones 259 (azaisoflavones) (Scheme 78).188 The desired compound 259 can also formed by the use of 15 mol% of AgOTf as co-catalyst along with 5 mol% of AuCl3 but only of in trace amount. Use of 1.0 equivalent of AgOTf alone as a catalyst gives the product in 25% yield in 24 hours but, the yield of the product can be improved to 65%, by using 1.0 equivalent of TfOH instead of AgOTf by using 3.0 equivalents of TfOH the yield can be further improved to 90%. This may be attributed to the stoichiometric amount of H2O formed (1.0 mol of H2O/1.0 mol of chalcone epoxide) in the reaction, and to the acidity of the system dropping significantly.189 This low acidity prevents further transformations from being catalyzed.
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| | Scheme 78 TfOH-promoted synthesis of azaisoflavones from 2′-amino chaloconeepoxide. | |
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| | Scheme 79 Proposed mechanism for the formation of azaisoflavone. | |
Under the reaction condition electron-releasing substituents 258 gave 90–94% yields of products in a shorter reaction time. In comparison, the electron-withdrawing substrate gave moderate to good 73–87% yields of products 259 in longer reaction time. This is because of the greater stabilization of the phenonium ion intermediate by the electron-releasing substituents compared to their electron-withdrawing complements. The reaction may proceed (Scheme 79) through the initial protonation of the trans-2′-amino chalcone epoxide 258 that results in the protonated intermediate 260. Six-membered intermediate 261 was formed by nucleophilic attack of the tethered amino group, which, upon deprotonation, may afford 2-aryl-3-hydroxy-tetrahydro-4(1H)-quinolone (262), where the aryl and hydroxyl groups are located trans to each other. Intermediate 263 was formed by the protonation of the hydroxyl group of 262. Phenonium ion190 264 (in resonance stabilization with 264′) may be formed by the anchimeric assistance by the C2-aryl group of 263. Electron-donating amino group facilitated migration of the aryl group from the C2 to the C3 position and pushes electron density from the aryl group towards the C3 position, leading to the intermediate 265. Deprotonation at the α-carbon of the intermediate 265 resulted in the formation of 3-aryl-4(1H)-quinolone 259.
L. Peng et al. reported a sequential hydration–condensation–double cyclization reaction for the construction of quinoline-based tetracyclic scaffolds 267.191 The environmentally benign and one-pot, atom-economical process catalyzed by p-toluenesulfonic acid in ethanol gave moderate to excellent yields 35–96% of 267 starting with readily available pyridine-substituted o-alkynylanilines 266 and β-keto esters 205 (Scheme 80). In the absence of β-keto esters, multisubstituted quinolines 268 are formed bimolecularly in reasonable yields 31–64% (Scheme 81).
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| | Scheme 80 Synthesis of multicyclic heterocycles containing pyridines and quinolines. | |
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| | Scheme 81 Synthesis of multi substitute quinolines. | |
Nitric acid can be used as an efficient catalyst for simple and efficient synthesis of polysubstituted 1,2-dihydroquinoline 270 or 271 derivatives including tricyclic ones via a one-pot tandem reaction of α-ketoesters 269 with primary 101 or secondary aromatic amines 109.192 This HNO3-catalyzed protocol offers various advantages, such as operation simplicity, cheap and readily available catalyst, atom efficiency as well as low toxicity and provide an expedient access to construct versatile dicyclic and tricyclic 1,2-dihydroquinoline building blocks, which could be further used for the synthesis of new drug molecules and other potential biologically active molecules (Scheme 82).
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| | Scheme 82 Nitric acid-mediated synthesis of substituted quinoline derivatives. | |
1-Acetyl N-aryl cyclopentanecarboxamides 272 via H2SO4-mediated tandem cyclization/ring-opening/recyclization reaction gave pyrano[2,3-b]quinoline derivatives 273 during which a novel ring-cleavage of the cyclopentane unit was involved.193 The reaction is simple and proceeded by dissolving 272 (1.0 mmol) in a small amount of 98% concentrated H2SO4 (0.5 mL) at 50 °C for 1.5 h, various substituted pyrano-[2,3-b]quinoline derivative 273 (Scheme 83) were obtained in 67–96% yields. The reaction does not depend on electron-donating or electron-withdrawing groups on the aryl ring.
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| | Scheme 83 Synthesized of pyrano[2,3-b]quinolines. | |
Similarly Xiang et al. used trifluoromethanesulfonic acid under solvent-free condition for the efficient one-pot synthesis of pyrano[2,3-b]quinolines 275 via the Combes-type reaction starting from readily available enaminones, 2-arylamino-3-acetyl-5,6-dihydro-4H-pyrans 274 (Scheme 84).194 The reactions in the presence of 4 mmol trifluoromethanesulfonic acid at 80 °C were equally efficient for enaminones bearing both electron-donating and electron-withdrawing groups on the aromatic ring and gave pyranoquinolines 275 in 64–95% yield.
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| | Scheme 84 Reactions of enaminones with CF3SO3H under solvent-free-conditions. | |
Jiang et al. synthesized a series of saccharide-binding arylboronic acid derivatives of indoloquinoline 279.195a Among various steps the key synthetic step was polyphosphoric acid-mediated cyclization (Scheme 85). Kouznetsov et al. used phthalic acid promoted domino Povarov reaction between anilines and N-vinylamides for the efficient synthesis of 2-methyl-1,2,3,4-tetrahydroquinolines with C4 amide fragments.195b
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| | Scheme 85 Synthesized of saccharide-binding arylboronic acid derivatives. | |
Seidel et al. showed that indoles display a diverse pattern of reactivity upon reaction with different classes of aminobenzaldehydes.196 Previous report demonstrated that indolobenzazepines formed via acid-catalyzed annulation cascade reaction of indole with aminobenzaldehydes. Whereas the acid-promoted reaction with secondary aminobenzaldehyde 280 triggers indole 109 annulation/oxidation cascades that lead to the rapid formation of neocryptolepine and analogues 281. The condensation of 280 with indole 109 in the presence of 1 equivalent p-TSA in EtOH under refluxing condition provides neocryptolepine 281 in 77% yields. Electronically diverse indoles 109 and modified aminobenzaldehydes 280 were readily engaged under this reaction condition to give neocryptolepine derivatives 281 in moderate to good yield 56–77% (Scheme 86). Under similar reaction condition primary aminobenzaldehydes 280 resulted in the formation of synthetically useful 3-(2-aminophenyl)quinolines 282 via a remarkably facile indole ring opening in good to excellent yields and typically in brief reaction time. Indoles with various substituents 109 well tolerated this reaction condition but for sterically encumbered substrates required longer time (8 h). Various aminobenzaldehydes 280 with different substitutions 109 were also suitable for this reaction and p-TSA or TFA were found as best acid promoter for these reactions (Scheme 87).
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| | Scheme 86 Synthesis of neocryptolepine and analogues. | |
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| | Scheme 87 Reactions of various aminobenzaldehydes with various indoles. | |
2-Indolinone-tethered allenols 283, when reacted with NPSP (N-phenylselenophthalimide), a source of seleniranium ion and catalytic amounts of PTSA in dichloromethane at room temperature gave rise to quinoline-2,3-dione 284a as major product; quinoline-2,4-dione 285a and spirocycle 286a were also isolated as minor components.197 The comparative studies of quinolone formation with addition of PTSA demonstrated that the presence of Brønsted acid gave higher yields and that the acid additive acts as an activator but not as a catalyst (Scheme 88). The same reaction without any PTSA gives the spirocycle 286a as major product (thermodynamic control product), while the quinoline-2,3-dione 284a was obtained as a minor product (kinetic control product).
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| | Scheme 88 Rearrangement reactions of indonlione-tethered allenols. | |
Kulkarni et al. developed an efficient method for the preparation of 3-methylquinoline-4-carbaldehydes 288 by reductive cyclization of nitroaldehydes 287 (Scheme 89).198a Nitroaldehydes 287 was obtained from 2-nitro benzaldehyde after a series of reactions such as Wittig-olefination–Claisen-rearrangement etc. similarly in Ramesh et al. used iron/acetic acid-mediated carbon degradation reaction for efficient synthesis of substituted quinoline derivatives.198b
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| | Scheme 89 Synthesis of 3-methylquinoline-4-carbaldehydes. | |
Brønsted acid MsOH played an extremely important role along with pyridine-N-oxide (external oxidant), for the metal-free oxidation/C(sp3)–H functionalization of unactivated aryl alkynes 289. 4.0 equivalent MsOH along with 2.0 equivalent of pyridine-N-oxide in CH2Cl2 at 30 °C plays significant role towards the activation of the alkynes (Table 13).199
Table 13 Synthesis of dihydroquinolin-4(1H)-one
Mphahlele et al. used acid-mediated cyclization of 1-(2-amino-3,5-dibromophenyl)-3-aryl-2-propen-1-ones 291 with orthophosphoric–acetic acid mixture to afford isomeric 2-aryl-6,8-dibromo-2,3-dihydroquinolin-4(1H)-ones 292 under refluxing condition for 2 h (Scheme 90).200
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| | Scheme 90 Synthesis of 2-aryl-6,8-dibromo-2,3-dihyroquinolin-4(1H)-ones. | |
Ethyl pyrroloquinoline 4-carboxylates 294 could be synthesized by one-pot Doebner–Von Miller quinoline synthesis through reaction of 4- or 5-aminoindoles 293, aldehydes, and ethyl pyruvate in refluxing ethanolic HCl.201 By heating a mixture of an equimolar amount of methyl pyruvate and an aldehyde in ethanolic HCl at 80 °C for 1 h under N2 atmosphere gave corresponding β,γ-unsaturated α-ketoester. Dropwise addition of a solution of aminoindole 293 in EtOH to the β,γ-unsaturated α-ketoester and heating at 100 °C for additional 2–10 h afforded the desired pyrroloquinoline 4-carboxylates 294 in 15–50% yields (Scheme 91).
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| | Scheme 91 Synthesis of ethyl pyrroloquinoline 4-carboxylates. | |
Orejarena et al. used Clauson–Kaas reaction of a substituted ortho-allylaniline 295 followed by acid-catalyzed regioselective intramolecular Friedel–Crafts alkylation of the resulting 1-(2-allylaryl)-1H-pyrroles 296 for the synthesis of substituted 4-methyl-4,5-dihydropyrrolo[1,2-a]-quinolines 297. The synthesized pyrrolo[1,2-a]quinolines 297 have potential antitumor activity (Scheme 92).202 Polyphosphoric acid can also be used for this cyclization of N-aryl allyl anilines 298 (Scheme 93).203 A series of 4-phenyl-1,2,3,4-tetrahydroquinolines, 6-aryl-1,2,5,6-tetrahydro-4H-pyrrolo[3,2,1-ij]quinolines, and 4-aryl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolines 299 were prepared by this PPA-mediated cyclization starting from easily accessible 3-arylprop-2-en-1-yl amine 298 derivatives.
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| | Scheme 92 Synthesis of 1-(2-allyaryl)-1H-pyrroles and substituted 4-methyl-4,5-dihydropyrrolo[1,2-a]-quinolines. | |
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| | Scheme 93 Synthesis of 1,2,3,4-tetrahydroquinolines. | |
Very recently Zhang group reported Brønsted acid (TfOH)-mediated reactions of aldehydes 109 with 2-vinylaniline 300 and biphenyl-2-amine 302 for the efficient synthesis of nitrogen-containing heterocycle quinoline derivatives 301 and phenanthroline derivatives 303 (Scheme 94).204 The electronic properties of the substituents on the aldehydes 109 and 2-vinylaniline 300 had no effect on these cyclization processes i.e., both electron-donating and -withdrawing substituents were perfectly suitable substrates for this transformation. The expected quinolines 301 and phenanthrolines 303 were obtained in moderate to excellent yields 48–90% and 32–71%, respectively.
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| | Scheme 94 Triflic-catalyzed synthesis of quinoline and phenanthroline derivatives. | |
Gao et al. used p-toluenesulfonic acid as an effective acid-catalyst for Povarov reaction of isatin-3-imines 305 with β-enamino esters 304 for the efficient synthesis of polysubstituted spiro[indoline-3,2′-quinolines] 306 in high yields and with high diastereoselectivity (Scheme 95).205 The compound 304 was in turn generated in situ from the reaction between arylamines 101 and methyl propiolate 79 in ethanol. Shorter reaction time, readily variable substrates, and easiness of handling made this domino reaction very useful for the synthesis of structurally diverse and medicinally important heterocyclic compounds.
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| | Scheme 95 Synthesis of poly substituted spiro[indoline-3,2′-quinolines]. | |
Recently our group reported the synthesis of a series of tricyclic pyrano[3,2-f]quinoline and phenanthroline derivatives 308 by HCl-mediated 6-‘endo–trig’ Michael type ring closure reaction of 6-amino-5-(3-hydroxy-3-methylbut-1-ynyl)-2H-chromen-2-one 307 in excellent yields 89–99% under both conventional heating and microwave heating.206 The process is very simple, facile, inexpensive and can provide a diverse range of substituted quinoline derivatives from simple and easily available starting materials (Scheme 96). Moreover, the synthesized derivatives exhibited staining property to the cultured HeLa cells after fixing and can be used as fluorophores which can bind with protein molecule. Various substituents on the heterocyclic ring or on the amine group have no significant effect on this cyclization. First acid-catalyzed de-acetylation occurred for the starting materials 309a, 309g (where R = COMe) followed by acid-mediated cyclization to give 308a, 308g (Scheme 97). The synthesis of 308a–i can also be accomplished under microwave irradiation with the reduction of reaction time from several min to 1 min only for the cyclization step and slightly better yields were obtained under microwave irradiation.
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| | Scheme 96 Synthesis of substituted quinoline derivative. | |
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| | Scheme 97 Acid mediated deacetylation & cyclization reactions. | |
Although the starting materials 307 were easily prepared by Sonogashira cross coupling of 310 with 2-methyl but-3-yn-2-ol. 308 can be prepared by a one-pot, two step procedure starting from 310. When 310 was treated with 2-methyl but-3-yn-2-ol under standard Sonogashira reaction condition at 90 °C for 3 h followed by addition of 2 equiv. of HCl to the reaction mixture and heating the reaction mixture at the same temperature for further 30 min (Scheme 98). Quinoline derivatives 308 were obtained in excellent yields without isolation of the intermediate.
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| | Scheme 98 One-pot synthesis of various tetrahydroquinoline derivatives. | |
The reaction may precede through the initial formation of aldols I via addition of a water molecule to the alkyne moiety. Next the α,β unsaturated ketones II are formed from I by regioselective dehydrative rearrangement which by 6-‘endo–trig’ Michael-type ring closure may give the products 308 (Scheme 99).
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| | Scheme 99 Possible pathway. | |
Brønsted superacids (CF3SO3H, HSO3F) can also be useful similar to strong Lewis acids AlX3 (X = Cl, Br) for superelectrophilic activation of N-aryl amides of 3-arylpropynoic acids 311 for the formation of 4-aryl quinolin-2(1H)ones in 312 in quantitative yields (Scheme 100).207 The vinyl triflates 313 may be formed as side products under this reaction condition. Various amides 311 reacted with benzene 235 under the superelectrophilic activation gave 4,4-diaryl 3,4-dihydroquinolin-2-(1H)ones 314.
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| | Scheme 100 Transformations of N-arylamindes of 3-arylpropynoic acids under the action of TfOH, HSO3F. | |
Cellulose-supported sulfonic acid or green AMCell-SO3H catalyst has several advantages. It can be easily recovered by filtration and reused several times without a significant loss in activity. Arenas et al. used this AMCell-SO3H as an efficient catalyst for solvent-free imino Diels–Alder/intramolecular amide cyclization cascade reaction.208 Highly functionalized isoindolo[2,1-a]quinolinone derivatives 316 were obtained in good to excellent yields 45–82% using different arylamines 101, inexpensive phthaldehydic acid 315 and alkenes 80 (Scheme 101). Up to three new stereogenic centers were generated, with excellent regioselectivities and diastereoselectivities under this Brønsted acid-catalyzed multicomponent reaction.
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| | Scheme 101 Synthesis of isoindolo[2,1-a]quinolinone. | |
Hamada group developed two different cascade cyclization processes using aryl group-substituted propargyl alcohol derivatives with a p-hydroxybenzylamine unit as common substrates 317.209 An intramolecular ipso-Friedel–Crafts alkylation of phenol derivatives, formation of an iminium cation via- a rearomatization-promoted C–C bond cleavage, an aza-Prins reaction, and a 6-membered ring formation proceeded sequentially by using TFA as an acid promoter to access a variety of fused tricyclic dihydroquinoline derivatives 318 in 45–99% yield. In addition, fused-tricyclic indole/benzofuran derivatives 319 in 66–89% yields were formed by a one-pot sequential silver acetate-catalyzed hydroamination/etherification-acid-promoted skeletal rearrangement by using the same series of substrates (Scheme 102). The cascade cyclization process proceeded through acid-promoted intramolecular ipso-Friedel–Crafts-type addition of the phenol unit in 317 to a propargyl cation through an SN2′ mechanism, and gave an allenyl spirocyclohexadienone intermediate I. Next an iminium cation intermediate II was formed by rearomatization-promoted C–C bond cleavage. Subsequent aza-allyl cation intermediates III or IV was formed by Prins reaction of intermediate II. Finally, the tetrasubstituted carbon center was formed through a 6-membered ring formation, produced fused-tricyclic dihydroquinoline derivatives 318 (Scheme 103). Similarly, the same group also synthesized 4,5-fused tricyclic quinoline derivatives based on TFA-promoted intramolecular Friedel–Crafts allenylation of anilines in 31–84% yield.210
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| | Scheme 102 Cascade cyclization via ipso-Friedel–Crafts alkylation of phenol derivatives. | |
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| | Scheme 103 Plausible reaction pathway for tricyclic dihydroquinoline derivatives. | |
Mehra and co workers disclosed one-pot synthesis of C-3 functionalized quinolin-4(1H)-ones 321 or 322 via triflic acid promoted Fries rearrangement of C-3 vinyl-/isopropenyl substituted azetidin-2-ones 320.211 The reaction of 320 at 0 °C in dry CHCl3 in presence of TfOH resulted in the tautomeric mixture of products with a preference for the formation of the conjugated product 321. At higher temperature 60 °C gave exclusively product 321. DFT calculations showed that compound 321 were energetically more favourable than 322. At higher temperatures, the faster C–C rotation destabilizes 322 and leads to the exclusive formation of 321 (Scheme 104).
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| | Scheme 104 Synthesis of 2-aryl-2,3-dihydro-1H-quinolin-4-ones 2 and 3. | |
The reaction proceeded through initial protonation of 3-vinyl/isopropenyl-β-lactam 320 by TfOH to generate the carbenium ion intermediate 324. Fries rearrangement via an ortho attack of the aromatic substituent on the nitrogen atom of intermediate 324 readily gave a ring expanded intermediate 325. Aromatization followed by proton abstraction generates the intermediate 326. Intermediate 326 on [1,5] sigmatropic shift/tautomerization yielded a mixture of 3-ethylidene/isopropylidene-2-aryl-2,3-dihydro-1H-quinolin-4-ones 321 and 3-vinyl/isopropenyl-2-aryl-2,3-dihydro-1H-quinolin-4-ones 322 (Scheme 105).
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| | Scheme 105 Plausible mechanism for the formula of 2-aryl-2,3-dihydro-1H-quinolin-4-ones. | |
Recently, Tummatorn et al. applied formal [4 + 2] cycloaddition of N-aryliminium ions, generated in situ from the benzylic azide rearrangement, and haloacetylene analogues for the regioselective synthesis of 3-haloquinoline compounds 328 or 329.211 1.0 equivalent of arylmethyl azide 4, 2.0 equiv. of haloacetylene 327 and 1.2 equiv. of TfOH in DCE in the first step and 1.0 equiv. of DDQ in EtOAc in the second step were optimal combination for these sequence of reactions (Scheme 106). Starting from appropriate nucleophilic alkyne derivatives 327, 3-bromo-, 3-chloro-, and 3-iodoquinolines as well as 3-alkyl-, 3-aryl-, 3-carbonyl and 3-unsubstituted quinoline derivatives could be obtained by applying this methodology (Scheme 107). Moreover, synthesis of tetrahydroquinolines 331 with high regio- and diastereoselectivity using proper nucleophilic alkenes 330 were also successful by this method (Scheme 108). Readily availability of the starting materials, metal-free conditions, a green chemistry approach and tolerance of substrates containing various substituents under reaction conditions for the synthesis of quinoline derivatives in moderate to high yields made this methodology a useful one.
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| | Scheme 106 Synthesis of quinoline derivatives by using various arylmethylazide substrates. | |
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| | Scheme 107 Synthesis of quinoline derivatives by using various alkynes. | |
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| | Scheme 108 Synthesis of quinoline derivatives by using various alkene substrates. | |
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| | Scheme 109 Synthesis of various hydroxmic acid. | |
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| | Scheme 110 Solid-phase intramolecular N-acyliminium Pictet–Spengler reaction for the synthesis of pyranoisoquinolines. | |
Catalytic amount (20 mol%) of o-benzenedisulfonimide, a Brønsted acidic organocatalyst could be used for synthesis of tetrahydroisoquinolines 340 in 72–89% and tetrahydro-β-carbolines 342 in 80–86% yield, using the Pictet–Spengler reaction (Scheme 111).221 The catalyst has great economic and ecological advantages because it could be easily recovered, purified and reused. The reaction conditions were mild, green and gave target products in good yields.
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| | Scheme 111 Synthesis of tetrahydroisoquinolines and tetrahydro-β-carbolines. | |
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| | Scheme 112 Synthesis of isoquinoline N-oxides. | |
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| | Scheme 113 Synthesis of isoquinoline in one-pot. | |
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| | Scheme 114 Synthesis of β-carbolines via Pictet–Spengler reaction. | |
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| | Scheme 115 Thiourea/benzoic acid dual catalyzed enantioselective protio-Pictet–Spengler reaction. | |
Wu et al.234 synthesized highly substituted indoloquinolizidines 355 in moderate to good yields 31–88% with good to excellent enantioselectivities 77–96% ee by an asymmetric organocatalyzed (353 and BzOH) one-pot three-component cascade reaction of tryptamines 351, alkyl propiolates 79, and α,β-unsaturated aldehydes 61. Spontaneous cyclization of aldehydes afforded hemi-aminoacetals 354, a Pictet–Spengler reaction took place via the imminium ion intermediate in the presence of the acid additive to give the desired compound 355 (Scheme 116).235
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| | Scheme 116 One-pot three-component synthesis of indoloquinolizidines. | |
2.1.5. Seven membered azepine. (−)-Tuberosutemonin, (−)-cephalotaxine, SB-462795, neostenine, neo-tuberostemonine, diazepam, flumazenil, temazepam, nimetazepam, nitrazepam, alprazolam, bretazenil, brotizolam, quazepam, and clobazam are few examples of biologically active natural products and pharmaceuticals containing seven membered nitrogen-containing heterocycles azepine or diazepine derivatives.236–241The enantioselective synthesis of both enantiomers of 4,5,6 and 3,4,5,6-substituted azepanes (359 and 362) was achieved by Lee and Beak from highly diastereo- and enantioenriched ene-carbamates 356.242 The ester was converted to the amides 357 and 360 followed by acid hydrolysis by aqueous HCl of the enamides to the aldehydes and subsequent cyclization provided the corresponding N-alkyl lactams 358 and 361 (Scheme 117). Various enantioenriched 356, were converted to the corresponding 4,5,6-substituted N-Boc azepanes 359 in high yields via aminolysis, hydrolysis, reduction, debenzylation, and addition of the Boc group.
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| | Scheme 117 Asymmetric synthesis of 4,5,6 and 3,4,5,6-substituted azepanes. | |
Stefani and co-workers reported US-promoted and 10 mol% of p-toluenesulfonic acid (PTSA) catalyzed condensation of o-phenylenediamine derivatives 363 with 2,4-pentadione 364 or ketones 189, respectively for efficient synthesis of 1,5-benzodiazepinic rings 365 or 366 (Scheme 118).243 Azepine derivatives were obtained in 77–87% or 77–85% yields, containing either electron-withdrawing or electron-donating groups attached to the diamine 363.
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| | Scheme 118 Synthesis of 1,5-benzodiazepinic rings. | |
p-Nitrobenzoic acid is also a versatile Brønsted acid promoter that was tested for the preparation of benzodiazepine derivatives 367. A wide range of o-phenylenediamines 363 and ketones 189 in the presence of 1 mmol p-nitrobenzoic acid gave good yields 62–92% of 1,5-benzodiazepine derivatives 367 under mild conditions using acetonitrile as solvent at room temperature and the used catalyst could be recovered for further use (Scheme 119).244
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| | Scheme 119 p-Nitrobenzoic acid-catalyzed synthesis of benzodiazepine derivatives. | |
Balázs et al. described the synthesis of cycloalkane-fused and phenyl substituted 1,4-diazepin-5-ones 370, 373 and 376 by a process which involved oxidative cleavage of a terminal olefin and 10 mol% p-TsOH-catalyzed microwave-assisted intramolecular condensation step (Scheme 120).245
 |
| | Scheme 120 Synthesis of cycloalkane-fused and phenyl substituted 1,4-diazepin-5-ones. | |
Functionalized dibenzo[c,f]thiazolo[3,2-a]azepines 381, a fused ring system of thiazole and azepines was successfully synthesized in a four-step protocol starting from readily available substituted N-allyl-N-benzylanilines 377.246 First 2-allyl-N-benzylanilines 378 were prepared from N-allyl-N-benzylanilines 377 by aromatic amino-Claisen rearrangement. The substituted 2-allyl-N-benzylanilines 378 on acid-catalyzed intramolecular Friedel–Crafts alkylation produced dihydromorphanthridines 379 which was the crucial step of these series of reactions. The synthesis of the title compounds was accomplished by cyclocondensation of mercaptoacetic acid with morphanthridines 380 which in turn, were prepared by selective oxidation of dihydromorphanthridines 379 with pyridinium chlorochromate in dichloromethane (Scheme 121).
 |
| | Scheme 121 Reagents and conditions: (i) BF3·OEt2 (1.5 equiv.), 138–145 °C, 3–5, 74–88%, (ii) 95% H2SO4, 70–90 °C, 5–60 min or 70% HClO4, 85–120 °C, 30–60 min (iii) PCC (1 equiv.), CH2Cl2, °C, 30–40 min, 85–95%; (iv) Dean–Straktrap, HSCH2CO2H (3 equiv.), BF3·OEt2 (trace), toluene, reflux, 24–40 h, 50–65%. | |
Pictet–Spengler condensation of 4-(2-anilinophenyl)-7-azaindole or deazapurine 382 with α-oxoesters in MeOH using HCl (4 N in dioxane) at 100 °C for 30 min afforded the corresponding fused benzoazepine derivatives 383 in more than 90% yield (Scheme 122).247 The synthesized benzoazepines exhibited potent Janus kinases (JAK) inhibitory activity.
 |
| | Scheme 122 Synthesis of fused benzoazepine derivatives. | |
2.2. Oxygen heterocycles
2.2.1. Five membered furan. The furan ring is a constituent of several important natural products, as well as furanoflavonoid, furanolactones, furanocoumarins and many natural terpenoids.248,249 The incorporation of a furan nucleus is an imperative synthetic strategy in drug discovery. The most widely used strategy to furan synthesis is the Paal–Knorr method in which 1,4-dicarbonyl compounds are converted to furan derivatives via acid-mediated dehydrative cyclization.250 In this century-old reaction for furan synthesis, all the carbons as well as one oxygen necessary for the furan ring formation is provided by the substrate 1,4-dicarbonyl compounds.
2.2.1.1. Brønsted acid as catalyst. Pradilla et al. used Katsuki–Jacobsen oxidation–epoxydation of acyclic α-silyloxy sulfinyl dienes, followed by CSA-promoted cyclization for the synthesis of 2,5 trans sulfonyl dihydrofurans with good selectivities. This methodology is applicable for the formal synthesis of (6S,7S,9R,10R)-6,9-epoxynonadec-18-ene-7,10-diol.251Shanmugam et al. synthesized spiro-α-methylene-γ-butyrolactone-oxindoles 386 starting from MBH adducts of isatins 384.252 The products were obtained in excellent yields by a sequence of three reactions of them the last step were p-TsOH-catalyzed lactonization (Scheme 123).
 |
| | Scheme 123 Synthesis of spiro-α-methylene-g-butyrolactone-oxindoles. | |
Bossharth et al. synthesized furo[2,3-b]-pyridin-4(7H)-ones 388 in 45–70% yield by AcOH-promoted 5-endo-heteroannulation of N-alkyl-4-alkoxy-3-alkynylpyridin-2(1H)-ones 387.253 Under this acid-promoted reaction furopyridinium intermediates was formed which in situ dealkylation provides the corresponding furo[2,3-b]-pyridin-4(7H)-ones 388 (Scheme 124). Similarly, synthesis of furo[2,3-b]quinolin-4(9H)-ones derivatives 390 could also be possible in 77% yield by this reaction condition starting from 389. The alkynylpyridone was activated by AcOH and formed intermediate A which undergoes intramolecular nucleophilic attack by the carbonyloxygen of the amide group to form the furo-pyridinium intermediate B. Intermediate B undergoes cleavage of the O-benzyl group by the counter an ion and resulted in the formation of the neutral furopyridone 388.
 |
| | Scheme 124 Reaction of 3-alkynyl-2-pyridones in AcOH. | |
Contiero et al. used one-pot condensation–rearrangement–cyclisation reaction sequence for the efficient synthesis of benzofuran derivatives by the reaction of O-arylhydroxylamine hydrochlorides with either cyclic or acyclic ketones in the presence of 2 equivalent of methanesulfonic acid.254
The common route to the synthesis of furan is undoubtedly the cyclization of o-(1-alkynyl)phenol compounds through a transition metal-catalyzed activation of the triple bond.255 The benzofurans are ubiquitous structural motifs in both natural products and synthetic pharmaceuticals,256 Alami and co workers developed a rapid and metal-free access to 2-arylsubstituted benzo[b]furans 391, according to the green chemical philosophy257 (Scheme 125). A variety of 2-arylbenzo[b]furans 391 were readily prepared in good to excellent yields from the cyclization of o-(1-alkynyl)anisole derivatives 344 under mild reaction condition using an alcohol, p-toluenesulfonic acid under microwave irradiation. The presence of an ortho- and/or para-methoxy groups on the aromatic rings bearing the alkyne significantly activated the triple bond of the substrate for the cyclization. The suitable condition required the use of PTSA (1.0 equiv.) in EtOH or MeOH under microwave irradiation at 130 °C for 1 h, the expected benzofurans 391 were obtained in satisfactory yields devoid of any trace of the hydration product 392.
 |
| | Scheme 125 Synthesis of 2-arylsubsutituted benzo[b]furans. | |
Li group described a Brønsted acid (TfOH) catalyzed rearrangement of tert-butyl peroxides 393 to provide 2,3-disubstituted furans 394 as well as 2,3-dihydrofuran in one step via 1,2-aryl migration.258 Highest 81% yield of the furan derivatives was obtained in the presence of 0.3 equiv. of triflic acid. A variety of aryl groups could migrate smoothly, and the furan product 394a along with the corresponding phenolic compounds 395 were obtained in good to excellent yields. Substrates with electron-donating or electron-withdrawing groups on the aryl ring reacted well under the optimized reaction conditions. Notably, bulky phenol could also be obtained in 83% yield through this TfOH catalyzed 1,2-aryl migration reaction. Moreover, 2-naphthyl substituted peroxide also gave the furan 394a (Scheme 126).
 |
| | Scheme 126 TfOH-promoted rearrangement. | |
In addition, tert-butyl peroxides could also be transformed into 2,3,5-trisubstituted or 2,5-disubstituted furans 397 through a sequence of base-catalyzed Kornblum–DelaMare rearrangement and acid-promoted Paal–Knorr reaction (Table 14).
Table 14 Synthesis of various 2,3,5-trisubstituted or 2,5-disubstituted furans
Dhiman and Ramasastry described sacrificial benzofuran facilitating the formation of tri- and tetra-substituted furans under simple, effective, air- and moisture insensitive conditions by unusual TfOH-catalyzed benzofuran ring opening and furan ring closure (Scheme 127).259 Benzofuranyl carbinols 398 and 1,3-dicabonyls 364 in the presence of 20 mol% TfOH produced functionalized, polysubstituted furans 399 in 50–80% yields. This benzofuran ring opening and furan recyclization process provided a direct and facile access to the synthetically useful and medicinally important polysubstituted furan derivatives. The proposed mechanism (Scheme 128) rationalizes the transformation of furyl and benzofuranyl carbinols to tri- and tetra-substituted furans 399 under acidic conditions. The reaction proceeded with an initial reversible protonation of the benzofuran ring at C-3, which prompted reversible attack of the enol oxygen at the positively charged C-2. Protonation and subsequent ring opening delivered the product. This reversibility also explained the transformation of 364 and its analogues into 2-(2-hydroxybenzyl)furan which proceeded slowly.
 |
| | Scheme 127 Synthesis of tri- and tetrasubstituted furans. | |
 |
| | Scheme 128 Proposed mechanism for the formation of tri- and tetra-substituted furans. | |
Recently, Ruano and Alemán et al. used Brønsted acid TFA (5 mol%) at room temperature for the synthesis of a variety of dihydro-2H-cyclohepta[b]furan derivatives 402 and 402′ by formal [8 + 2] cycloaddition reaction of tropones 400 with azlactones 401 (Scheme 129).260
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| | Scheme 129 Synthesis of dihydro-2H-cyclohepta[b]furan derivatives. | |
Brønsted acid TfOH was found to be an excellent catalyst for Prins cyclization reaction of α-prenylated alcohols 403 with aldehydes 189 for the synthesis of five-membered furan derivatives 404 (Scheme 130).261 Prins reaction utilizing the homoallylic alcohol affords six-membered THP, but use of the α-prenylated alcohol as the alcohol source and reaction with aldehydes constructs five-membered THF. This operationally facile cyclization protocol required only 0.1 mmol of TfOH for synthesis of biologically and synthetically interesting highly functionalized heterocycles.
 |
| | Scheme 130 Synthesis of substituted tetrahydrofurans through Prins cyclization. | |
Clark and co-workers used chloroacetic acid as a Brønsted acid for efficient and diastereoselective synthesis of 2,3,5-tri-substituted furan fused with cyclopropyl substituent at the 5-position 406.262 Intramolecular cascade reaction of electron-deficient ynenones 405 in the presence of 1 equiv. chloroacetic acid in CH2Cl2 (0.25 M) under refluxing condition for 18–27 h furnished fused furan derivative in 58–94% yields (Scheme 131). Various substituents on the pendent alkene tethered to the electron-deficient enyne substrates were tolerated and synthesis of polycyclic products incorporating oxygen or nitrogen was also successful. Alkene containing ketone, ester, phosphonate, or a sulfone group, gave corresponding cyclopropyl furans in good to high yields. Mixture of 2-alkynal 407, acetylacetone 364, chloroacetic acid, and MgSO4 in toluene at 60 °C for 18 h, gave the cyclopropyl furans 406 in 57–78% yields (Scheme 132). Synthetically relevant polycyclic building blocks featuring rings of various sizes and heteroatoms have been synthesized in high yields using this mild acid-catalyzed reaction.
 |
| | Scheme 131 Synthesis of 2,3,5-trisubstituted furan fused with cyclopropyl substituent. | |
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| | Scheme 132 Synthesis of cyclopropyl furans 406. | |
The reaction proceeded by protonation of one of the carbonyl groups of 405 and formed intermediate 408 which, is converted to another intermediate 409 by resonance. Intermediate 409 underwent cyclization by intramolecular nucleophilic attack of the allenic carbon by the enol to form carbene 410. Carbene reacted with the alkene or undergoes allylic C–H insertion to give the products 406 and also 411a/411b (Scheme 133).
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| | Scheme 133 Proposed mechanism for the acid-catalyzed reaction of 405. | |
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| | Scheme 134 Synthesis of the 2-isopropenyl-2,3-dihydrobenzofuran. | |
Combination of [Ru(η-3-2-C3H4Me)(CO)(dppf)][SbF6] (dppf = 1,1′-bis(diphenylphosphino)ferrocene) 417 and trifluoroacetic acid (TFA) is useful catalytic system to promote the coupling between secondary propargylic alcohols 416 and cyclic 1,3-diketones 364.264 The nature of the resulting products was found to be dependent on the ring size of the dicarbonyl compound employed. Thus, whereas 6,7-dihydro-5H-benzofuran-4-ones 418, 420 and 422 have been selectively obtained starting from 1,3-cyclohexanediones 416, 419 and 421, via furan-ring formation. The use of 1,3-cyclopentanedione led to 6,7-dihydro-4H-cyclopenta[b]pyran-5-ones via a pyran-ring formation process. The process, which proceeded in a one-pot manner, involves the initial trifluoroacetic acid promoted propargylic substitution of the alkynol by the 1,3-dicarbonyl compound and subsequent cyclization of the resulting γ-ketoalkyne catalyzed by the 16-electron allyl-ruthenium(II) complex (Scheme 135).
 |
| | Scheme 135 TFA-mediated synthesis of furans from alkynols and 1,3-dicarbonyl compounds. | |
Trost group reported an atom-economical method for the synthesis of tetrahydropyrans and tetrahydrofurans 425.265 Tetrahydrofurans were obtained in excellent yields 72–77% from enones and enals derived from the [IndRu(PPh3)2Cl2]-catalyzed redox isomerization of primary and secondary propargyl alcohols followed by intramolecular conjugate addition (Scheme 136). 3 mol% 424 with 3 mol% indium(III)triflate and 20 mol% CSA is perfect to get various tetrahydrofuran 425 as sole product (Table 15).
 |
| | Scheme 136 Tandem redox isomerization-conjugate addition to generate cyclic aldehydes. | |
Table 15 Scope of formation of a hydropyransa
| Entry |
Propargyl alcohol |
Product |
Isolated yield |
| All reactions run as in Scheme 136. |
| 1 |
 |
 |
77% |
| 2 |
 |
 |
77% |
| 3 |
 |
 |
72% |
| 4b |
 |
 |
75% |
| 5 |
 |
 |
72% |
2.2.2. Six membered.
2.2.2.1. Chromone or pyran/pyrone/benzopyran derivatives. The chromane skeleton is found in a myriad of medicinally important compounds that have a broad range of biological activities e.g., antibacterial, antitumor, antioxidant, antileishmanial, and potent cytotoxic activities.266–272 Dihydropyranones occur widely as key structural subunits in numerous natural products, such as obolactone, cyclocurcumin, vitamin E, diversonol, afzelechin, cordotolide A, lotthanongine, and flavonoids.273–276
Brønsted acid as catalyst.
Various conventional methods were used for preparing the dihydropyranone skeleton because of their interesting pharmacological and bioactive properties.277 A large number of natural products incorporate a hydroxyl group (or glycoside linkage) at C-4, one such compound is polycavernoside A, a toxin isolated from the red alga Polycavernosatsudai.278 Barry et al. accomplished279 a concise and stereoselective synthesis of the tetrasubstituted tetrahydropyran core 427 of polycavernoside A in 55% overall yield from 3-benzyloxypropanal 426. A stereoselective allyl transfer reaction was used in the synthesis of enol ether followed by a TFA-mediated cyclization to create the three new asymmetric centers in the tetrahydropyran 427 with complete stereocontrol in a single-pot process (Scheme 137).
 |
| | Scheme 137 Synthesis of tetrahydropyran. | |
Hoveyda and groups reported a Brønsted acid-catalyzed reaction of enol ethers 428 to form cis-2,6-disubstituted tetrahydropyrans 429.186g The TfOH-catalyzed reaction is direct and operationally simple, occurred in less than 10 minutes at room temperature in the presence of just 0.01–0.1 mol% TfOH catalyst (Table 16). Treatment of 428 with higher (5 or 10) mol% of TfOH resulted in rapid polymerization, but decreasing the catalyst loading considerably (≤1 mol%), the desired tetrahydropyrans 429 were formed in major amount. In the presence of 0.01 mol% of TfOH loading three olefin isomers 429:430:431 were affected formed in 4:1:1 ratio. Further lowering the catalyst loading resulted in >98% recovery of the starting material. Interesting features of this reaction is that the TfOH-catalyzed reactions could be carried out efficiently in various common solvents, with similar levels of efficiency. Notably, the product selectivity was consistently higher in benzene. The reaction can be carried out at −78 °C and longer reaction times led to an increase in endocyclic product. The limitation of this TfOH-catalyzed reaction is that there is <2% conversion with substrates bearing terminal alkene.
Table 16 Diastereoselective TfOH-catalyzed synthesis of cis-2,6-disubstituted-4-methylene tetrahydrofuransa
Formation of the functionalized pyran-4-ones 434, such as 5,6-dihydrobenzo[h]chromones and 5,6,7,8-tetrahydrochromones could be achieved by acid-mediated ring-transformations of 5-alkylidene-2,5-dihydropyrrol-2-ones, available by cyclization of 1,3-diketone dianion 364 with bis(imidoyl) dichlorides of oxalic acid 432 (Scheme 138).280
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| | Scheme 138 Synthesis of 5-alkylidene-2,5-dihydropyrrol-2-ones 433 and of pyran-4-ones 434. (i): (1) 2.2 equiv. LDA, (2) THF, −78–20 °C; (ii): HCl (1 M), 1 h, 40 °C. | |
In, 2008, Ramachary et al. used multi-catalysis for the sequential one-pot synthesis of highly functionalized chromone and xanthone derivatives.281 This multi-component aniline-, self- and Brønsted acid-catalyzed process describes the sequential one-pot synthesis of highly substituted 2-(2-hydroxy-aryl)-cyclopentane-1,3-diones, 3,9-dihydro-2H-cyclopenta[b]chromen-1-ones 438 and 3,3-dimethyl-2,3,4,9-tetrahydro-xanthen-1-ones 440. Direct combination of aniline- and self-catalyzed cascade olefination–hydrogenation (O–H) of 1,3-diones 220 or 419, salicylic aldehydes 435 and dihydropyridine 436 first produced substituted 2-(2-hydroxy-aryl)-cyclopentane-1,3-diones 437 followed by Brønsted acid (p-TSA 30 mol%)-catalyzed cascade oxy-Michael-dehydration (OM-DH) in one-pot to furnish the highly functionalized 3,9-dihydro-2H-cyclopenta[b]chromen-1-ones 438 in 75–99% yields and 3,3-dimethyl-2,3,4,9-tetrahydro-xanthen-1-ones 440 in 65–99% yields, respectively (Scheme 139).
 |
| | Scheme 139 One-pot synthesis of highly functionalized chromone and xanthone derivatives. | |
The reaction of 2-hydroxynaphthalene-1,4-dione 441 and isatin 219 in 2:1 molar ratio in the presence of a catalytic amount of p-TsOH under refluxing condition in water for 24 h proceeded smoothly to furnish the spiro[dibenzo[b,i]-xanthene-1,3,3′-indoline]-2′,5,7,12,14-pentaone 442 in 80% yield (Scheme 140).282 A series of such heterocyclic compounds in good yields 75–82% have been synthesized from the reactions of N-methylisatin, N-benzylisatin, 5-bromoisatin, 5-nitroisatin, and N-bromo-5-methylisatin with compound 219.
 |
| | Scheme 140 Synthesis of 442. | |
In 2010, a stereoselective total synthesis of pyrone containing natural product obalactone283 444 was reported via a Brønsted acid-mediated tandem cyclization of 443 as the key reaction to realize a methylene bridged bis-dihydropyrone ring skeleton in one-pot and in a highly efficient manner (Scheme 141). This strategy may be useful for the synthesis of similar ring-containing natural products.
 |
| | Scheme 141 Stereoselective total synthesis of pyrone containing natural product obalactone. | |
Normally high temperature and lengthy reaction time are required for formation of xanthene derivatives. However xanthene derivatives have recently been synthesized using a catalytic amount of perchloric acid in water.284
The thermal ([1,3]-alkyl shift) of α-naphthyl geranyl ether (445) and β-naphthyl geranyl ether (449) were followed by acid-catalyzed cyclization to give benzoxanthene (446, 447) and benzochromane derivatives (450, 451) (Scheme 142) in moderate yields.285
 |
| | Scheme 142 Rearrangement of naphth-1-ylgeranylether: (a) PTSA, toluene, rt, 7 days; (b) reflux in chlorobenzene, 24 h. | |
Flavans are a class of naturally occurring flavonoids possessing a 2-phenyl-3,4-dihydro-2H-chromene nucleus. Perchloric acid supported on silica can be used as a recyclable heterogeneous catalyst for the synthesis of flavans derivatives. First, Bharate et al. reported286 a simple and economical tandem one-pot protocol for the synthesis of flavans 453 directly from substituted phloroglucinol precursors 452 (Scheme 143). The method involved a Knoevenagel-type condensation leading to in situ formation of transient o-quinone methide which underwent [4 + 2]-Diels–Alder cycloaddition with styrene 80 to yield a flavan skeleton 453. The developed protocol provided a shorter route to access a variety of flavan natural products such as catechins, flavonoids, anthocyanins, grandinal etc. using the appropriate dienophile 80, aldehyde 189 and phloroglucinol precursor 452.
 |
| | Scheme 143 One-pot synthesis of flavan and flavone derivatives. | |
Yang et al. developed287 a Brønsted acid-catalyzed highly regioselective cycloisomerization of 5-en-2-yne-1-ones 454 under mild condition, that provided a metal-free straightforward route to highly substituted dihydropyranones 455. The reaction facilitates multistep bond formation in a one-pot manner to achieve an economically useful transformation. In addition, a Brønsted acid acting as a dual catalyst was used to activate both the carbonyl and alkene moieties in a cascade manner (Scheme 144). On the basis of observations, a plausible mechanism (path a or path b) was suggested for this transformation as follows. Path a: initial interaction of the proton from the Brønsted acid with the carbonyl oxygen atom of 454 may form the complex A. Methanol acts as a nucleophile and attacks the carbon–carbon triple bond to form B.288,289 The release of a proton followed by a keto–enol tautomerization may lead to the formation of intermediate C and regenerate the acid catalyst. Owing to the steric hindrance, upon heating C undergoes subsequent conversion to D, followed by activation of the alkene moiety by the Brønsted acid to form the stable tertiary carbocation E. Attack of the oxygen of the carbonyl group at the cationic center of E may afford the oxonium ion F,290,291 that undergoes protonation and demethylation to produce dihydropyranone 455 (path a). Alternatively the alkene moiety of 454 may isomerize into conjugation with the ynone, followed by nucleophilic addition, which is then followed by electrocyclic ring closure to give the dihydropyranone (path b). The fact that only intermediate C was isolated and characterized favour the path a as the more reasonable pathway.
 |
| | Scheme 144 Cascade Brønsted acid catalyzed synthesis of various pyran-4-ones derivatives. | |
Acid-mediated cyclizations of hydroxy alkenes represent a particular class, as they are promoted by stoichiometric or catalytic amounts of Brønsted acids. Their regioselectivity is usually controlled by the Markovnikov rule and most of them involve strong Brønsted acids. Actually, they are commonly carried out in the presence of large quantities of these reagents. During the synthesis of platensimycin (458 in Scheme 145),292 treatment of a 2:1 mixture of diastereomers 456a and 456b with TFA (TFA–CH2Cl2 2:1) afforded the cage-like structure 457 in 87% yield, based on the isomer 456a present in the mixture. Notably, the undesired diastereomer 456b was recovered unchanged.293
 |
| | Scheme 145 Synthesis of platensimycin. | |
These cyclizations could be carried out under catalytic conditions.294–296 One of the most sophisticated methods to induce the formation of the oxocarbenium ion consisted of treating a silylated cyclopropylmethylcarbinol 459 (as an equivalent of homoallyl alcohol) with trifluoroacetic acid in the presence of an aldehyde 189. Ring opening generated the corresponding unsaturated β-silylated carbenium ion, which was trapped by the aldehyde basic oxygen atom. Three stereogenic centers were introduced in one step in the final tetrahydropyranol 460, isolated as a unique isomer (Scheme 146).297 The bulky silylmethyl group occupies the equatorial position in the chair-like six-membered cyclic transition state. The cyclization was followed by selective axial trapping of the resulting benzylic carbocation by water.
 |
| | Scheme 146 Synthesis of tetrahydropyranol 460. | |
Alcohol 461 contains a mono-substituted and a 2,2-disubstituted olefin, in the presence of 1 mol% or 10 mol% of triflic acid for 12 h in toluene at 80 °C formed only the product from cyclization at the more substituted olefin 462. This product 462 was isolated in 61% yield by using 1 mol% of HOTf without formation of any 463 (Scheme 147).186d
 |
| | Scheme 147 TfOH-catalysed cyclization contains a mono-substituted and a 2,2-disubstituted olefin. | |
The use of trifluoroacetic acid-mediated Prins cyclization followed by subsequent hydrolysis of the trifluoroacetate, has been widely applied in natural product syntheses.298,299 The same experimental conditions applied by Lee in the total synthesis of (+)-exiguolide 465 from 466 (Scheme 148).146 The aldehyde was introduced as an enol ether in the precursor, as in the approach developed by Nussbaumer and Fráter.300
 |
| | Scheme 148 Total synthesis of (+)-exiguolide. | |
The pyran ring in the core structure of neopeltolide was also fashioned, by Maier, via Prins cyclization in the presence of TFA followed by hydrolysis of the resulting trifluoroacetate (Scheme 149).301
 |
| | Scheme 149 Synthesis of neopeltolide. | |
Jadav et al. reported the synthesis of various pyran derivatives,302–306 one such example is the synthesis of 4-arylthio-tetrahydropyrans 471 by a three-component procedure involving an aromatic aldehyde 189, a homoallylic alcohol 470 and an aromatic or heteroaromatic thiol 469 using trifluoroacetic acid catalyst at room temperature.307 Good yields and all cis-selectivity were reported (Scheme 150). Other Lewis acids including acetic acid, BF3·OEt2, metal halides, metal triflates, and heterogeneous catalysts such as montmorillonite KSF clay were found to be ineffective or did not lead to the expected sulfide. When sodium azide was used instead of heteroaromatic thiol 4-azido-tetrahydropyrans derivatives 471 were obtained.308 Under the experimental condition, HN3 was generated in situ from trifluoroacetic acid and NaN3.309 The same group successfully used phosphomolybdic acid (H3PMo12O40, a heteropoly acid) as an efficient catalyst for Prins cyclization of homoallylic alcohols with aldehydes in water at room temperature to synthesize tetrahydropyran-4-ol derivatives in high yields with all cis-selectivity.310
 |
| | Scheme 150 Synthesis of 4-substituted tetrahydropyrans. | |
Jadav et al. used intramolecular-Prins-cyclization strategy and p-toluenesulfonic acid as an efficient catalyst for the coupling of (Z)-hex-3-ene-1,6-diol 472 with a series of aldehydes or ketones 189 to provide the corresponding hexahydro-2H-furo[3,2-c]pyran derivatives 473 in 40–74% yields with complete cis-selectivity. Similarly, the coupling of (E)-hex-3-ene-1,6-diol 472 with aldehydes under the same reaction condition gave trans-fused bicyclic furopyrans 473 (Scheme 151).311
 |
| | Scheme 151 p-TSA catalyzed synthesis of 2-substituted furanopyrans. | |
Brønsted acidic ionic liquids containing hydrogen sulfate and dihydrogen phosphate as anions can be used as an acidic catalysts for the synthesis of 1,8-dioxo-octahydroxanthene 474 in water medium (Scheme 152). Salvi et al. synthesized six different ionic liquids ([BMIM][BF4], [CMIM][HSO4], [NMP][HSO4], [BMIM][HSO4], [(CH2)4SO3Hmim][HSO4], [BMIM][H2PO4] and used as catalysts for the synthesis of 1,8-dioxo-octahydroxanthene 474 in high yields by the reaction of benzaldehyde 189 and dimidone 419.312 The products could be separate from the reaction mixture by simple filtration and the dissolved catalyst could be regenerated and recycled. The presence of a SO3H group in the anions of BAILs brought about a significant promoting effect on the formation of 1,8-dioxo-octahydroxanthene.
 |
| | Scheme 152 Synthesis of 1,8-dioxo-octahydroxanthene. | |
Scheidt group developed a modular and highly stereoselective approach to construct spirooxindole annulated pyrans 477 in high yield 51–94% in the presence of a catalytic amount of TfOH (20 mol%).313 The Prins-type cyclization of an aldehyde, an acetoacetate fragment (dioxinone), and isatin provided an efficient and diastereoselective route for diversified products with complete transfer of stereochemistry from the β-hydroxy intermediate to the products (Scheme 153). The high levels of 2,6-cis selectivity observed for this catalytic process is attributed to the equatorial preference of the oxindole moiety 477. Catalytic amount of a Brønsted acid facilitate this cyclization reaction. The reaction was unsuccessful in the presence of H3PO4, p-TSA·H2O, TFA, or Tf2NH; however, 50 mol% of TfOH, H2SO4, and MeSO3H all catalyzed the reaction. The best result was obtained with 20 mol% of TfOH, 1 equiv. of isatin diketal 475, and 1.5 equiv. of dioxinone 476, affording the desired product with excellent level of diastereo-selectivity favoring the 2,6-cis isomer. Addition of flame-dried 5 Å molecular sieves to the reaction mixture was also crucial to achieve the high yield for this process. The process tolerated both electron-donating and withdrawing substituents at either 5 or 6 position when the nitrogen of the isatin is methyl or benzyl group protected. The unprotected isatin substrates are also compatible with the reaction conditions, but failed completely with more sterically demanding substrates, such as 4-bromoisatin. Substitution on dioxinones with alkyl (linear and branched) or aromatic groups gave moderate to high yields with good diastereoselectivity of the resulting spiro-tricyclic product 477. The reaction proceeded through the initial formation of the oxocarbenium ion intermediate which was formed by the combination of isatin ketal 475 and β-hydroxy dioxinone 476.
 |
| | Scheme 153 TfOH-catalyzed synthesis of spirooxindole pyrans. | |
Asthasko and Tyvorskii reported the synthesis of 2,6-disubstituted 2,3-dihydro-4H-pyran-4-ones 480 via Kulinkovich cyclopropanation of protected 3-oxocarbocyclic acid esters 478 followed by cleavage of the resulting cyclopropanols to form corresponding β-hydroxyketones 479 and their subsequent acid-promoted cyclization. β-Hydroxyketones 479 with cold 50% H2SO4 in methylene chloride at rt gave the corresponding 2,6-dialkyl-2,3-dihydro-4H-pyran-4-ones 480 in 74–93% yields (Scheme 154).314
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| | Scheme 154 Synthesis of 2,6-dialkyl-2,3-dihydro-4H-pyran-4-ones. | |
2H-Pyran-2-ones 482 were synthesized by sulphuric acid-catalyzed domino reaction of 3-hydroxyhexa-4,5-dienoates 481 (Scheme 155).315 Modification of the substitution patterns of the substrates 481, 3,4,6-Trisubstituted 485, 4,5,6-trisubstituted 487, 490 and 3,4,5,6-tetrasubstituted α-pyrones 491 can also be synthesized with high efficiency (Schemes 156 and 157). When substrates contained a methyl group on the internal position of the allene moiety, indene derivatives 488 were also formed together with 4,5,6-trisubstituted or 3,4,5,6-tetrasubstituted α-pyrones 490 or 491. The main advantages of this H2SO4-catalyzed substituted pyran synthesis are ready availability of the substrates, diversely substituted products, cheap catalyst and mild reaction conditions. The reactions proceeded through the acid promoted dehydration of 481 to produce 2,4,5-trienoate I or alkyne A. Hydration of I or A may give an enol intermediate (II) or its ketone isomer B. Intramolecular trans-esterification of II may produce α-pyrone 482. Product 482 could also be formed through the separable alkyne A and A on further treatment with H2O in CH2Cl2 at rt for 0.5 h in the presence of H2SO4 (10 mol%) gave 482 in 88% yield (Scheme 158).316
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| | Scheme 155 Synthesis of 4,6-disubstituted 2H-pyran-2-ones. | |
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| | Scheme 156 Synthesis of 3,4,6-trisubstituted and 4,5,6-trisubsituted α-pyrones. | |
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| | Scheme 157 Synthesis of 3,4,5,6-trisubstituted α-pyrones. | |
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| | Scheme 158 Probable pathway. | |
Symmetrical 2,3,6,7-dibenzo-9-oxabicyclo[3.3.1]nona-2,6-diene analogues 493 bearing various functionalities were synthesized by Brønsted acid-promoted dimerization of o-alkynylbenzaldehydes 492 (Scheme 159).317 In the presence of 45% aq. HBF4 in acetic acid the cascade methodology provided a convenient one-step synthesis of Kagan's ether which could be potentially transformed into another Kagan's ether. The hydrolysis product derived from the o-alkynylbenzaldehydes 492 participates in the dimerization process. The hydrolysis products (494 and 496) from o-alkynylbenzaldehydes (Scheme 160) were heated with 45% aq. HBF4 in acetic acid at 75 °C. 496 afforded the corresponding ICTB 497 in 3 h in 73% yield; but the corresponding reaction of 494 did not provide any of the expected dimer or ICTB salt. However, reaction of a mixture of 496 (1 equiv.) and 492 (1 equiv.) resulted in smooth dimerization under the same conditions. However, the combination of 495 and 496 still afforded the corresponding ICTB 497. Limitations of this methodology in scope of substrates are that only acyl groups could be directly introduced into the C4 and C8 positions of Kagan's ethers.
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| | Scheme 159 Synthesis of 2,3,6,7-dibenzo-9-oxzbicyclo[3.3.1]nona-2,6-diene analogues. | |
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| | Scheme 160 Examination of the proposed intermediates. | |
Qiu et al. reported p-TsOH-catalyzed tandem cyclization for effective preparation (yield up to 99%) of a series of polysubstituted 4-pyrones 499 from diynones 498.318 4-Pyridone derivatives 500 were also selectively synthesized by employing NIS and p-TsOH (Scheme 161). The reaction was rationalized as depicted in Scheme 162. p-TsOH may activate the carbonyl, followed by methanol attacking the carbon–carbon triple bond to generate the intermediate A. Upon heating, A may undergo subsequent conversion to B by the configuration turning. Once again, attack of the intramolecular methoxyl group onto the carbon–carbon triple bond of B may afford C, which underwent demethylation to give 4-pyrones 499.
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| | Scheme 161 p-TsOH-catalyzed synthesis of 4-pyrone and 4-pyridone. | |
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| | Scheme 162 Proposed mechanism for p-TsOH-catalyzed synthesis of 4-pyrone. | |
Reddy and co-workers used p-TSA at ambient temperature for smooth coupling of 5-(hydroxymethyl)-4,6,6-trimethylcyclohex-3-enol 501 with various aldehydes 189 for efficient synthesis of hexahydro-8,8-dimethyl-1H-isochromen-7-ol derivatives 502 in good yields 60–87% and high selectivity (Scheme 163).319 This reaction occurred in single step and involved three consecutive reactions-double bond isomerization, Prins cyclization and proton elimination.
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| | Scheme 163 p-TSA-promoted synthesis of 1H-isochromen-7-olderivatives. | |
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| | Scheme 164 Organocatalytic oxa-Diels–Alder reaction for synthesis of substituted tetrahydropyran-4-ones. | |
Brønsted acids could be used to effect cyclization of simple diene precursors in a “one-pot” procedure. Kalbarczyk et al. reported321 an efficient means of heterocycle synthesis through the acid-promoted end functionalization of an acyclic diene. The net reaction resulted in both cyclization and the addition of nitrogen and hydrogen to the ends of the diene, so it can be described as a formal 1,4-hydroamination. The authors showed the effectiveness of this reaction as a heterocycle synthesis, for both dehydropiperidines (tetrahydropyridines) and tetrahydropyrans 508 (Scheme 165). Since the dienes 505, 80 are readily accessible by catalytic enyne metathesis, a facile new route to heterocycle synthesis from alkynes was provided by the application of this methodology.
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| | Scheme 165 One-pot approach to heterocycles: enyne metathesis and acid-promoted cyclization. | |
Trost group reported an atom-economical method for the synthesis of tetrahydropyrans and tetrahydrofurans.322 These cyclic ethers were obtained in moderate to high yields 50–86% and 72–77% respectively from enones and enals derived from the [IndRu(PPh3)2Cl2]-catalyzed redox isomerization of primary and secondary propargyl alcohols followed by intramolecular conjugate addition. In the presence of 3 mol% indium(III)triflate and 20 mol% CSA tetrahydropyran was obtained as sole product with complete selectivity of the cis diastereomer.
Shindo et al. showed how addition of Brønsted acids dramatically switched the mode of cyclization.323 Torquoselective olefination of alkynoates provided functionalized tetrasubstituted olefins, (E)-2-en-4-ynoic carboxylic acids 511 which on treatment with 0.1 equiv. of Ag(I) underwent 5-exo mode of cyclization to provide tetronic acid 512. Addition of 0.5 equiv. of Brønsted acids (AcOH or TFA) along with 0.1 equiv. Ag(I) catalyst dramatically switched the mode of the cyclization to 6-endo from 5-exo to provide 2-pyrones 513 (Scheme 166).
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| | Scheme 166 Tetronic acid derivatives (5-exo) vs. pyrones (6-endo) cyclization. | |
Jørgensen and co workers reported the use of 10 mol% PhCO2H as additive along with diarylprolinol ether 353 as catalyst (10 mol%) for tandem conjugate addition/acetalization reaction of 1,3-diketones 220 with α,β-unsaturated aldehydes 61 for the synthesis of 3,4-dihydropyrans 514.324 This reaction was investigated with 1,3-cyclopentadione 220 as the substrate with a broad range of different types of α,β-unsaturated aldehydes 61 (R = aliphatic, aromatic, ester, heteroaromatic and alkene) and all were well tolerated under the reaction condition (Scheme 167).
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| | Scheme 167 Tandem Michael addition/acetalization reaction for enantioselective synthesis of 3,4-dihydropyrans. | |
Similarly, Xiao et al. used a combination of diarylprolinol ether 353 and TsOH·H2O for enantioselective intramolecular hydroarylations of electron rich arenes 515.325 The arylation of ω-aryloxy-tethered α,β-unsaturated aldehydes 515 produced functionalized pyrans 516 in moderate to good yields 50–78% and with enantioselectivities of upto 96% ee (Scheme 168).
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| | Scheme 168 Organocatalytic intramolecular hydroarylation for synthesis of pyrans. | |
Chandrasekhar et al. used enantioselective tandem Michael addition/ketalization strategy for the construction of cycloalkane-fused tetrahydropyrans 520 using cyclohexanone 517 as nucleophile and hydroxymethyl nitroolefins 518 as Michael acceptors. Only moderate yields 41–62% were obtained and best result in terms of enantioselectivity (up to 99% ee) were obtained using pyrrolidine-triazole 519 (20 mol%) together with TFA (10 mol%) (Scheme 21) (Scheme 169).326
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| | Scheme 169 Tandem Michael addition/ketalization reaction for enantioselective synthesis of cycloalkane-fused tetrahydropyrans. | |
Prins cyclization proceeded well under the influence of Sc(OTf)3, but enhanced reaction rate and high conversion was observed using a mixture of Sc(OTf)3 and TsOH (1:3).327 Homoallylic substrates such as (E)-6-arylhex-3-enyl alcohols 521 underwent smooth cross coupling with various aldehydes 189 in the presence of 10 mol% Sc(OTf)3 and 30 mol% TsOH to afford the trans-fused hexahydro-1H-benzo[f]isochromenes 522 in 56–92% yields. However, the cross-coupling of (Z)-olefins such as 6-arylhex-3-enyl alcohols 521′ with aldehydes afforded the corresponding (Z)-hexahydro-1H-benzo[f]isochromenes 522′ in 56–86% yields with complete cis selectivity via intramolecular Prins/Friedel–Crafts cyclization (Scheme 170). The combination of Sc(OTf)3 and TsOH (1:3) worked more effective than either Sc(OTf)3 or TsOH alone in terms of reaction time and yield. The high catalytic activity of this catalytic system can be explained by means of a cooperative catalysis between Sc(OTf)3 and an organic co-catalyst (TsOH).
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| | Scheme 170 Synthesis of hexahydro-1H-benzo[f]isochromenes. | |
Combination of primary–secondary diamines 524/(L)-CSA (10 mol%) was also found as an efficient catalyst for asymmetric intramolecular oxa-Michael reactions of α,β-unsaturated ketones 523 to produce synthetically useful tetrahydrofurans/2H-pyrans 525 in good yields and with high enantioselectivities (up to 90% ee) (Scheme 171).328
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| | Scheme 171 Synthesis of tetrahydrofurans/2H-pyrans. | |
Very recently Ascic et al. reported a ruthenium and/Brønsted acid-catalyzed reaction sequence for the synthesis of pyran derivatives 527.329 Readily available allylic ethers 526 may first undergo ruthenium catalyzed sequential isomerization followed by Brønsted acid-catalyzed (TsOH) endo cyclization with tethered nucleophiles (Scheme 172).
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| | Scheme 172 Synthesis of fused oxacycles. | |
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| | Scheme 173 Synthesis of dihydrocoumarins and dihydroquinolone derivatives. | |
Later on Krawczyk et al. have identified CF3SO3H as an efficient catalyst for Friedel–Crafts reaction of electron-rich hydroxyarenes 395 with (E)-3-aryl-2-(diethoxyphosphoryl)acrylic acids 533.334 The reaction features high efficiency of the catalyst regardless of electronic character of the aryl substituent on the acrylic acid moiety, high yield, and excellent regio- and diastereo-selectivity providing a practical method for the synthesis of trans-3-diethoxyphosphoryl-4-aryl-3,4-dihydrocoumarins 534 (Scheme 174).
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| | Scheme 174 Synthesis of trans-3-diethoxyp hosphoryl-4-aryl-3,4-dihydrocoumarins. | |
The Brønsted acid-catalyzed cyclization process for the synthesis of coumarin derivatives has been reported by Uchiyama et al.335 Heating 535 with 1 mol% trifluoromethanesulfonic acid in toluene for 12 h afforded isocoumarin 536 in quantitative yield (Scheme 175). This process has been used for the synthesis of “Thunberginol A” (538) via cyclization and deprotection sequence from 537.
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| | Scheme 175 Synthesis of “Thunberginol A”. | |
In the same year (2008) Hellal and co-workers reported regiocontrolled ‘6-endo–dig’ cyclization of 2-(2-arylethynyl)heteroaryl esters under Brønsted acidic conditions and in the presence of a catalytic amount of Lewis acids such as Cu(OTf)2, AuCl3, or (CF3CO2)Ag (Scheme 176).336 Under acidic conditions, the electronic bias on both carbons of the triple bond favors Michael-type (6-endo–dig) cyclization and good agreement was observed when methyl 2-(2-arylethynyl)benzoate 535 reacted in trifluoroacetic acid (TFA), as Brønsted acid. At room temperature and in less than 1 h to the corresponding isocoumarins 539 was obtained in excellent yields 83–98% (Scheme 177). No 5-exo–dig cyclization product was formed in this particular reaction and the presence of electron-donating groups on the aryl moiety seems to have favored the cyclization.
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| | Scheme 176 Acid promoted cyclization of various heterocycles. | |
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| | Scheme 177 Acid promoted isocoumarin cyclization. | |
Green and noncorrosive solid acids e.g. HZSM-5 zeolite, tungestophosphoric acid (H3PW12O40) and tungestosililic acid (H4O40SiW12) were used in three-component, one-pot reaction of 453, 540 and 363 for the synthesis of coumarin dye 541 (Scheme 178).337
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| | Scheme 178 Synthesis of coumarin derivatives 541. | |
Remarkable catalytic performance of hydrobromic acid was used by Yuan and co-workers as a unique catalyst for C–C bond forming reactions of readily available functionalized ketene dithioacetals 542 with various electrophiles.338 The reactive species sulfur-stabilized carbonium ylide 542 was proposed as the key intermediate in the unique catalysis of hydrobromic acid for the synthesis of coumarins 543 or benzothiofurans 545. By the application of this efficient C–C bond-forming reaction of 542 with salicylaldehydes (435) and p-quinones (544), generated coumarins 543 and benzothiofurans 545, respectively were successfully synthesized (Scheme 179).
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| | Scheme 179 Synthesis of coumarins 543 and benzofurnas 545. | |
The preparation of 3-methyl coumarins was reported by a sequence of reactions, microwave assisted Baylis–Hillman reactions of substituted salicylaldehydes 435, followed by one-pot hydrogen iodide-mediated cyclization and reduction of the corresponding adducts 546, to afford 3-methylcoumarins 547 in up to 94% yield.339 Subsequent microwave-assisted selenium dioxide oxidation of selected 3-methylcoumarins 547 has provided convenient access to the corresponding coumarin-3-carbaldehydes 548 and permitted a significant improvement over the yield obtained using conventional heating (Scheme 180).
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| | Scheme 180 Synthesis of coumarin-3-carbaldehydes. | |
Another Brønsted acid catalyzed synthesis of coumarin derivatives has been developed by Kim et al.340 Substituted coumarins 550 were obtained from the condensation of electron-rich arenes 395 with allenes 549 in the presence of TfOH in good yields 61–90% (Scheme 181). Readily available allenes 549 were employed as the three-carbon atom source for construction constituting the coumarin skeleton. Depending on the substituent pattern of allenes employed, various 4-substituted and 3,4-disubstituted 3-arylcoumarins 550 were obtained by applying this methodology.
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| | Scheme 181 Synthesis of 3,4-dissubstituted and 4-substituted coumarin derivatives. | |
Shaterian et al. used Pechmann condensation of pholoroglucinol 551 with β-keto ethyl/methyl ester 205 in the presence of Brønsted acidic ionic liquids, namely 2-pyrrolidonium hydrogen sulfate, N-methyl-2-pyrrolidonium hydrogen sulfate, N-methyl-2-pyrrolidonium dihydrogen phosphate, (4-sulfobutyl)tris(4-sulfophenyl)phosphonium hydrogen sulfate, and triphenyl(propyl-3-sulfonyl)phosphonium toluenesulfonate as catalyst for efficient synthesis of 5,7-dihydroxy-4-methylcoumarin and 5,7-dihydroxy-4-phenylcoumarin 552 in good to excellent yields (Scheme 182).341
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| | Scheme 182 Synthesis of 5,7-dihydroxy-4-substituted coumarin derivatives. | |
Naganaboina et al. reported a trifluoroacetic acid promoted (0.6 mmol) simple and efficient protocol for the synthesis of pyrrolobenzoxazine 555 and 3-arylamino coumarin derivatives 556 (Scheme 183).342 The reaction probably proceeded through the Michael addition of the vinylogous carbamates which takes place at the α-position of β-nitrostyrene 554. This further underwent elimination of H2O from species A and gave pyrrolobenzoxazine derivatives 555. Similarly 1,4-benzoxazinone derivative 553 in the presence of trifluoroacetic acid underwent Michael addition with p-benzoquinone 544. The species B underwent subsequent re-aromatization and successive intramolecular ring opening of oxazinone ring and cyclization to pyranone ring to produce 3-arylamino coumarin derivative 556 (Scheme 184).
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| | Scheme 183 Synthesis of pyrrolobenzoxazine and 3-arylamino coumarin derivatives. | |
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| | Scheme 184 Probable pathway for the formation pyrrolobenzoxazine and 3-arylamino coumarin derivatives. | |
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| | Scheme 185 Synthesis of 3-substituted isocoumarin. | |
2.3. Sulphur heterocycles
2.3.1. Three membered thiirane. Sulphur containing heterocycle thiiranes played central role in various synthetic transformations and are commonly used in pharmaceutical, polymer, pesticide, and herbicide industries.344,345In 2007, Bandgar et al. reported fluoroboric acid adsorbed on silica gel (HBF4–SiO2) as a catalyst for efficient and convenient synthesis of thiiranes 560 from various oxiranes 559 with potassium thiocyanate in high yields 80–98% under solvent-free conditions at room temperature (Scheme 186).346 Reactions completed in 10–20 minute and equally efficient in variety of aliphatic, aromatic and allylic epoxides, including those with electron-withdrawing and electron-donating substituents to give smooth conversion into the corresponding thiiranes 560.
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| | Scheme 186 HBE2-SiO2 catalyzed conversion of epoxides to thiiranes at room temperature. | |
2.3.2. Five-membered thiophene. Thiophenes are frequently found in various pharmaceuticals and drug candidates,347 semiconductors,348 liquid crystals349 and other molecular functional materials.350 Owing to their wide application in academia and industry, new methods and strategies for the generation of functionalized thiophene derivatives are in demand. Environment friendly procedure351 of furan synthesis was extended to synthesize 2-aryl-substituted benzo[b]thiophene derivatives 562 (Scheme 187), that are prevalent in many compounds of biological interest.352 The synthesis was accomplished from o-(1-alknyl)thioanisole 561. A similar trend in selectivity was also observed with the alkyne 561c having an ortho hydroxy substituent as 561c led to a mixture of easily separable 562c (71%) and a furan derivative (13%).
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| | Scheme 187 Synthesis of 2-aryl-substituted benzo[b]thiophene derivatives. | |
A straightforward strategy353 for the introduction of thiophene as an end caps on aromatic analogues, that is, naphthothiophenes, naphthodithiophenes, pyrenothiophene, and benzotrithiophene, can be prepared from commercially available hydroxyarenes 395 in two steps. This includes a consecutive acid-mediated nucleophilic aromatic substitution of hydroxyarenes 395 with 2-mercaptoethanol 563, followed by cyclization to form an arene-fused dihydrothiophene 564 on oxidation of the dihydrothiophene unit gave the thiophene 565 (Scheme 188).
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| | Scheme 188 Synthesis of arene-fused dihydrothiophene and thiophene derivatives. | |
2.3.3. Six-membered thiochromen. Zhong et al. used sulfuric acid or triflic acid-promoted intramolecular Friedel–Crafts reaction of the Moritae–Baylise–Hillman adducts 567 derived from 2-arylthioquinolin-3-carbaldehydes for the synthesis of 12H-thiochromeno[2,3-b]quinolines 568.354 Interestingly, the substrates with electron-donating groups on the aromatic ring gave six-membered fused-ring 12H-thiochromeno[2,3-b]quinolines 568 in good yields in presence of sulfuric acid, while those with electron-withdrawing groups afforded eight-membered fused-ring 5H-benzo[7,8]thiocino[2,3-b]quinolines 569 in moderate yields. Triflic acid instead of sulfuric acid under the similar reaction condition gave only products 568 (Scheme 189).
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| | Scheme 189 Synthesis of 12H-thio chromeno[2,3-b]quinolines and 5H-benzo[7,8]thiocino[2,3-b]quinolines. | |
3. Heterocycles with two hetero-atoms
3.1. Heterocycles with two nitrogen atoms
3.1.1. Five membered. Pyrazole and pyrazoline derivatives exhibit antibacterial, antiviral, antidiabetic, antitubercular, antitumor, anti-inflammatory, and antidepressant activities.355–359 They are found in a number of natural products, including asomnine and 4-hydroxy with asomnine, pyrazofurine, formycin A and B, oxoformycin B, nostocine A, fluviol, and 3-n-nonylpyrazole. Pyrazoles are also used in supramolecular and polymer chemistry, in organic synthesis as ligands, in the food industry, and as cosmetic colorings.360–362
3.1.1.1. Pyrazole. The most notable method for the synthesis of pyrazolin-5-ones involves the reactions of β-oxo esters, or their derivatives, with hydrazines.363 Other reported methods are air oxidation of 1,2-hydrazinohydrazones,364 palladium-catalyzed carbonylation of 1,2-diaza-1,3-buta-dienes,365 or rearrangement of an oxadiazole derived from a 2-cyanoacetanilide.366 Bagley et al. used concentrated hydrochloric acid–methanol (1.5% v/v) for efficient synthesis of 1,3- and 1,5-disubstituted pyrazoles by reaction of α,β-ethynyl ketones and hydrazine derivatives.367 In 2008, Deng and Mani developed a regioselective synthesis of 1,3,5-tri- or 1,3,4,5-tetra-substituted pyrazoles 571 from electron-deficient N-arylhydrazones 570 and nitroolefins 518 in good to moderate yields under acidic condition using CF3CH2OH/TFA (Scheme 190).368 This pyrazole synthesis tolerated various substituents with respect to hydrazones and nitroolefins. Various substitutions at the R4 position and various electron-donating and electron-withdrawing groups on the aryl ring of R3 are compatible with the reaction conditions. An aliphatic nitroolefin, sterically demanding nitroolefin and heterocycles such as a thiophene also afforded reasonably good yields of the pyrazole products. But, the electronic properties of the hydrazone substituents found to be a dominant factor scheming reactivity. Hydrazones with electron-donating substitution at R1 and R2 position afforded the pyrazole products in decent yields. However; the yield was much lower with electron-deficient hydrazones. Electron-poor hydrazone needed higher temperature than electron-rich hydrazone and the yield was also low.
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| | Scheme 190 Synthesis of 1,3,5-tri- or 1,3,4,5-tetrasubstituted pyrazoles. | |
The reaction preceded through (Scheme 191) an initial formation of a neutral intermediate 572 by a nucleophilic attack of hydrazone 570 on nitroolefin 518. Either trans- or cis-nitro-olefin afforded the same intermediate 572 by free rotation of C–C single bond between R3 and R4 and which results in the nonstereospecificity of the pyrazolidine formation step. Intermediate 572 could be converted to two different products in two different reaction pathways. One could be an irreversible protonation to afford the Michael addition product 573. The other one could be an intramolecular cyclization to furnish the pyrazolidine intermediate 574 followed by a slow oxidation by air and fast elimination of HNO2 to furnish the pyrazole product 571.369
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| | Scheme 191 Possible reaction pathway for synthesis of 1,3,5-tri- or 1,3,4,5-tetrasubstituted pyrazoles. | |
Among recent acid-promoted pyrazolines synthesis Wang group developed a convenient and efficient synthesis of spiro-fused pyrazolin-5-one N-oxides 576 starting from readily available 1-carbamoyl-1-oximylcycloalkanes 575. This general protocol370 featured a novel and facile way to access the five-membered azaheterocycles by the formation of a new N–N single bond (Scheme 192). The key cyclization step utilized the formation of an N-oxonitrenium intermediate, mediated by the hypervalent iodine reagent phenyliodine(III)-bis(trifluoroacetate), and its subsequent intramolecular trapping by the amide moiety under rather mild experimental conditions.
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| | Scheme 192 Synthesis of spiro-fused pyrazoline-5-one N-oxides. | |
Recently, Deng and co-workers reported a TFA-catalyzed cycloaddition reaction of hydrazones 570 with β-bromo- or β-chloro-β-nitrostyrenes 518 for the regioselective synthesis of 4-nitro- or 4-chloro tetrasubstituted pyrazoles 577.371 The reaction proceeded through a common 4-halo-4-nitropyr-azolidine intermediate and the identity of the pyrazole product formed was dependent on the relative leaving group abilities of the halo and nitro substituents (Scheme 193).
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| | Scheme 193 Synthesis of 4-nitro-or 4-chloro tetrasubstituted pyrazoles. | |
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| | Scheme 194 Synthesis of 1-methyl-2-(hetero)arylbenzimidazoles. | |
p-Toluenesulfonic acid catalysed one-pot, three-component condensation reaction of 2-aminoazine 580, aldehyde 189 and isocyanide 581 at room temperature afforded a number of 3-aminoimidazo[1,2-a]pyridines or 3-aminoimidazo[1,2-a]pyrazines 582 in 86–98% yields (Scheme 195).377a Similarly Rahmati et al. synthesized novel series of N-alkyl-2-aryl-5H-imidazo[1,2-b]pyrazol-3-amines in good to high yields 70–97% by three-component condensation of an aromatic aldehyde, an aminopyrazole, and an isocyanide in acetonitrile in the presence of 4-toluenesulfonic acid (0.3 mmol) as a catalyst at room temperature for 3–7 h.377b
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| | Scheme 195 Synthesis of 3-aminoimidazo[1,2-a]-pyridines and -pyrazines. | |
Mert-Balci et al. synthesized pyrido[2′,1′:2,3]imidazo[4,5-c]isoquinolin-5(6H)-ones 583 by MeSO3H catalyzed (0.2 equivalents) microwave-assisted three-component reaction between 2-aminopyridines 580, isocyanides 581, and 2-carboxybenzaldehydes 315 (Scheme 196).378
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| | Scheme 196 Microwave-assisted synthesis of pyrido[2′,1′:2,3]imidazo[4,5-c]isoquinolin-5(6H)-ones. | |
1-Methylimidazoluim triflouroacetate ([Hmim]TFA), a reusable Brønsted acidic ionic liquid (BAIL) was used by Dabiri et al.379 to synthesize 2-aryl-1-arylmethyl-1H-1,3-benzimidazoles from the reaction of o-phenylenediamines and aromatic aldehydes. Similarly, Mukhopadhyay et al. reported the synthesis of 2-alkyl substituted benzimidazoles 584 under microwave irradiation using 62% aqueous hexafluorophosphoric acid (10 mol%) as catalyst.380 The reaction of the o-phenylenediamines 364 with the various aliphatic acids 115 was highly efficient in most cases 65–96%. When an electron-withdrawing group like Cl was present in the aromatic diamine part, the yield was affected and the yield 65% could not be enhanced even when the reaction time was prolonged (Scheme 197).
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| | Scheme 197 Selective synthesis of benzimidazole. | |
p-Toluenesulfonic acid in methanol was found to be an efficient catalyst for the synthesis of bis-3-aminoimidazo[1,2-a]-pyridines, -pyrimidines and -pyrazines 585 as extended π-conjugated systems in 56–99% yields by a novel pseudo five-component condensation of 2-amino-pyridine, -pyrimidine and -pyrazins derivatives 580 with terephthalaldehyde or isophthalaldehyde 189 and isocyanides 581 (Scheme 198).381
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| | Scheme 198 Synthesis of bis-3-aminoimidazo[1,2-a]-pyridines, -pyrimidines and pyrazines. | |
Sharma and co-workers used HClO4 in MeOH at rt as acid-catalyst for incorporation of glyoxylic acid as formaldehyde equivalent in the 3C–C reaction towards synthesis of 3-aminoalkyl imidazo-azines.382 Hutt et al. used three-component coupling reaction of substituted picolinaldehydes 586, amines 101, and formaldehyde 189 for efficient synthesis of imidazo[1,5-a]pyridinium ions 587.383 This acid-catalyzed reaction gave high yields under mild condition with diverse functional group and chiral substituents tolerance and provided an efficient method for the preparation of N-heterocyclic carbenes (NHCs) (Scheme 199). Various multidentate NHC ligands with wide applications were also synthesized by higher order condensations.
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| | Scheme 199 Synthesis of imidazo[1,5-a]pyridine N-heterocyclic carbene precursors. | |
Chen et al. reported a synthesis of benzo[4,5]imidazo[2,1-a]isoindole 589 and 1,2-dialkyl-2,3-dihydrobenzimidazoles 590 under metal free conditions by using HCOOH-catalyzed coupling of dialdehyde 588 and o-diaminobenzene 363.384 A series of imidazoles were obtained in good yields in the presence of 0.5 mL HCOOH, 2 mL MeOH at 0 °C for 2 h (Scheme 200). However, the reaction of glutaraldehyde 591 with various o-diaminobenzene proceeded at 120 °C to give imidazole derivatives 592 and 593 (Scheme 201).
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| | Scheme 200 Synthesis of benzo[4,5]imidazo[2,1-a]isoindole and 1,2-dialkyl-2,3-dihydrobenzimidazoles. | |
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| | Scheme 201 Reaction of glutaraldehyde with various o-diaminobenzene. | |
In 2013, Eycken group used Groebke–Blackburn–Bienaymé reaction for the synthesis of 1H-imidazo[1,2-a]imidazol-5-amines 595 by microwave-assisted multi-component reaction between 2-aminoimidazole 594, aldehyde 189 and isocyanide 581 (Scheme 202).385 The reaction gave 77% (maximum) isolated yield by using 20 mol% TsOH·H2O in toluene under microwave heating at 110 °C for 30 minutes. The reported methodology tolarated various aldehydes 189, isocyanides 581 and 2-aminoimidazoles 594 under this reaction condition and a series of 1H-imidazo[1,2-a]imidazol-5-amines 595 having four points of diversity were synthesized in 17–77% yields.
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| | Scheme 202 Synthesis of imidazoles by Groebke–Blackburn–Bienaymé reaction. | |
Chen and co-workers described the synthesis of tri- and tetra-substituted imidazole derivatives 597 and 598 by simple, efficient, and eco-friendly methodology using pivalic acid mediated multicomponent reaction between various internal alkynes 344, aldehydes 189, and anilines 101 (Scheme 203).386 4 equiv. of NH4OAc in DMSO and H2O along with 1 equiv. PivOH at 140 °C was optimal for this reaction. Under this acid-catalyzed reaction condition various alkyl and aryl aldehydes and internal alkynes including electron-withdrawing and -donating functional groups in the aromatic moieties gave a wide range of imidazole derivatives 597 or 598 containing various substituents in 68–90% yields. Interestingly when 1,4-bis(phenylethynyl)benzene 599 was reacted with 2 equiv. of benzaldehyde under this acid-catalyzed reaction conditions 1-(2,5-diphenyl-1H-imidazol-4-yl)-4-(2,4-diphenyl-1H-imidazol-5-yl)benzene (600) was obtained in 55% yield.
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| | Scheme 203 Synthesis of tri- and tetrasubstituted imidazole derivatives. | |
Recently Brønsted acid (TsOH·H2O) has been used by Mayo and co-workers for the synthesis of various 2-substituted benzothiazoles/benzimidazoles in satisfactory to excellent yields.387 This Brønsted acid catalyzed cyclization reactions of 2-amino thiophenols/anilines 363 with β-diketones 364 under oxidant-, metal-, and radiation-free conditions well tolerated different groups such as methyl, chloro, nitro, and methoxy linked on benzene rings. No reaction was observed when 2-amino thiophenol 363 and 2,4-pentanedione 364 were treated in acetonitrile (CH3CN) at room temperature in the absence of an acid catalyst. But, the desired product 2-methyl benzothiazole (601) was obtained in more than 99% yield simply by using p-toluene sulfonic acid (TsOH·H2O) as a catalyst. Although CF3COOH and TsOH·H2O are equally effective for this reaction, (TsOH·H2O) was chosen as a catalyst due to its low cost and simplicity in handling. The best reaction condition for this reaction condition is to use TsOH·H2O as a catalyst at room temperature under solvent-free conditions for 16 h. Under this (TsOH·H2O) catalyzed cyclization reactions condition various 2-amino thiophenols 363 and various β-diketones 364 give excellent yields. When the same reaction was performed in CH3CN at 80 °C gave benzothiazole in 63% yield. Similar reactions of 2-amino anilines 363 with various β-diketones 364 in the presence of TsOH·H2O as a catalyst at room temperature under the same reaction condition as mentioned above did not give any reaction. But when the reaction was carried out at 80 °C it furnished benzimidazoles 602 in satisfactory to excellent yields (Scheme 204). Benzimidazole products 602 were isolated in 55–95% yields from the reactions of 2-amino aniline 363 with β-diketones 364 bearing aliphatic groups (Me, Et, and iPr) on their 1,3-positions. 2-Phenyl benzimidazole 602 was obtained in low yield (49%) because of the low reactivity of the β-diketone 364 bearing aromatic groups on its 1,3-position. For benzimidazole synthesis substituent property (electron-donating or -withdrawing) had almost no influence on the reactivity of 2-amino aniline substrates.
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| | Scheme 204 Synthesis of benzothiazole and benzimidazoles. | |
The reaction may proceed through the initial Brønsted acid catalyzed condensation reaction of 363 with 364 to generate a ketimine intermediate 603. Intermediate 603 may be converted to ketiminium intermediate 604 by TsOH·H2O. The intramolecular nucleophilic addition of 604 produced adduct 605. The C–C bond cleavage reaction of adduct 605 finally occurred to generate the product 601 (Scheme 205).
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| | Scheme 205 Proposed mechanism for 601. | |
Polyphosphoric acid (PPA) was also used as a catalyst for the reaction between aryl 2,3-epoxy esters 559 and 2-amino-pyridines 580 which involved multiple C–O/C–N bond-breaking/formation for efficient synthesis of (Z)-2-methyleneimid-azo[1,2-a]pyridin-3-ones 606 (Scheme 206).388 Polyphosphoric acid played a multifunctional role in this intermolecular cascade reaction at 110 °C and exploited the tendency of the oxirane ring to act as a bi-electrophile which is different from the known reactions of α,β-epoxy esters, which take place through oxiranyl C–O or C–C bond cleavage. The reaction may proceed through the epoxide C–O bond cleavage, formation of an α-enamine ester, and intramolecular transamidation with chemo-, regio- and diastereo-selectivity (Scheme 207). The reaction allows various 2,3-epoxy esters 559 as well as 2-aminopyridines 580 possessing either electron-donating or with-drawing functionalities for the synthesis of biologically relevant (Z)-2-methyleneimid-azo[1,2-a]pyridin-3-ones 606 where water and ethanol were the only byproducts.
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| | Scheme 206 Synthesis of (Z)-2-methyleneimid-azo[1,2-a]pyridin-3-ones. | |
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| | Scheme 207 Proposed mechanism for (Z)-2-methyleneimid-azo[1,2-a]pyridin-3-ones. | |
Another PTSA-catalyzed benzimidazole 608 synthesis was developed by Vidavalur et al.389 Various S-ethylated-N-acylthioureas 607 in the presence of 30 mol% PTSA and PEG-400 mediated straightforward reaction with o-phenylenediamine 363 at 120 °C afforded a wide variety of 2-(N-acyl)aminobenzimidazoles and 2-(N-acyl)amino-benzothiazoles 608 in 45–78% yields (Scheme 208). The reaction perhaps proceeded through the initial protonation of nitrogen of morpholine by PTSA, followed by the addition of o-phenylenediamine or o-amino-thiphenol to form 609 through the elimination of morpholine. Finally, intramolecular cyclization of 609 occurred through the elimination of ethyl mercaptan to form 2-(N-acyl)aminobenzimidazoles and 2-(N-acyl)amino-benzothiazoles 608.
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| | Scheme 208 Synthesis of 2-(N-acyl)aminobenzimidazoles and 2-(N-acyl)amino-benzothiazoles. | |
Table 17 Direct synthesis of imidazoles from nitriles and α-amino acetals
3.1.2. Six membered.
3.1.2.1. Pyridazine. Pyridazines are of significant interest due to their occurrence in natural products, and a wide range of bioactivities as plant virucides,391,392 antitumor agents,393–395 fungicides,396–398 insecticides,399,400 herbicides401–405 and anti-inflammatory agents.406,407Small and medium-sized fluoroalkyl substituted 1,2-diaza-3-one heterocyclic ring skeletons 615 has been developed by Wan and co-workers by a sequential condensation and ring-closure of ω-fluoroalkylated ketoesters 613 with hydrazines 127 catalyzed by 10–20 mol% TsOH (Scheme 209).408 Trifluoromethyl substituted seven- and eight-membered 1,2-diazapinone 617, 1,2-diazocinone 619 were also obtained in 85% and 45% yields respectively via this sequential reaction of δ-(or ε-)trifluoromethyl ketoesters with hydrazine hydrates in acidic condition but, the reaction of 613 with aryl hydrazines catalyzed by stoichiometric TsOH could not yield desired diazepinone or diazocinone derivatives, instead, 2-trifluoromethyl indole-3-propanoate 620 or 2-trifluoromethyl indole-3-butanoate was obtained in 72% or 78% yield, respectively (Scheme 210). Pharmaceutically interesting trifluoromethylated triazolopyridazine derivative 623 was also synthesized starting from 4,5-dihydro-6-trifluoromethyl pyridazinones 621 where last step is TsOH-catalyzed cyclization (Scheme 211).
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| | Scheme 209 Synthesis of hydrazones and pyridazinones. | |
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| | Scheme 210 Synthesis of fluoroalkylated 1,2-diazaones and indoles. | |
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| | Scheme 211 Transformation of 4,5-dihydro-6-trifluoromethyl pyridazinone to triazolopyridazine. | |
In 2012, Salunkhe et al. developed a new catalytic system Brønsted acid hydrotrope combined catalyst (BAHC) and applied this catalyst for the synthesis of pyrido[2,3-b]pyrazines 625 and quinoxalines in an aqueous medium at ambient temperature in excellent yields 80–96% (Scheme 212).409 The catalyst can be easily recovered after the reaction and can be reused. BAHC catalyst worked well and avoids the use of organic solvents.
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| | Scheme 212 Synthesis of quinoxaline and pyrido[2,3-b]pyrazine derivatives. | |
Wojaczyńska et al. disclosed the synthesis of pyrazine-2-ones 629 and 630 by the reaction of glyoxalate imine 627, derived from (1R,2R)-diaminocyclohexane 626, with a series of dienes.410 Reaction of in situ generated glyoxalate imine 627 with freshly distilled diene (1.2 equiv.) mediated by TFA (1 equiv.) and BF3·Et2O (1 equiv.) in DCM at room temperature for 20 h gave the corresponding pyrazine-2-ones 629 and 630 in good yields (Scheme 213).
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| | Scheme 213 Synthesis of pyrazine-2-ones. | |
The majority of synthetic routes to this family of aza-heterocycles involve strategies based on the condensation of amines and carbonyl compounds.416,417 Heteropolyacids are extremely useful and highly efficient heterogeneous solid acids for the synthesis of biologically potent aryl 3,4-dihydropyrimidinones. Yadav and co-workers used silver salt of heteropolyacid (HPA), i.e. Ag3PW12O40, in water for the three-component condensation of an aldehyde 189, 1,3-dicarbonyl compound 364, and urea or thiourea 631 in a one-pot operation (Scheme 214).418 This method tolerated a wide range of substrates, including aromatic, aliphatic, α,β-unsaturated, and heterocyclic aldehydes, and provided a variety of biologically relevant dihydropyrimidinones 632 in good yields after short reaction times by the use of water-tolerant and recyclable heteropoly acid as a catalyst.
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| | Scheme 214 Synthesis of 3,4-dihydropyrimidinones. | |
Sujatha et al. reported the synthesis of a series of biologically important 4-(substituted)-3,4-dihydropyrimidinone derivatives under microwave heating by the multicomponent reaction of 1,3 dicarbonyl compounds, urea, and aromatic aldehydes in acetic acid.419
A high yielding, low cost, simple and convenient procedure for the synthesis of benzo[4,5]imidazo[1,2-a]pyrimidine derivatives 634 has been developed by Chang-Sheng et al. through a three-component reaction of aldehydes 189, β-dicarbonyl compounds 364 and 2-aminobenzimidazole 633 catalyzed by sulfamic acid in a solvent-free condition (Scheme 215).420
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| | Scheme 215 Synthesis of benzo[4,5]imidazo[1,2-a]pyrimidine derivative. | |
Nag et al. synthesized substituted imidazo[1,2-a]pyrimidin-7-ylamines 637 from allylamine derivatives 636. Allylamine derivatives 636 were obtained from Baylis–Hillman acetates of substituted benzaldehydes and heterocyclic aldehydes by treatment with cyanamide. The secondary allylamines 636, were converted into 6-arylmethylimidazo[1,2-a]pyrimidin-7-ylamines 637 in a one-pot procedure by reacting with cyanamide under acidic conditions (Scheme 216).421 Other aldehydes also furnished similar products in a two-pot procedure. In two step procedure first allylamines reacted with cyanamide under acidic conditions to produce the 2-amino imidazoles (638), which under basic conditions underwent intramolecular cyclization to afford 637.
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| | Scheme 216 Reagents and conditions: (i) 2,2-dimethoxyethylamine, MeOH, room temp., 1 h; (ii) (1) NH2CN, AcOH/H2O (1:1, v/v), 90–100 °C, 2 h, (2) HCl (conc.), 90–100 °C, 30 min (one-pot), (iii) (1) NH2CN, AcOH/H2O, 90–100 °C, 2 h, (2) HCl (conc.), 90–100 °C, 5 min; (iv) NaOMe, MeOH, rt, 1 h. | |
Cyclic aminals can be prepared by Brønsted acid-promoted reaction. In 2009, Seidel and co-workers synthesized a series of substituted pyrimidine derivatives 639 by the reaction of aminobenzaldehyde 280 with different aromatic amines 101 in presence of 0.2 equiv. or 1.2 equiv. of CF3COOH (Scheme 217).422 Electronically diverse anilines with various substitution patterns and structurally diverse aminobenzaldehydes 280 provided access to products 639 in moderate to good yields in the presence of trifluoroacetic acid. Bulky substituents on both ortho-positions of the aniline 101 moiety were readily accommodated under this reaction condition. This redox neutral process involved first iminium ion formation followed by 1,5 H-transfer and ring closure (Scheme 218).
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| | Scheme 217 Synthesis of various cyclic aminals. | |
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| | Scheme 218 Proposed mechanism of formation of cyclic aminals. | |
Another multicomponent reaction for the synthesis of pyrimidine derivatives was reported.423 Efficient synthesis of 3,4-dihydropyrimidin-2(1H)-ones and thiones were carried out by the condensation of ethylacetoacetate, aldehydes (aromatic or aliphatic), and urea or thiourea in the presence of a Brønsted acidic ionic liquid (methylimidazolium hydrogensulfate).
Recently, a conceptually new three-component reaction was developed by Kumar et al. to construct a six-membered fused N-heterocyclic ring affording (pyrazolo)pyrimidines/pyridines 641 as potential inhibitors of PDE4.424 The reaction is catalyzed by triflic acid in acetic acid in the presence of aerial oxygen. The reaction proceeded well with a variety of aromatic aldehydes 189 and terminal alkynes 79 to give a range of 5,7-diaryl substituted derivatives 641 (Scheme 219). The reaction was assumed to proceed through four steps, e.g. (i) in situ generation of imine via condensation of amine 640 and aldehyde 189, (ii) subsequent nucleophilic addition of the alkyne 79 to imine leading to the key propargyl amine Z, (iii) cycloisomerization of the amine Z via intramolecular nucleophilic attack of pyrazole nitrogen (N-1) or carbon (C-4) in a 6-‘endo–dig’ fashion depending on the nature of X present and, finally, (iv) aerial oxidation of the resulting dihydropyrazolopyrimidine intermediate affording the product 641 (Scheme 220).
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| | Scheme 219 Synthesis of 5,7-diaryl substituted pyrazolo[1,5-a]pyrimidines. | |
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| | Scheme 220 Proposed reaction mechanism. | |
Recently, TFA:DIPEA (1:1) catalyst has been used for a simple, efficient and economic synthesis of tetrazolo [1,5-a]pyrimidine-6-carboxylates 643.425 Three-component reaction of β-ketoesters 364 or dimidone 419 with a mixture of aromatic aldehyde 189 and 5-aminotetrazole 642 gave tetrazole derivatives 643 or 644 (Scheme 221). The synthesized compounds were evaluated for their antimicrobial and antioxidant activity.
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| | Scheme 221 Diisopropylammonium trifluoroacetate mediated synthesis of tetrazolo pyrimidines. | |
Very recently Safari and co-workers reported the synthesis of a novel magnetic nanoparticle supported hydrogen sulfate ionic liquid catalyst (MNPs-IL-HSO4) and used this catalyst in Biginelli reaction under mild condition for the synthesis of a diverse range of 3,4-dihydropyrimidin-2(1H)-ones 645 (Scheme 222).426 The catalyst maintained satisfactory catalytic activity for the synthesis of Biginelli compounds after 8 rounds of recycling and gave pyrimidine derivatives 645 in 90–98% yield. Similarly, Brønsted acidic ionic liquid (4-sulfobutyl)tris(4-sulfophenyl)phosphonium hydrogen sulfate was used as an efficient catalyst for the synthesis of a series of 8,9-dihydro-2-(2-oxo-2H-chromen-3-yl)-5-aryl-3H-chromeno[2,3-d]pyrimidine-4,6(5H,7H)-diones (647) by the reaction of 2-amino-5,6,7,8-tetrahydro-5-oxo-4-aryl-4H-chromene-3-carbonitrile (646) with coumarin-3-catboxylic acid under neat conditions (Scheme 223).427 Brønsted-acidic ionic liquid 3-methyl-1-(4-sulfonic acid)butylimidazolium hydrogen sulfate [(CH2)4SO3Hmim][HSO4], is also a green and reusable catalyst under solvent-free conditions for similar type of reactions.428
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| | Scheme 222 Synthesis of dihydropyrimidinones. | |
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| | Scheme 223 Synthesis of chromeno pyrimidinones using ionic liquid. | |
Another one-pot reaction has recently been described by B. Karami et al. by using a safe and recyclable Brønsted acid catalyst under solvent-free conditions. Novel functionalized pyrimido[4,5-d]pyridazines 649 has been synthesized from aryl glyoxals 100 acetylacetone 364 and urea 631 using molybdate sulfuric acid (5 mol%).429 The reaction proceeded by formation of 5-acetyl-4-(aryloyl)-3,4-dihydropyrimidinones 648, which readily underwent the Knorr condensation with hydrazines to produce new pyrimido[4,5-d]pyridazines 649 (Scheme 224).
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| | Scheme 224 Synthesis of aryloyl-3,4-dihydropyrimidinones and pyrimido[4,5-d]pyridazines. | |
Three-component condensation reaction of o-phenylenediamines 363, diverse carbonyl compounds 189 and isocyanides 581 in the presence of a catalytic amount of p-toluenesulfonic acid (5 mol%) gave highly substituted 3,4-dihydroquinoxalin-2-amine derivatives 650 in good to excellent yields 75–95% (Scheme 225). Similarly, the same group also synthesized highly substituted imidazo[1,5-a]pyrazine derivatives.435
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| | Scheme 225 Synthesis of 3,4-dihydroquinoxalin-2-amine derivatives. | |
N-4-Substituted 2,4-diaminoquinazolines 652 synthesis has been developed by Yin et al. by employing tandem condensation of cyanoimidate–amine 651 and reductive cyclization using iron–HCl system (Scheme 226).436 This method involved intramolecular N-alkylation and produced two fused heterocycles in a one-pot procedure. This is a facile two-steps synthesis of tricyclic quinazolines 654, which is effected by potent cyanoimidation and tandem reductive cyclization. Moreover, the ring forming process of tricyclic quinazolines 654 has been investigated from the ring-opening/ring-closing cascade point of view. It was found that the preparation of tricyclic quinazolinones 654 in good yields relies on the selective hydrolysis of the tricyclic quinazolines 653 in base or acid system (Scheme 226).
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| | Scheme 226 Synthesis of N-4-substituted 2,4-diaminoquinazolines. | |
Recently, another acid-mediated quinoxalines synthesis have been described by Pereira et al.437 Synthesis of 4,7-substituted pyrrolo[1,2-a]quinoxalines and related heterocyles 656 have been developed through a cascade of redox reactions/imine formation/intramolecular cyclization (Scheme 227). This procedure tolerated various readily available substituted 1-(2-nitrophenyl)pyrrole 655 derivatives and aliphatic or benzylic alcohols 395 as starting materials using iron powder and acidic conditions. This was the first example of constructing N-heterocycles via iron-mediated aryl nitro reduction and aerobic oxidation of alcohols in one pot.
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| | Scheme 227 Synthesis of 4,7-substituted pyrrolo[1,2-a]quinoxalines. | |
3.2. Heterocycles with nitrogen and oxygen atom
3.2.1. Five membered oxazole. Natural products and drugs, such as siphonazole, madumycin I, muscoride A, telomestatin, hennoxazole A, ulapualide A, diazonamde A, leucamide A, disorazole C1, phorboxazole A, bengazole A, and oxaprozin contain nitrogen and oxygen containing important heterocyclic compound oxazole.438 Additionally, oxazolines are also used in organic synthesis as synthetic intermediates439 or protecting groups440 and also as metal ligands in asymmetric catalysis.441Sulfamic acid is a common organic mild acid and is dry, nonvolatile, nonhygroscopic, odorless and easy to handle as a catalyst. Madhav and co-workers442 used this sulfamic acid as catalyst and dimethyl formamide as a solvent under conventional method and microwave irradiation for rapid and efficient preparation of 3-(4,6-dimethyloxazolo[4,5-c]quinolin-2-yl)-chromen-2-ones starting from 3-amino-2,8-dimethyl-quinolin-4-ol and 2-oxo-2H-chromen-3-carboxylic acid and 3-amino-2,8-dimethyl quinoline-4-ol.
In 2014, Majumdar group reported triflic acid catalyzed synthesis of benzoxazoles 657 and other heterocycles by three-component reaction between 2-amino phenols 363, isocyanides 581, and ketones 189 (Scheme 228).443 The isocyanide-based heterocycle-forming reaction progressed via a benzoxazine intermediate formed by intramolecular nucleophilic trapping of the reactive nitrilium intermediate by an adjacent phenolic group. The reaction gave best yield in the presence of 0.1 equiv. of TfOH in 2,2,2-trifluoroethanol (TFE) at 55 °C. Stable products were formed from nitrilium trapping but in that particular examples the trapped intermediates could be opened up by bis-nucleophiles. Different ketones 189 and various substituted 2-aminophenols 363 gave the desired benzoxazole 657 in good yields 28–100%. The reaction showed good functional group tolerance. Multifunctional semisynthetic natural product-like naloxone can also be synthesized by using this triflic acid catalyzed reaction. In this reaction isocyanide 581 contributes only one carbon atom toward benzoxazole and almost all isocyanides were effective in the reaction, except 2,6-dimethylphenyl isocyanide. When 2,6-dimethylphenyl isocyanide was used as isocyanide yielded spiro[benzo[b]-[1,4]oxazine]imine (benzoxazine) scaffold 659 in good yield 33–79%. Another interesting observation was that when 2-chloro-6-methyl isocyanide was used, benzoxazine scaffold 659 was obtained but the reaction, also afforded benzoxazole 657 and the yield of benzoxazine 659 was lower (Scheme 229). Various heterocyclic scaffolds including benzimidazoles, dihydrothiazoles, and benzothiazoles were also synthesized by using the reactivity of benzoxazine to bis-nucleophiles (Scheme 230).
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| | Scheme 228 Synthesis of benzoxazoles with various ketones, isocyanides and substituted 2-aminophenol. | |
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| | Scheme 229 Isolation of trapped nitrilium intermediates benzoxazines. | |
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| | Scheme 230 Conversion of benzoxazine intermediate to diverse heterocycles with bis-nucleophiles. | |
From the above observations the authors concluded that the synthesis of benzoxazine proceeded by the formation of Schiff base (imine, A) from the reaction of ketone 189 and 2-aminophenol 363, which in turn was activated by the triflic acid and accelerate an attack by the isocyanide yielding the highly reactive nitrilium B. Intermediate B converted to intermediate C by the intramolecular nucleophilic attack of phenolic OH of 2-aminophenol on the reactive nitrilium carbon. Intermediate C could be isolated and was very susceptible to nucleophilic attack by a second molecule of the bis-nucleophile 2-aminophenol 363 and gave benzoxazole derivatives 657 (Scheme 231).
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| | Scheme 231 Proposed mechanism for the formation of benzoxazole 657. | |
Very recently, H. Zhou and co-workers described an efficient and practical method for diversity oriented synthesis of 3-oxazolines 661 in good to excellent yields (up to 99%) by TfOH-catalyzed chemoselective [3 + 2] cycloaddition of donor–acceptor oxiranes 559 and nitriles 610.444 Due to protonation of nitriles by Brønsted acid (TfOH), an excess of nitriles 610 along with 0.5 equiv. of TfOH is required for this transformation. Due to the destabilization of the incipient positive charge on the adjacent carbon of the dipole by the electron-deficient aryl moiety during the course of the reaction electron-deficient aryl substituted ones works less efficiently than the electron-rich aryl substituted oxiranyl dicacarboxylates. Similarly, the 1-naphthyl substituted starting material also furnishes the corresponding product in an excellent yield of 91%. Various aromatic nitriles afforded electron-donating or electron-withdrawing groups and alkyl nitrile giving the products in good to excellent yields (up to 99%). α,β-Unsaturated nitrile 610 also gave an excellent yield of 92% in this reaction condition (Scheme 232).
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| | Scheme 232 TfOH-catalysed [3 + 2] cycloaddition of various nitriles with various oxirane. | |
3.2.2. Six membered.
3.2.2.1. Oxazine. Oxazines are valuable intermediates in the synthesis of various natural and biologically important molecules. Various natural products, such as trichodermamides A–C, myrioxazine A, aspergillazine A, and gliovirin, various drugs, such as BW 306U, prepitant, phenmetrazine, phendimetrazine, 3-benzhydrylmorpholine, edivoxetine, morazone, fenbutrazate, reboxetine, which act as appetite suppressant, anti-inflammatory, anti-HIV, selective norepinephrine reuptake inhibitor, antihypertensive, antithromobotic, antibiotic, and anticonvulsant agents contain oxazine motif.445,4462-Amino-4H-1,3-oxazines 662 or 2-amino-4H-1,3-thiazines 663 with good yields was synthesized by novel three-component reaction in a one-pot procedure using TFA/AcOH (1 mL/3 mL).447 The reaction between alkynes (79), urea (631), and aldehydes (189) under refluxing condition for 10 h gave 2-amino-4H-1,3-oxazines derivatives 662 in 53–87% yields. Similarly reaction between alkynes (79), thiourea (631), and aldehydes (189) under the same reaction condition gave 2-amino-4H-1,3-thiazines derivatives 663 in 50–75% yields. This multicomponent reaction allowed considerable flexibility in the nature of both the alkyne R1 group and the aldehyde R2 group, facilitating the preparation of a diverse array of 2-amino-4H-1,3-oxazines 662 and 2-amino-4H-1,3-thiazines 663 (Scheme 233). The presence of electron-withdrawing or electron-releasing groups on the aromatic rings of the aromatic aldehydes did not exhibit significant effect on the yield. However, for aryl alkynes, those carrying electon-withdrawing groups led to lower yields, most likely due to the lower electron density of the alkyne group. The reaction was assumed to proceed via the initial condensation of aldehyde 189 and urea or thiourea 631 to give the intermediate 664 which was further converted to the reactive intermediate 665. Subsequently, the resulting reactive intermediate 665 underwent hetero Diels–Alder cycloaddition with the alkyne 79, affording 2-amino-4H-1,3-oxazinium salts or 2-amino-4H-1,3-thiazinium salts 666. Finally, the oxazinium salts or thiazinium salts 666 were deprotonated to furnish 2-amino-4H-1,3-oxazine 662 or 2-amino-4H-1,3-thiazine 663 (Scheme 234).
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| | Scheme 233 Multicomponent synthesis of 2-amino-4H-1,3-oxazines or 2-amino-4H-1,3-thiazines. | |
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| | Scheme 234 Proposed mechanism. | |
p-Toluenesulfonic acid has been used as a catalyst in one-pot, three-component reaction of an aromatic aldehydes 189, isocyanides 581 and o-aminophenols 363 for rapid and high yielding 87–91% synthesis of 3-aryl-4H-benzo[1,4]oxazin-2-ylamine 667 (Scheme 235).448
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| | Scheme 235 Synthesis of 3-aryl-4H-benzo[1,4]oxazin-2-ylamine derivatives. | |
An efficient and novel one-pot synthesis of six-membered N,O-heterocyclic skeleton 3,4,5-trisubstituted-3,6-dihydro-2H-1,3-oxazine 668 has been described via Brønsted acid-promoted (HCl) domino hydroamination/Prins reaction/cyclization/dehydration reactions of alkynoates 344, anilines 101, and formaldehyde 189 by Cao et al.449 The reported reaction gave highest yield in the presence of 3 mol% HCl in CH3OH at room temperature stirring for 15 h (Scheme 236). The reactions proceeded rapidly and afforded the desired products in good to excellent yields 70–84% in the presence of various substituents present in the aromatic ring of the amine derivatives 101. Although, substituents present on the p- and m- of aromatic group of aniline have no obvious effect on the reaction but more sterically hindered o-disubstituted amines such as 2,3-difluoro- and 2-methylaniline as substrates, the desired products could not be obtained. Ethanol was replaced by methanol as solvent in order to avoid ester exchange reaction when diethyl acetylenedicarboxylate was used as alkyne.
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| | Scheme 236 Domino synthesis of skeleton 3,4,5-trisubsti tuted-3,6-dihydro-2H-1,3-oxazine. | |
Next year Srikrishna and co-workers reexamined the structure assigned for the products obtained in the Brønsted acid catalyzed reaction of dimethyl but-2-ynoates 344 with anilines 101 and an excess of formaldehyde 189 in methanol and found that the actual product was methyl 1-(aryl)-3-(methoxymethyl)-4,5-dioxopyrrolidine-3-carboxylate 671 not dimethyl 3-(aryl)-3,6-dihydro-2H-1,3-oxazine-4,5-dicarboxylate (Scheme 237).450 The structure of 2,3-dioxopyrrolidine-3-carboxylate 671 was established from single crystal X-ray diffraction analysis.
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| | Scheme 237 Synthesis of 2,3-dioxopyrrolidine-3-carboxylate. | |
In 2012, Kumar et al. used SDS promoted cellulose sulfuric acid, a recyclable solid-supported acid catalyst in water for easy, eco-friendly one-pot synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one 672 and amidoalkylnaphthols.451 This multicomponenet reaction between aldehydes 189, urea/thiourea/amides 631 with 1/2-naphthol 395 using SDS promoted cellulose sulfuric acid as catalyst allowed for the synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one 672 and amidoalkyl naphthols in water (Scheme 238). A series of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one 672 and amidoalkyl naphthol derivatives were prepared with various aromatic aldehydes 189 containing either electron-withdrawing and electron-donating substituents, urea/thiourea/amide 631 and naphthol 395 (α and β) under the optimized reaction conditions and all of them gave good to excellent yields 80–92%.
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| | Scheme 238 CellSA catalyzed synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one derivatives in water. | |
Similarly R. Eligeti et al. used Brønsted acidic ionic liquid [Hmim]BF4 for the one-pot green synthesis of isoxazolyl-1,3-benzoxazines in short reaction time (30 min).452
He et al. synthesized 6H-pyrido[2′,3′:5,6][1,3]oxazino[3,4-a]indole derivatives 675 in two steps. First step was the synthesis of fused 1,3-oxazines 674. Oxazines 674 were obtained in 24–84% yields by the cyclocondensation of indoline 673 with α-oxoesters, such as ethyl glyoxalate, ethyl pyruvate, and diethyl oxomalonate in the presence of MsOH (0.1 equiv.) as Brønsted acid in THF at 50 °C for 2 h, or using p-TsOH (0.1 equiv.) in toluene at 80 °C for 1.5 h. In presence of DDQ (1.1 equiv.) in toluene at 80 °C for 4 h 674 underwent oxidative aromatization to afford 675 in 91–98% yields (Scheme 239).453
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| | Scheme 239 Synthesis of 6H-pyrido[2′,3′:5,6][1,3]oxazino[3,4-a]indole derivatives. | |
Zhu group reported a [3 + 2 + 1] cycloaddition reaction for the synthesis of 4,4,5-trisubstituted 1,3-oxazinan-2-ones 677. Formal [3 + 2 + 1] cycloaddition of α-substituted α-isocyanoacetates 540 with phenyl vinyl selenones 676 and water in the presence of a catalytic amount of base (DBU or Et3N, 0.05–0.1 equiv.) followed by addition of p-toluenesulfonic acid (PTSA, 0.1–0.2 equiv.) afforded the products 677 in good to excellent yields (>65%).454 This operationally simple transformation created four chemical bonds and proceeded by Brønsted base catalyzed Michael addition of α-isocyanoacetates to phenyl vinyl selenones to form 678 followed by Brønsted acid catalyzed domino oxidative cyclization sequence that converted the isocyano group to the carbamate function. The phenylselenonyl group has triple role and served as an activator for the Michael addition, a leaving group and a latent oxidant in this integrated reaction sequence. PPTS (pyridinium ptoluenesulfonate) can also able to catalyze the domino oxidative cyclization of Michael adduct 678 to 677 as efficiently as PTSA (p-toluenesulfonic acid). Because PTSA is a strong acid (pKa = 4.8 in H2O) capable of protonating most of the Brønsted bases leading to the corresponding ammonium p-toluenesulfonate similar to PPTS. The reaction does not depend on the electronic effect of the aromatic ring, and heteroarene (furan) is well tolerated under this reaction condition. Also, α-isocyanoacetates bearing arenes/heteroarenes with different electronic properties (electron-rich and -poor) all participated in the reaction to give the corresponding 4,4-disubstituted 1,3-oxazinan-2-ones 677 (Scheme 240).
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| | Scheme 240 Integrated one-pot synthesis of 1,3-oxazinan-2-ones. | |
The proposed rationalization for the formation of products is depicted in Scheme 241. The reaction preceded by the acid-catalyzed hydration of –NC of 678 to give N-alkylformimidic acid 680 by acid-catalyzed hydration and which is readily isomerized to 681. Intramolecular nucleophilic displacement of the phenylselenonyl group by amide oxygen455 afforded 5,6-dihydro-4H-1,3-oxazine 682 with concurrent release of benzeneseleninic acid. In another way the formation of 682 can also be explained by intermolecular displacement of phenylselenonyl group by water followed by intramolecular insertion of the isocyano group to the OH bond. Reaction of 682 with water under acidic conditions produce 683 which could be oxidized by BSA to 677 via intermediate 684.456 The intermediate 682 can also be converted to the intermediate 685 by trap of water by methanol. Oxidation of the secondary amine to imine by benzeneseleninic anhydride provided 2-methoxy-5,6-dihydro-4H-1,3-oxazine (679).457
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| | Scheme 241 Possible reaction pathway of one-pot synthesis of 1,3-oxazinan-2-one. | |
Brønsted acid catalyzed intramolecular aza-Michael reaction can be used for the robust stereodivergent synthesis of substituted morpholines (Scheme 242).461 It was found that Pd(II) complexes were also effective to promote the reaction by hydrolysis to release proton as the active catalyst. Zhong et al. showed that the acidity of Brønsted acids determined the influence of diastereoselectivity in the synthesis of 3,5-disubstituted morpholines 687. Higher acidic Brønsted acid (TfOH) yielded the trans isomer, while relatively lower acidic (TFA) provided the cis isomer (Table 18). Various 2,5- and 2,3-di-substituted morpholines 687 can be synthesized by using the same acids but the selectivities were opposite. This catalysts was also used for the synthesis of 2,3,5-tri-substituted morpholines 687 with excellent diastereoselectivity (Table 19).
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| | Scheme 242 Selective accesses to both diastereomers. | |
Table 18 Brønsted acids in aza-Michael reaction
Table 19 Diastereoselectivity comparision between Pd(MeCN)2Cl2, TFA-and TfOH-catalysis
3.3. Heterocycles with nitrogen and sulphur atom
3.3.1. Five-membered thiazole. Thiazoles are important structural moieties found in many natural products (largazole, bacillamide A–C, telomestatin, neobacillamide A, urukthapelstatin A, hexamollamide, venturamide A and B, bisebromoamide, aerucyclamides A–D, hoiamide A etc.) and compounds of current biological and material interest as well as versatile building blocks in organic synthesis especially in diastereoselective reactions as chiral auxiliaries, and in organic functional materials such as fluorescent dyes and liquid crystals.438–441 Among the numerous reactions devoted to the construction of this aromatic heterocycles, the most often relied upon synthetic method is the Hantzsch thiazole synthesis. This protocol makes the use of α-halo ketones and thioamides as the substrates.438
3.3.1.1. Brønsted acid as catalyst. Zhang et al. reported p-TsOH·3H2O-catalyzed cyclization of trisubstituted propargylic alcohols 416 with thioamides 688 which did not require air- and moisture-free conditions462 (Scheme 243). The 2,4-di- and tri-substituted thiazole products 689 were obtained as a single regioisomer in moderate to excellent yields. The acid-mediated formations of thiazoles are depicted in Scheme 244. At first Brønsted acid-mediated dehydration of 416 generates the alkynyl cation species B and its allenic resonance form C. The cationic species then reacts with thioamide to give the intermediate D which then cyclize to afford the thiazoles 689 via D′.
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| | Scheme 243 p-TsOH·H2O-catalyzed formation of 2,4-di and 2,4,5-trisubstituted thiazoles from propargylic alcohols and thioamides. | |
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| | Scheme 244 Tentative mechanism for p-TsOH·H2O-catalyzed formation of 2,4-di and 2,4,5-tri-substituted thiazoles. | |
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| | Scheme 245 Synthesis of 2-aminobenzothiazole. | |
The probable reaction pathway as given by the authors (Scheme 246) is promoted by protonation. Cyclohexanone 517 first enolizes into I which can then undergo α-iodination to generate II. Thiourea 631 tautomerize to III and α-sulfur substituted cyclohexanone IV was formed as a result of nucleophilic substitution reaction of III with II. Intramolecular nucleophilic attack of nitrogen atom of the imine group of intermediate IV on the carbonyl provide V. Dehydration of V gives intermediate VI, which on dehydrogenation by iodine gives the final product 2-aminobenzothiazole 690.
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| | Scheme 246 Probable pathway for the formation of 2-aminobenzothiazole. | |
3.3.2. Six-membered thiazines. Thiazine nucleus is important because the compounds possessing thiazine nucleus exhibit wide pharmacological activities, such as calcium channel blockers, phosphodiesterase 7 inhibitors, 5-HT3 antagonists, and antipsychotics agents. Thiazine is found in many natural products, such as thiaplakortones A–D, antimalarial thiazine alkaloids.464–473One-pot, three-component reaction of aromatic aldehydes 189, isocyanide 581 and substituted o-aminothiophenols 363 using p-toluenesulfonic acid as a catalyst afforded 3-aryl-4H-benzo[1,4]thiazin-2-ylamines 691 in 87–91% yields (Scheme 247).474
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| | Scheme 247 Synthesis of 3-aryl-4H-benzo[1,4]thiazin-2-ylamines derivatives. | |
In 2013, a mixture of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) (20 mol%) and H2SO4 (10 mol%) has been used as a catalyst system for a cascade addition/cyclization of 2-alkynylaniline 266 and carbon disulfide 692 at room temperature for the synthesis of variety of benzo[d][1,3]thiazine-2(4H)-thiones 693 in high yields 26–86% with high regio- and stereoselectivity (Scheme 248).475 Both electron-donating groups (such as methyl and methoxy groups) and electron-withdrawing groups (chloro and fluoro groups) give good yields in the reaction condition. However, 2-alkynylaniline 266 having strong withdrawing groups like CO2Et and CN present on the para-position failed to give any cyclized product. Substrates having electron-donating and electron-withdrawing groups at the aryl group attached to the triple bond and various substituents at the end of alkynes proceeds smoothly in the dual functionalized catalyst system. The reaction proceeds through the activation of CS2 by DBUH+ (formed from DBU and H2SO4). Nucleophilic nitrogen of 2-alkynylaniline 266 attack the carbon atom of CS2 to form intermediate A and release DBUH+. Intermediate A is converted to intermediate B by proton transfer and complexation of carbon–carbon triple bond. Intermediate B on nucleophilic addition converted to benzo[d][1,3]thiazine-2(4H)-thiones 693 (Scheme 249).
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| | Scheme 248 Reaction between various 2-alkynylbenzenamine. | |
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| | Scheme 249 Proposed route for DBUH+-catalyzed reaction. | |
3.3.3. Seven membered thiazepine. Thiazepine moieties are found in a number of pharmaceuticals and biologically active compounds, such as diltiazem, temocapril, discorhabdin-Q, CGP 37157, omapatrilat, and tianeptine, with a wide range of medicinal properties.476–479 Natural products, such as rubiothiazepine alkaloid, exhibiting cytotoxic and anti-HIV activity also contain thiazepine motif.480Chen et al. used p-toluenesulfonic acid as a catalyst for the three-component reaction of isatins 219, 1-phenyl/methyl-3-methyl-5-amino-pyrazoles 694, and mercaptoacetic acids 695 to afford the spiro-thiazepinone-oxindoles 696 in 71–89% yields (Scheme 250).481 A probable pathway for this reaction was proposed where Baylis–Hillman type adduct 697 was formed by the nucleophilic addition of pyrazoles to isatin and which undergoes protonation of the hydroxyl group followed by dehydration to produced another intermediate 698. The Michael addition of mercaptoacetic acids to intermediate 698 formed another intermediate 699. Cyclization of intermediate 699 leads to the spiro-fused heterocycle 700 and finally dehydration gave the final product 696 (Scheme 251).
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| | Scheme 250 Synthesis of spiro-thiazepinone-oxindoles. | |
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| | Scheme 251 Proposed mechanism for formation of spiro-thiazepinone-oxindoles. | |
4. Heterocycles with three hetero-atoms
4.1. Triazole
1,2,3-Triazoles synthesis became important in modern heterocyclic synthesis because of their wide range of uses in biological science, material chemistry, medicinal chemistry, and agrochemicals.482–485 Among various methods most attractive ways to prepare 1,2,3-triazole moieties are thermal 1,3-dipolar cycloaddition of azides with alkynes.486,487
Sulfamic acid (SA) was successfully utilized by Kidwai and co-workers as a green, cost-effective and reusable catalyst for one-pot, three-component condensation of urazole 701, aldehydes 189 and cyclic β-diketones 419 in water for the synthesis of triazole[1,2-a]indazoletrione derivatives 702 in high yields 62–92%.488 The methodology is useful because it is cost effective and the protocol uses catalyst that is easily available, cheap and recyclable (Scheme 252). Aryl aldehydes with both electron-donating and electron-withdrawing groups are equally efficient for the synthesis of the corresponding triazolo[1,2-a]indazolone derivatives 702 in good yields. But, expectedly the aliphatic aldehydes reacted slowly as compared to the aryl aldehydes and gave lower yields of the products. Ketones like acetophenone, cyclohexanone etc. in place of aldehyde as the carbonyl source are ineffective and the reaction did not proceed under the reaction condition.
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| | Scheme 252 Synthesis of library of triazole[1,2-a]indazole-trione derivatives. | |
Schmidt and co-workers used 1.5 equiv. of acetic acid at mild condition at 50–70 °C for one-pot reaction between 2-hydrazinopyridines 703 and ethyl imidates 704 to rapidly synthesizing of [1,2,4]triazolo[4,3-a]pyridines 707 (Scheme 253).489 Various 2-hydrazinopyridines 703 were efficiently cyclized with various ethyl imidates 704 containing aryl or alyl substituents in good yields 72–92%. Highly electron deficient 2-hydrazinopyridines 703 gave rearranged product [1,2,4]triazolo[1,5-a]pyridines 707 and the reaction was found to be strongly dependent on the steric and electronic properties of the hydrazine and imidates.
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| | Scheme 253 One-pot triazolopyridine synthesis from hydrazinopyridines and ethylimidates. | |
N. Mangarao et al. developed an inexpensive and environmentally benign methodology by using PEG as a solvent and employing PTSA as the catalyst for the facile and highly efficient synthesis of 3,4,5-trisubstituted 1,2,4-triazoles 710 and 3,5-disubstituted 1,2,4-oxadiazoles 712 from 2,2,2-trichloroethyl imidates 709 and 708 or 711 (Scheme 254).490 40 mol% of PTSA is sufficient to carry out this reaction smoothly. The rate of the reaction increases on increasing temperature but the product yields are not satisfactory. This reaction tolerates a wide substrate scope of amidrazone derivatives 708 or 711 with either electron-donating or electron-withdrawing substituents on the aryl residue and 2,2,2-trichloroethyl imidates 709 with aryl or alkyl substituents.
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| | Scheme 254 Synthesis of various 3,4,5-trisubstituted 1,2,4-triazoles and 3,5-disubstituted 1,2,4-oxadiazoles. | |
4.2. Azoles derivatives
Azoles are of paramount importance in medicinal chemistry and pharmaceutical industry in numerous ways and find wide applications in agricultural and material research.491 Recently Chowdhury et al. reported the synthesis of various potentially bioactive azoles.492 o-Iodoxybenzoic acid (IBX) has been used for the oxidative desulfurization to trigger domino reactions of readily accessible thiosemicarbazides 713, bis-diarylthiourea 714, 1,3-disubstituted thiourea 715, and thioamides 716 leading to new methodologies for the synthesis of highly substituted oxadiazoles 717, thiadiazoles 718, triazoles 719, and tetrazoles 720 in good to excellent yields (Scheme 255).
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| | Scheme 255 IBX-mediated synthesis of various azoles. | |
5. Miscellaneous
5.1. Brønsted acid as catalyst
Total synthesis of desbromoarborescidines A and C has been illustrated by the application of Brønsted acid-catalyzed highly stereoselective arene–ynamide cyclization by Y. Zhang and co-workers (Scheme 256).493 The Brønsted acid-catalyzed cyclization reaction constitutes a keteniminium variant of the Pictet–Spengler cyclization, leading to efficient synthesis of nitrogen heterocycles and related alkaloids (Scheme 257). Only 1 mol% HNTf2 is sufficient for this reaction and dihydroamino-naphthalene can be isolated in 91% yield after stirring in CH2Cl2 at room temperature for 3 min. A series of cyclized products were obtained in good yields from their respective C-tethered arene–ynamides but with an additional methylene unit present in the substrate present required 5 mol% HNTf2 for effective cyclization. For indole-tethered ynamides 721 and 724, PNBSA [p-nitrobenzenesulfonic acid] 20 mol% was found to be suitable Brønsted acid, while HNTf2 was found to be ineffective. This is most probably due to contending protonation of the indole ring when a much stronger Brønsted acid such as HNTf2 is used. But for protonations of N-acyl ynamides a stronger Brønsted acid is necessary as the substrate are much less reactive than N-sulfonyl ynamides given that the nitrogen lone pair is more delocalized into the acyl carbonyl. Ynamide with a nosyl [Ns] group substitution worked equally well in the presence of PNBSA.
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| | Scheme 256 Synthesis of 10-desbromoardorescidine A and 11-desbromoadorescidine C. | |
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| | Scheme 257 Keteniminium Pictet–Spengler cyclization. | |
Brønsted acid promoted transannular alkylation of an enol with an unactivated alkene has been used by Clarke et al. They achieved the synthesis of bicyclo[4.3.0]nonane ring system 732 of the pinguisane-type sesquiterpenoid natural products, including the vicinal quaternary stereocentres, as a single enantiomer.494 Lactone 732 (Scheme 258), can be prepared in 71% yield by HF deprotection of the silyl ether and removal of the tert-butyl ester and in situ lactonisation of 731 with TFA.
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| | Scheme 258 Acid-catalyzed lactone synthesis. | |
Padwa and group developed495 a general and efficient strategy for synthesis of the seven-membered ring skeleton found in selaginoidine. Various substituted bicyclic lactams were obtained by TFA-induced Pictet–Spengler reaction of tetrahydroindolinones bearing tethered heteroaromatic ring. The cyclization depended on the position of the furan tether, tether length, nature of the tethered heteroaromatic ring, and also on substituent group present on the 5-position of the tethered heteroaryl group. Treatment of 733 with trifluoroacetic acid produced the tetracyclic-substituted lactam 735 in 78% yields via the formation of a single lactam diastereomer 734. This is perhaps the result of stereoelectronic preference for axial attack by the furan ring on the N-acyliminium ion 733 from the least hindered side 734 (Scheme 259).496,497 Tetrahydroindolinone 738 containing a methyl group at the 5-position of the furan ring could be formed in 62% yield and the reaction of 738 with TFA at 25 °C afforded the bicyclic lactam 739 in 95% yield. Here the cyclization occurred at 3-position of the furan ring as a consequence of the presence of the methyl group, which blocks the dimerization pathway. Treatment of the compound 741 with trifluoroacetic acid afforded the bicyclic lactam 742 in 96% yield (Scheme 260). Similarly, (3-furan-2-yl)ethyl-substituted tetrahydroindolinone 743 also underwent TFA-induced cyclization and afforded the corresponding tetracyclic-substituted lactam 744 in 98% yield (Scheme 260). Thiophen-3-yl-substituted system in the presence of trifluoroacetic acid at 25 °C for 4 h afforded the closely related tetracyclic lactam in 73% yield.
 |
| | Scheme 259 Mechanism of the acid-promoted cyclization. | |
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| | Scheme 260 Acid-promoted cyclization of various heteroaromatic systems. | |
Youn et al.498 reported TFA-mediated reaction of 2-arylanilines 745 with arylaldehydes 189 to afford 6-substituted phenanthridines 746. Notably this process (Scheme 261) can tolerate various functional groups such as methoxy, bromo, nitro, furyl, and thienyl groups. All 6-arylsubstituted phenanthridines 746 prepared from this facile reaction were highly fluorescent and will have important applications in material science and as DNA-intercalating agent. While limitation of this process includes the requirement of harsh reaction conditions (in TFA at 120–140 °C for 1.5–7 days) and a specific substrate class (such as arylaldehydes), the operational simplicity and use of simple reactants without requirement of synthesis of complicated precursors, strictly anhydrous conditions, air-sensitive organometallic reagents, or expensive metal catalysts make this particularly attractive for library construction of fluorescent molecules.
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| | Scheme 261 Synthesis of 6-substituted phenanthridines. | |
Tanner et al. synthesized framework of the cylindricine and lepadiformine alkaloids.499 Tricyclic pyrroloquinoline derivative 749 was obtained in single operation via a transannular Mannich reaction involving a macrocyclic diketoamine 748. Macrocyclic diketoamine 748 under acidic conditions, typically with TFA, followed by basic work up gives the desired tricycle 749 in 50–55% yields (Scheme 262).
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| | Scheme 262 Preparation of tricycle 749. | |
Martin and co-workers achieved a TFA-mediated condensation between the substituted allylamine 750 and monoprotected dialdehyde 751 via an iminium ion cascade reaction and subsequent trapping with cyanide for the synthesis of aza-fused bicycles 752 (Scheme 263).500 This methodology can be used for the synthesis of various indolizidine and quinolizidine based natural products. The cyclization of the intermediate imine proceeded at −40 °C in acetonitrile in presence of TFA.
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| | Scheme 263 Synthesis of aza-fused bicycles. | |
For drug discovery novel pyrimidine-fused tetracyclic, structurally diversified compounds 754 were designed and synthesized by Yang and co-workers using intramolecular inverse electron demand hetero-Diels–Alder reaction of an imine or iminium ion formed in situ from N-allylaminopyrimidinealdehyde 753 with suitable primary or secondary aryl amines 101 (Scheme 264).501 The reactions yielded exclusively cis-configured products in good to excellent 49–98% yields in the presence of 2 equiv. TFA in CH3CN/H2O (1:1) at 25 °C. Various substituted aromatic amines 101 reacted with pyrimidines 753 to give the desired products 754 in good to excellent yields. Under the reaction condition aromatic amines containing electron-withdrawing groups reacted faster than those with electron-donating groups, which hints that the cycloaddition reactions are in the category of inverse electron demand Diels–Alder reactions. Anilines with an ortho substituent, reacted very sluggishly or giving no desired product. When R1 is H, the reaction stopped at the stage of imine intermediate and When R1 is phenyl, no desired product was formed.
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| | Scheme 264 Synthesis of pyrimidine-fused tetracyclic heterocycle. | |
Another acid-mediated convenient synthesis of N,N-disubstituted 4H-3,1-benzothiazin-2-amines 757 from aryl(2-isothiocyanatophenyl)methanones 755 using a two-pot procedure has been achieved by Kobayashi group.502 Treatment of isothiocyanato ketones with secondary amines gave the corresponding keto thiourea, that were allowed to react in one pot with sodium borohydride or methylmagnesium bromide to afford 1,1-dialkyl-3-{2-[aryl (hydroxy)methyl]phenyl}thioureas or 1,1-dialkyl-3-[2-(1-aryl-1-hydroxyethyl)phenyl]thioureas, respectively. Hydrobromic acid-mediated cyclization of these hydroxy thiourea precursors 756 provided the desired 4H-3,1-benzothiazin-2-amines 757 (Scheme 265).
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| | Scheme 265 Synthesis of N,N-disubstituted 4H-3,1-benzothiazin-2-amines. | |
Balamurugan and co-workers used four-component domino reaction of isatin 219, phenylhydrazine, 3-aminocrotononitrile 758, and cyclic β-diketones/amide/thioamide 759 in the presence of (±)-camphor-10-sulfonic acid in water at 100 °C (2–3 h) for efficient synthesis of spiro-fused 2-oxindoles 760 (Scheme 266).503
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| | Scheme 266 Synthesis of spiro-fused 2-oxindoles. | |
Dibenzo[b,f][1,5]diazocines were synthesized via a novel, efficient acid-catalyzed reaction of 2-acylbenzoisocyanate 761 at room temperature by N. Zhao and co-workers.504 TFA (0.1 equiv.) was found as the best catalyst and THF or MeCN as best solvent for this reaction to give the best yield of diazocine derivatives 762 (Scheme 267). The phenyl rings bearing either electron-donating or electron-withdrawing groups and heterocyclic substituent, such as thiophene ring also gave a high yield 65–92% of the diazocine derivatives. Sterically hindered tert-butyl substituted diazocine 762 was also obtained in 85% yield. Heterocycles bearing a ketone and an isocyanate moiety at adjacent positions also afforded the corresponding diazocines derivative 762 under harsher condition.
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| | Scheme 267 TFA-catalyzed diazocine synthesis. | |
Poly-functionalized two-carbon-tethered fused acridine/indole pairs 763 can be synthesized via Brønsted acid-promoted domino reactions between indoline-2,3-dione 219 and C2-tethered indol-3-yl enaminones 99.505 HOAc is most efficient at 110 °C under microwave irradiation in a sealed vessel for 20 min to give 84% yield of 763 (Scheme 268). The reaction is also useful for the synthesis of C-tethered fused acridine/pyridine pairs, N-substituted amino acids, N-cyclopropyl and N-aryl substituted fused acridine derivatives, as well as bis-furan-3-yl-substituted indoles. The reaction proceeded through the domino construction of a fused acridine skeleton with concomitant formation of two new rings in a one-pot operation.
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| | Scheme 268 Synthesis of two-carbon-tethered fused acridine/indole pairs. | |
2-Amino-1,3,4-thiadiazoles 766 have been efficiently synthesized utilizing TMSNCS 765 and acid hydrazides 764 as starting materials. The method involves in situ generation of thiosemicarbazides, which undergo H2SO4-catalyzed cyclodehydration to give 2-amino-5-aryl 1,3,4-thiadiazoles 766 (Scheme 269).506
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| | Scheme 269 Synthesis of 2-amino-1,3,4-thiadiazoles. | |
An acid promoted novel cascade cyclization was reported by Yokosaka et al.507 Use of 8 equiv. of trifluoro acetic acid or a catalytic amount of Lewis acid as the promoter afforded structurally diverse polycyclic cyclopenta[b]indoles 768 in moderate to excellent yields 42–99% (Scheme 270). 5-Membered ring-fused tetracyclic cyclopenta[b]indoles as well as 6-membered ring-fused pentacyclic indole derivatives were obtained in 42–83% yields. Oxygen-tethered substrate was also converted into the corresponding pentacyclic indole derivative containing a chroman moiety. 15 equiv. of TFA was required for a smooth reaction of substrate having a (CH2)2 unit rather than CH2 and afforded the corresponding 7-membered ring in excellent yield with good diastereoselectivity. This cascade process was extremely effectual for the synthesis of 8-membered ring-fused cyclopenta[b]indole derivatives 768 and also provided access to polycyclic molecular frameworks and scaffolds for drug design. Substrates bearing various electron-donating and electron-withdrawing functionalities and various protecting groups, such as p-toluenesulfonyl (Ts), Boc, benzyloxycarbonyl (Cbz), allyloxycarbonyl (Alloc) and benzoyl (Bz), on the nitrogen atom in the indole unit were tolerated under this reaction condition and the corresponding products 768 were obtained in good to high yields.
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| | Scheme 270 Synthesis of various polycyclic cyclopenta[b]indoles. | |
In 2014, Theunissen and co-workers synthesized polycyclic nitrogen heterocycles 770 possessing up to three contiguous stereocenters and seven fused cycles by efficient keteniminium-initiated cationic polycyclization. The clean polycyclization of N-benzyl-ynamides 769 required only TfOH/CH2Cl2 (0.8:1) at 0 °C for 5 min or 20 mol% of bistriflimide in CH2Cl2 at room temperature for 20 h as a promoters for this reaction (Scheme 271).508 The cyclization gives exclusively cis-isomers with full diastereoselectivity under both stoichiometric and catalytic conditions. A variety of tolyl groups on the starting ynamide including deactivated ones were suitable for this polycyclization. The nature of the aromatic ring on the benzyl moiety in 769, which act as terminal nucleophile showed strong impact on the polycyclization. Desired polysubstituted indenotetrahydroisoquinolines 770 were readily obtained from N-benzyl-ynamides containing electron-donating groups at the meta position but electron-donating substituents at para position resulted in a more substrate-dependent cyclization. The reaction was inefficient with a more donating methoxy group but successful with a methyl substituent. Electron-withdrawing groups on the starting ynamide were also equally efficient.
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| | Scheme 271 Scope of the polycyclization of N-benzyl-ynamides. | |
Replacement of the arene by a suitably functionalized alkene as the terminal nucleophile (ynamides 771) in the polycyclization also provided desired tricyclic indenotetrahydropyridines 770. Structurally simple “flat” starting materials bisynamides 772 and 774 gave double cyclization to provide heptacyclic nitrogen heterocycles 773 and 775 (Schemes 272 and 273).
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| | Scheme 272 Polycyclization of N-allyl-ynamides. | |
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| | Scheme 273 Synthesis of heptacyclic nitrogen heterocycles. | |
The reaction is initiated by protonation of the electron-rich alkyne of the starting ynamide I giving a highly reactive N-tosyl- or N-acyl-keteniminium ion II. A [1,5]sigmatropic hydrogen shift of intermediate II generates conjugated iminium III on which is in resonance formed the bis-allylic carbocationic form IV. 4π conrotatory electrocyclization of carbocationic form IV gave V (in the manner of the Nazarov reaction). A second cyclization between this intermediate benzylic carbocation V and the arene/alkene subunit would finally account for the formation of polycycle VI as well as its cis ring junction (Scheme 274). The use of a strong acid promoter such as TfOH or Tf2NH is essential to avoid trapping the intermediate keteniminium ion by the conjugated base, i.e., the weakly nucleophilic anions TfO− or Tf2N−.
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| | Scheme 274 Proposed mechanism for the cationic polycyclization. | |
Yokosaka et al. reported the synthesis of six, seven and eight membered ring fused indole derivatives 777 with a 3-aminomethyl indole motif in 31–99% yield through a cascade reaction involving ipso-Friedel–Crafts alkylation of phenols 776, rearomatization of the spirocyclohexadienone unit and ipso-Pictet–Spengler reaction. TFA was used as an acid promoter and both electron donating and withdrawing group on the aromatic ring of indole derivatives gave the desired heterocycles 777 in high yields (Scheme 275).509
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| | Scheme 275 Cascade reaction for synthesis of fused indole derivatives. | |
Recently, Schuster et al. disclosed efficient synthetic routes to murrayamine E in which one of the key step is CSA-catalyzed isomerization of 778 to 779. 779 Could be converted to murrayamine E in 2 steps in 54% yield (Scheme 276).510
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| | Scheme 276 Syntheses of murrayamine E. | |
5.2. Brønsted acid as co-catalyst
Chan and co-workers reported two methods to prepare benzo[b]oxepin-3(2H)-ones 781 and (Z)-1,2-dihydro-1-tosylbenzo[b]azepin-3-ones 782 by the application of gold-mediated heterocyclization/Petasise Ferrier rearrangement Brønsted acid-assisted debenzoxylation starting either from 2-(prop-2-ynyloxy)benzaldehydes or 2-(N-(prop-2-ynyl)-N-tosylamino)benzaldehydes 780 (Scheme 277).511 The p-TsOH·H2O (20 mol%) catalyzed reactions are fast, mild and operationally straightforward for a wide variety of aldehyde substrates containing electron-withdrawing, electron-donating, and sterically demanding functional groups and afforded the corresponding heterocycles in moderate to excellent yields.
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| | Scheme 277 Brønsted acid-assisted synthesis of nitrogen and oxygen heterocycle. | |
Li et al. synthesized tryptamine-fused polycyclic privileged structures 785 by the treatment of substituted tryptamines 783 and 2-ethynylbenzoic acids or 2-ethynylphenylacetic acids 784 using gold(I)/TFA-catalyzed one-pot cascade reaction (Scheme 278).512 Formation of one C–C bond and two C–N bonds with high yields and broad substrate tolerance are the key features of this reaction. Reaction of 783 with 1.2 equiv. of 784 in toluene in the presence of 5 mol% of Au[P(t-Bu)2(o-biphenyl)][CH3CN]SbF6 and 20 mol% of TFA in a sealed tube at 110 °C for 6 h gave the best result. Substrate scope of this cascade reaction was investigated under the optimized reaction condition. The reaction was successful with various substituted tryptamines 783 and 2-ethynylbenzoic acids 784 to give the desired products 785 in good to excellent yields 75–91%. Electronic nature of substituents on the tryptamines moiety influences the yield. Tryptamines with substituted methyl- or methoxyl-groups gave excellent yields but, introducing chloro- or fluoro-substitution resulted in decreased yields. Various substituents in 2-ethynylbenzoic acid 784 had no significant influence on the yield. In addition, replacement of 2-ethynylbenzoic acid 784 was 2-ethynylphenylacetic acid, correspondingly, gave six-membered ring products 785 in good yields 66–86%. Electron-donating substituents on the substrates 783 were more beneficial to the yield than that of the substrates with electron-withdrawing substituents.
 |
| | Scheme 278 Synthesis of tryptamine-fused polycycle. | |
Under the reaction condition Au catalyst first activated the alkyne terminal of 784, which then underwent an intramolecular cyclization with the carboxylic acid end to form A, followed by regeneration of the Au catalyst. Primary amino group of tryptamine 783 attract the enol lactone A to form the ammonolysis product B. Subsequently, Brønsted acid (TFA) catalyzed N-acyliminium ion formation gives C. Finally, the electron-rich carbon atom on indole attacked the iminium ion through a nucleophilic process to give the target molecule 785 (Scheme 279).
 |
| | Scheme 279 Probable reaction mechanism. | |
6. Recent updates
Among recent literature, C. Huo et al. reported a green and efficient multi-component procedure for the synthesis of highly functionalized pyrrolidones 788 using dilute sulfuric acid as the catalyst from easily available anilines 104, α-oxoaldehydes 786, and α-angelicalactone 787 as starting materials (Scheme 280).513 Pyrrolidone derivatives 788 were obtained in good to high yields 28–71% at room temperature and under solvent-free conditions and also useful for multigram scale with the same efficiency.
 |
| | Scheme 280 Synthesis of highly functionalized pyrrolidones. | |
A. V. Vasilyev and co-workers used acidic zeolite CBV-720 in benzene at 130 °C for intramolecular cyclization of N-aryl amides of 3-arylpropynoic acid 311 and efficiently synthesized 4-aryl quinolin-2(1H)-ones 312 in 1–98% yield (Scheme 281).514
 |
| | Scheme 281 Synthesis of 4-aryl quinolin-2(1H)-ones. | |
Recently, B. Saoudi et al. reported synthesis of interesting potentially biologically active 2,3-disubstituted thieno[2,3-b]quinoxalines 790 by using H2SO4 or HCl via the condensation of 3-methylquinoxalines 789 and aldehyde 189 in EtOH in high 48–91% yield (Scheme 282).515
 |
| | Scheme 282 Synthesis of 2,3-disubstituted thieno[2,3-b]quinoxalines. | |
S. J. Gharpure and co-workers showed that γ-hydroxy lactam 791 on treatment with H2SO4 in acetone gave the N-fused indoline derivative 792 as the sole product in 52% yield. Interestingly, treatment with formic acid gives pyrrolo[1,2-a]indole derivative 793 in 88% yield (Scheme 283).516
 |
| | Scheme 283 Synthesis of N-fused indoline derivative. | |
Y. Yamaoka et al. developed a Brønsted acid-promoted arene–ynamide cyclization strategy for the synthesis of 3H-pyrrolo[2,3-c]quinolines 795 (Scheme 284).517 This TfOH (1.2 equiv.) promoted reaction of 794 consists of the generation of a highly reactive keteniminium intermediate from arene–ynamide activated TfOH and electrophilic aromatic substitution reaction to give arene-fused quinolines 795 in high 52–92% yields. The developed Brønsted acid-promoted methodology enabled facile access to marinoquinolines A and C and aplidiopsamine A.
 |
| | Scheme 284 Synthesis of arene-fused quinolines. | |
Very recently, a variety of biologically relevant phenanthridines and quinolines 798 or 800 have been synthesized by Brønsted acid and photocatalyst under visible light at room temperature with satisfactory yields (Scheme 285).518 This one-pot synthesis of phenanthridines and quinolines 798 or 800 proceeds through the formation of O-acyl oximes by using p-Cl-benzenesulfonic acid (CBSA) as Brønsted acid and commercially available or easily prepared aldehydes 796 or 799 and O-(4-cyanobenzoyl)hydroxylamine 797 as the nitrogen source starting materials. O-Acyl oximes on visible light photoredox catalyzed cyclization via iminyl radicals produced aza-arenes.
 |
| | Scheme 285 One-pot synthesis of phenanthridines and quinolines. | |
7. Conclusion
In conclusion, we have discussed spectacular advancement of the synthesis of heterocycles that involve Brønsted acid-catalyzed reactions as the key step. This review shows that the application of easily available Brønsted-acid as catalysts for the synthesis of common heterocycles is an active area of research. It is also emphasized that, in addition to the conventional metal catalysts, chiral phosphoric acids and chiral Brønsted-acid are also becoming a powerful tool for the synthesis of heterocycles. Chiral Brønsted-acid catalysis is restricted to academic synthesis but, industrial applications are fastly growing due to economic and environmental benefits. Despite remarkable progress of the transition metal-catalyzed and chiral Brønsted-acid catalyzed chemistry of heterocycles there is still a high demand for common and sustainable, methodologies for the synthesis of these important molecules by efficient formation of carbon–carbon and carbon–heteroatom bonds. In addition, development of techniques that use low-cost acids such as common Brønsted-acid in heterocyclic synthesis is very interesting in terms of economic and environmental factors. Common acids are used to increase efficiency of traditional non-catalyzed and metal-catalyzed transformations for the synthesis of heterocycles, such as addition of nucleophilic heteroatoms to multiple bonds, reaction of nucleophilic heteroatoms with carbonyl compounds, cycloaddition reactions, Michael addition reaction, and other usual and unusual types of transformations. The most important fact of acid catalysis lies in its efficacy, the reaction can be performed at various (lower to higher) catalyst loading and at different temperatures. Moreover, the reaction proceeds under relatively mild condition and can tolerate a wide variety of functional groups. The use of Brønsted acid catalysts are used in various novel transformations, such as cross-coupling reactions, olefin metathesis, insertion of carbenoids into heteroatom–H bond, C–H activation, alkyne–allene rearrangement, and a ring-opening of small cycles and followed by rearrangement. Significant applications were also found in the area of complex cascade cyclo-isomerization reactions, for the regiodivergent synthesis of diversity oriented heterocyclic molecules. The substrate scopes of acid-catalyzed reactions are vast and mounting at an unique pace. There is no doubt that more and more useful synthetic reactions will be developed in the near future. In fact, lots of efforts and developments have transformed Brønsted acid-catalyzed heterocyclic synthesis into a very broad and exciting field with opportunities for research and development. Definitely, much more remains to be done in this field, and in the next few years. Brønsted-acid catalysts and metal-oriented chiral catalysts will complement each other and make significant contributions to synthetic organic chemistry. We believe that this small effort will provide appropriate background for such developments during last 10 years and encourage synthetic organic chemists to employ these Brønsted acid-catalyzed reactions in heterocyclic and medicinal chemistry.
Abbreviations
| ACN | Acetonitrile |
| AcOH | Acetic acid |
| Ac | Acetyl |
| AMCell | Amorphous milled cellulose sulfonic acid |
| Ar | Aryl |
| BAHC | Brönsted acid hydrotrope combined catalyst |
| BAIL | Brønsted acidic ionic liquid |
| Boc | Di-tert-butyl dicarbonate |
| BINOL | 1,1′-Bi-2-naphthol |
| Bn | Benzyl |
| BSA | Bovine serum albumin |
| CBSA | p-Cl-benzenesulfonic acid |
| CSA | Camphor sulphonic acid |
| CsF | Cesium fluoride |
| Cy | Cyclohexyl |
| DABCO | 1,4-Diazabicyclo[2.2.2]octane |
| DBU | 1,8-Diazabicyclo[5,4,0]undec-7-ene |
| DCE | 1,2-Dichloroethane |
| DCM | Dichloromethane |
| DDQ | 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone |
| DFT | Density functional theory |
| DIB | (Diacetoxyiodo)benzene |
| DIPEA | N,N-Diisopropylethylamine |
| DMF | N,N-Dimethylformamide |
| DNA | Deoxyribonucleic acid |
| Dppf | 1,1′-Bis(diphenylphosphino)ferrocene |
| dr | Diastereomeric ratio |
| EDC | 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide |
| EDG | Electron-donating group |
| ee | Enantiomeric excess |
| eq. | Equivalent |
| Et | Ethyl |
| EWG | Electron-withdrawing group |
| FG | Functional group |
| h | Hour |
| HIV | Human immunodeficiency virus |
| [Hmim]TFA | 1-Methylimidazoluim trifluoroacetate |
| HPA | Heteropoly acid |
| IBX | o-Iodoxybenzoic acid |
| ICTB | Isochromenylium tetrafluoroborate |
| IL | Ionic liquid |
| i-Pr | Iso-propyl |
| i-Bu | Iso-butyl |
| LDA | Lithium diisopropylamide |
| M | Molar |
| MBH | Morita–Baylis–Hillman |
| mCPBA | meta-Chloroperoxybenzoic acid |
| Me | Methyl |
| Mes | Mesithyl |
| MNPs-IL-HSO4 | Magnetic nanoparticle supported hydrogen sulfate |
| MS | Molecular sieve |
| MSA | Molybdate sulfuric acid |
| MsOH | Methanesulfonic acid |
| MW | Microwave |
| NHC | N-Heterocyclic carbene |
| NIS | N-Iodosuccinimide |
| NMR | Nuclear magnetic resonance |
| NPSP | N-Phenylselenophthalimide |
| nPr | n-Propyl |
| Ns | Nitrobenzene-sulfonyl |
| PEG | Poly(ethylene glycol) |
| PG | Protecting group |
| Ph | Phenyl |
| PIFA | Phenyliodinebis(trifluoroacetate) |
| PiVOH | Pivalic acid |
| PNBSA | p-Nitrobenzenesulfonic acid |
| PPA | Polyphosphoric acid |
| PPTS | Pyridinium p-toluenesulfonate |
| PTSA | p-Toluenesulfonic acid |
| RCM | Ring-closing metathesis |
| rt | Room temperature |
| SDS | Sodium dodecyl sulphate |
| SES | Trimethylsilylethanesulfonyl |
| STA | Silicotungstic acid |
| tBu | tert-Butyl |
| TEMPO | 2,2,6,6-Tetramethyl-1-piperidinyloxy |
| TFA | Trifluoroacetic acid |
| TFE | Trifluoroethanol |
| TfOH | Triflic acid |
| THF | Tetrahydrofuran |
| TIPS | Triisopropylsilyl |
| TMS | Trimethylsilyl |
| TsOH | Tosylic acid |
| Ts | p-Toluenesulfonyl |
| US | Ultra sonication |
| v/v | Volume/volume |
Acknowledgements
SP is grateful to CSIR, (New Delhi) for his fellowship and KCM is thankful to UGC, (New Delhi) for Emeritus Fellowship.
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