New route to 1,4-oxazepane and 1,4-diazepane derivatives: synthesis from N-propargylamines

Abstract

N-Propargylamines are one of the most useful and versatile building blocks in organic synthesis that are successfully transformed into many significant N-heterocycles. Due to high atom economy and shorter synthetic routes, the synthesis of 1,4-oxazepane and 1,4-diazepane cores from N-propargylamines have undergone an explosive growth in recent years. This review gives an overview of new developments in the synthesis of the title compounds from N-propargylamines in recent years. Mechanistic aspects of the reactions are considered and discussed in detail.

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Esmail Vessally

Esmail Vessally was born in Sharabiyan, Sarab, Iran, in 1973. He received his B.S. degree in Pure Chemistry from University of Tabriz, Tabriz, Iran, and his M.S. degree in organic chemistry from Tehran University, Tehran, Iran, in 1999 under the supervision of Prof. H. Pirelahi. He completed his Ph.D. degree in 2005 under the supervision of Prof. M. Z. Kassaee. Now he is working at Payame Noor University as Associate Professor. His research interests include Theoretical Organic Chemistry, new methodologies in organic synthesis and spectral studies of organic compounds.

Akram Hosseinian

Akram Hosseinian was born in Ahar, Iran, in 1973. She received her B.S. degree in Pure Chemistry from University of Tehran, Iran, and her M.S. degree in inorganic chemistry from Tarbiat Modares University, Tehran, Iran, in 2000 under the supervision of Prof. A. R. Mahjoub. She completed his Ph.D. degree in 2007 under the supervision of Prof. A. R. Mahjoub. Now she is working at University of Tehran as Assistant Professor. Her research interests include inorganic and organic synthesis, new methodologies in nano material synthesis.

Ladan Edjlali

Ladan Edjlali was born in Tabriz, Iran, in 1960. She received her B.S. degree in Applied Chemistry from University of Tabriz, Iran, and her M.S. degree in organic chemistry from University of Tabriz, Tabriz, Iran, in 1993 under the supervision of Prof. Y. Mirzaei. She completed his Ph.D. degree in 2000 under the supervision of Prof. Y. Mirzaei and Prof. S. M. Golabi. Now she is working at Islamic Azad University, Tabriz Branch as Associate Professor. Her research interests include organic synthesis and new methodologies in organic synthesis.

Ahmadreza Bekhradnia

Ahmadreza Bekhradnia is associated professor at Mazandaran University of Medical Sciences, Iran. His current research interests focus on pharmaceutical intermediates, ranging from synthesis to study of biological and photophysical properties, as well as application of molecular modeling in mechanistic investigation. He received his PhD degree in organic chemistry from Tarbiat Modares University – Tehran, Iran (2005). During his sabbatical period, he worked on the mechanism of transition metal-catalyzed cross-coupling reaction, both from an experimental and computational viewpoint under the supervision of Prof. Per-Ola Norrby in Gothenburg University, Sweden (2012–2013).

Mehdi D. Esrafili

Mehdi D. Esrafili, was born in Shabestar, Iran, in 1981. He received his MS & Ph.D (honor) at Tarbiat Modares University. Then, he joined Kyoto University under the supervision of Professor Keiji Morokuma. Dr Esrafili is currently working as Head of Laboratory and working group on computational chemistry at University of Maragheh, Iran. His researches are centered on the intermolecular interactions and reaction mechanism using electronic structure methods.

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1. Introduction

Heterocyclic compounds are highly important because of their abundance in numerous natural products¹ and synthetic pharmaceuticals.² Due to their diverse biological properties, the synthesis of medium-sized rings using new protocols is always interesting.³ Seven-membered rings with two heteroatoms are known to have various biological activities.⁴ Among them, 1,4-oxazepane and 1,4-diazepanes continue to play a pivotal role in the development of new pharmaceuticals. 1,4-Oxazepane is a seven-membered heterocyclic compound containing oxygen and nitrogen atoms in the 1,4-positions, which is a one-carbon homologue of morpholine. This ring is the base core for a number of natural and biologically active compounds (Fig. 1).^5a–f There are some methods used for the determination of diazepam and other 1,4-benzodiazepines, such as HPLC, capillary electrophoresis, electrochemiluminescence, chemiluminescence, electrochemistry and fluorimetry.^5g Among them, considerable attention has been paid to fluorometry because of its high sensitivity.^5h–k 1,4-Diazepane derivatives are also well-known important biological components. The marketed drugs containing 1,4-diazepane cores are known to have many biological properties such as antidepressant,⁶ antiplatelet,⁷ muscle relaxant,⁸ anticonvulsant⁹ activity and many more (Fig. 2). Despite the wide importance of these cores in the drug design of various pharmacological agents, very few synthetic methods have been reported to date.¹⁰ Thus, there is an urgent need for the development of methods for the efficient synthesis of the title compounds.

Fig. 1 Selected examples of some bioactive 1,4-oxazepanes.

Fig. 2 Chemical structure of some of the marketed drugs containing 1,4-diazepane ring system.

New methods that produce complex molecules from simpler materials in a single operation are important challenges in modern synthetic chemistry. N-Propargylamines are one of the most useful and versatile building blocks in organic synthesis that were successfully transformed into many significant N-heterocycles¹¹ and complex natural products.¹² More recently, we published four review papers that cover the synthesis of pyrrole,¹³ pyridine,¹⁴ pyrazine,¹⁵ and quinoline¹⁶ derivatives from N-propargylamines. Due to high atom economy and shorter synthetic routes, the synthesis of 1,4-oxazepane and 1,4-diazepane cores from N-propargylamines have undergone an explosive growth in recent years. To the best of our knowledge, a comprehensive review has not appeared on synthesis of these compounds from N-propargylamines in literature. This review is an attempt to summarize the data available from the literature about the synthesis of titled compounds from N-propargylamines (Fig. 3). It should be noted that we have not discussed synthesis of 1,4-oxazepane and 1,4-diazepane derivatives from N-propargylamides.

Fig. 3 Synthesis of 1,4-oxazepane and 1,4-diazepane cores from N-propargylamines.

2. 1,4-Oxazepanes

2.1. Highly substituted 1,4-oxazepanes

An interesting approach toward the synthesis of oxazepan derivatives by treatment of p-toluenesulfonyl azide with N-propargylamines in the presence of copper iodide as catalyst and triethylamine as base, was developed by Muthusubramanian et al. Thus, a variety of functionalized 2,4-diaryl-1,4-oxazepan-7-ones 3 were synthesized via the Cu-catalyzed reaction of 1-aryl-2-(aryl(prop-2-ynyl)amino)ethanols 1 and tolylsulfonyl azide 2 in dichloromethane at room temperature (Scheme 1a). The author proposed mechanism for the formation of compounds 3 is depicted in Scheme 1b. The copper acetylide A, formed from 1 and CuI, undergoes a 1,3-dipolar cycloaddition reaction with sulfonyl azide 2 to generate the triazole derivative B. This intermediate can then be converted into the ketenimine derivative C that after intramolecular cyclization and hydrolysis affords desired 1,4-oxazepans 3.¹⁷

Scheme 1 (a) One-pot synthesis of 2,4-diaryl-1,4-oxazepan-7-ones 3 via Cu-catalyzed cascade reaction of N-propargylamines 1 and tolylsulfonyl azide 2. (b) Possible reaction pathway for generation of 3.

Recently, Kamimura and co-workers showed that a series of optically active 1,4-oxazepan-7-ones 5 were formed from chiral N-propargyl-β-amino-α-methylene esters 4 through 7-exo-dig cyclization employing Ph₃PAuCl as catalyst and Cu(OTf)₂ as co-catalyst, in refluxing 1,2-dichloroethane. It is noted that the presence of co-catalyst is vital for this reaction. In the absence of Cu(OTf)₂ no reaction was observed. The reaction tolerated the substrates being both aryl and alkyl groups. The results showed that N-propargylamines with R = aryl groups gave higher yields than those with R = alkyl groups. It was also found that the electronic character of the aryl ring had little effect on the rate of the reaction. However, p-methoxyphenyl substituent, underwent decomposition in this reaction conditions and gave no corresponding cyclic product. The mechanism proposed to explain this reaction starts with the generation of the active intermediate A via coordination of the terminal alkyne unit with Au(I) catalyst, and then the 7-exo-dig cyclization of A furnishes intermediate B. Subsequently, elimination of isobutene or tert-butyl cation from intermediate B gives the intermediate C. Finally, the protonation of C affords the observed products 5 with concomitant regeneration of Au(I) catalyst (Fig. 4).¹⁸

Fig. 4 Mechanism proposed to explain the 1,4-oxazepan-7-one synthesis developed by Kamimura.

More recently, an efficient one-pot synthesis of the 1,4-oxazepane scaffold has been developed by Karunakar et al. they showed that N-propargylic β-enaminones 6 underwent an intramolecular cyclization reaction in the presence of AuCl₃/AgSbF₆ catalytic system in methanol. Under optimized conditions, the reaction is tolerant toward a variety of functional groups such as fluoro, bromo, chloro, cyano, methoxy and nitro on the aryl groups and gave the corresponding unsaturated 1,4-oxazepanes 7 in good yields (Scheme 2). They probed the mechanism of the reaction and found that the reaction proceeded via a 7-exo-dig cyclization.¹⁹

Scheme 2 Au-catalyzed intramolecular cyclization of N-propargylic β-enaminones 6 for the synthesis of 1,4-oxazepane derivatives 7.

2.2. Fused 1,4-oxazepanes

In 2000, Chaudhuri and Kundu reported a robust protocol to benzo 1,4-oxazepanones via the Cu(I)-catalyzed intramolecular cyclization of the corresponding N-propargylamines. Thus, the corresponding 3-benzylidene-benzo[e][1,4]oxazepan-5-ones 9 were synthesized in good yields from m-(propargylamino)benzoic acids 8 through regio- and stereoselective intramolecular cyclization using CuI/Et₃N system in refluxing acetonitrile (Scheme 3a).²⁰ In a closely related investigation, Saltiel, Marín, and Porcel also found that N-propargylamines 10 were converted to the corresponding 3-methylene-benzo[e][1,4]oxazepan-5-one 11 using AgSbF₆/Ph₃P/DCE or AuCl(PPh₃)/AgSbF₆/DCE catalytic systems. They observed that the substituent effects on the cycloisomerization afforded a mixture of products. Thus, the terminal alkynes afforded predominantly benzo 1,4-oxazepanone 11, but the internal alkynes yielded a mixture of 11 and 1,2-dihydro-4-methylbenzo[c][1,5]oxazocin-6-one 12 (Scheme 3b).²¹ Vaca's research team in 2013 has more improved the efficiency of this protocol by using iodoplladate as catalyst in CDCl₃ under argon atmosphere.²²

Scheme 3 (a) Cu(I)-mediated synthesis of benzo [1,4]oxazepan-5-ones developed by Kundu; (b) Ag(I) and Au(I)-catalyzed synthesis of benzo [1,4]oxazepan-5-ones described by Porcel.

In 2007, the group of Shi has reported a beautiful route for regio- and diastereoselective synthesis of fused bicyclic ketal derivatives 14 from the reaction of N-propargyl epoxides 13 with water, through a Au(I)-catalyzed three-membered ring-opening/6-exo-cycloisomerization/nucleophilic addition sequential process (Scheme 4). After studying a number of catalyst, such as AgOTf, NaAuCl·2H₂O, AgSbF₆, (Ph₃P)AuCl, (Ph₃P)AuCl/AgOTf, (Ph₃P)AuCl/AgSbF, CuI, Bi(OTf)₃, and HOTf in 1,4-dichloroethane, the system (Ph₃P)AuCl/AgSbF at room temperature was found to be optimum with respect to the yield of product isolated. The results revealed that the electronic character of the substituents on the aryl ring had little effect on the rate of the reaction and the use of substrates bearing an electron-withdrawing substituent on the alkyne gave higher yields than those with electron-donating substituent. The author proposed mechanistic course for this reaction sequence is depicted in Scheme 5.^23,24

Scheme 4 Gold(I)-catalyzed regio- and diastereoselective synthesis of fused bicyclic ketal derivatives 14 from N-propargyl epoxides 13 and water.

Scheme 5 Mechanism proposed to explain the ketal 14 synthesis.

3. 1,4-Diazepanes

3.1. Triazolo 1,4-diazepanes

The earliest methods for formation of triazolodiazepanes from N-propargylamines involved 1,3-dipolar cycloaddition of easy available N-protected N-(3-azidopropyl)prop-2-yn-1-amine derivatives [CH≡C–CH₂–N(protecting group)–(CH₂)₃–N₃]. In 2009, Lamaty and co-workers reported a selective synthesis of trans-disubstituted triazolodiazepanes 16 via intramolecular [3 + 2] Huisgen cycloaddition of SES-protected anti-disubstituted N-propargylamines 15 in toluene at 80 °C (Scheme 6). It was found that syn-disubstituted N-propargylamines did not work under this reaction conditions. This fact can be explained by unfavorable interactions between substituents, thus in stable conformer of syn diastereoisomer; alkyne and CH₂N₃ are distant and cannot cyclize to triazole.²⁵

Scheme 6 Synthesis of trans-disubstituted triazolodiazepanes 16 from Aza-Baylis–Hillman adducts 15.

The Batra group reported a related strategy toward 7-methylene substituted triazolodiazepanes 19 from N-propargylamines 18 in refluxing toluene (Scheme 7). Regardless of the geometry of the starting N-propargylamine (E/Z mixture), E-products were always obtained preferentially (the E : Z ratios were in the range of 50 : 50 to 100 : 0). It should be mentioned that the electron-donating groups on aromatic ring increase the ratio of Z-isomer.²⁶ Later, S. Ballet research team used this methodology to a series of amino-triazolodiazepine scaffolds.²⁷

Scheme 7 Synthesis of 7-methylene substituted triazolodiazepanes 19 from N-propargylamines 18.

Perićas has described an efficient method for the synthesis of trans-disubstituted triazolodiazepanes 22 by treatment of N-((3-phenyloxiran-2-yl)methyl)propargylamines 20 with NaN₃. Mechanistically, the reaction involves: (i) regioselective epoxide ring opening by addition of aizde to the benzylic carbon of epoxide which resulted intermediate 21; and (ii) intramolecular 1,3-dipolar cycloaddition of 21 to provide corresponding triazolodiazepanes 22 (Scheme 8).²⁸

Scheme 8 Synthesis of trans-disubstituted triazolodiazepanes 22 from N-propargylamine 20 and azide.

The group of Ballet has recently described a beautiful catalyst-free four-component synthesis of aminotriazoloazepanone-dipeptide derivatives 25 through a one-pot two-step sequence from propargylamine, isocyanide, aldehyde and Boc-protected 2-amino-3-azidopropanoic acid 23 as the key reagent (Scheme 9). A variety of aromatic and aliphatic isocyanides and aldehydes can be used, including heteroaromatic aldehydes. The mechanism proposed for this transformation involves the formation of a liner propargylamide intermediate 24 by Ugi-four-component reaction, followed by a thermal azide–alkyne Huisgen cycloaddition. The author extended their methodology for synthesis of tripeptide-derivatives. However, this methodology for synthesis of tripeptide-derivatives was problematic due to the requirement of a very low reaction temperature.²⁹

Scheme 9 Four-component synthesis of aminotriazoloazepanone-dipeptide derivatives 25.

Recently, X. Bi and co-workers reported an example of 8-methylene-triazolodiazepane preparation from N-propargyl-3-butynylamine. They showed that N-(but-3-ynyl)-N-(3-phenylprop-2-ynyl)benzenamine 26 underwent an Ag-catalyzed cascade hydroazidation and alkyne–azide 1,3-dipolar cycloaddition with trimethylsilyl azide in the presence of 2 equiv. H₂O in DMSO at 80 °C. The corresponding triazolodiazepane 27 was obtained in good yield of 78% (Scheme 10).³⁰

Scheme 10 Ag-catalyzed tandem hydroazidation/alkyne–azide cycloaddition of N-propargylamine 26 with TMS-N₃.

3.2. Triazolo-benzo 1,4-diazepanes

One of the earliest reports on the synthesis of triazole fused benzodiazepanes from N-propargylamines appeared in 2007, when o-(azidomethyl)propargylanilines 28 were cyclized to triazole fused 1,4-benzodiazepines 29 through catalyst-free intramolecular 1,3-dipolar cycloaddition in refluxing chloroform. The reaction tolerates both electron-withdrawing and electron-donating substituents at the arene ring and gave final product in good to excellent yields (Scheme 11).³¹

Scheme 11 Synthesis of triazole fused benzodiazepanes 29 from o-(azidomethyl)propargylanilines 28.

Recently, to develop an efficient protocol for the synthesis of triazole fused benzodiazepanes from N-propargylamines, S. Martin and co-workers have investigated reductive amination of o-azidobenzaldehydes 30 with N-propargylamine. Subsequently, thermal azide–alkyne Huisgen cycloaddition of generated N-(2-azidobenzyl)propargylamines A in toluene, and good yields of desired products was observed (Scheme 12a). The authors synthesized a number of N-functionalized triazole fused benzodiazepanes bearing various functional groups, including of amides, sulfonamides, ureas, thioureas, carbamate and amines from prepared N–H free benzodiazepanes 31.³² The same authors also extended this chemistry to cyclizations of N-propargylamines 32 with acyl chlorides 33 to cyanosubstituted triazole fused benzodiazepanes 34 in the presence of pyridine as a base and acetonitrile as solvent at room temperature (Scheme 12b).³³

Scheme 12 (a) Synthesis of N–H free triazole fused benzodiazepanes 31 through reductive amination/azide–alkyne Huisgen cycloaddition strategy; (b) synthesis of cyanosubstituted triazole fused benzodiazepanes 34 from cyclization of N-propargylamine 32 with 33.

Along this line, Majumdar and Ganai reported an efficient route for synthesis of triazole fused benzodiazepanes 37 from the reaction of N-propargylaniline 35 with o-azidobenzylbromide 36 through a Cu-catalyzed tandem Ullmann C–N coupling, and azide-alkyne cycloaddition strategy in the presence of CuI as catalyst and K₂CO₃ as base. This reaction was run in DMF at 110 °C and tolerated various functional groups (including fluoride, chloride, and methoxy), and in all cases provided triazolo-benzodiazepanes 37 in good to high yields (Scheme 13).³⁴

Scheme 13 Synthesis of triazole-fused benzodiazepanes 37 via Cu-catalyzed tandem Ullmann C–N coupling/azide–alkyne cycloaddition.

3.3. Benzo-imidazo-tiazolo 1,4-diazepanes

In 2005, Gracias and co-workers developed the synthesis of benzo-imidazo-tiazolo-diazepanes 41 from N-propargylamines 38, o-azidobenzaldehydes 39, and p-toluenesulfonylmethyl isocyanides 40 through a van Leusen/alkyne–azide cycloaddition sequence (Scheme 14). Thus, the base-catalyzed three-component reaction gives van Leusen intermediates A, which undergoes a Cu-catalyzed intramolecular alkyne–azide cycloaddition into polycyclic diazepanes 41. It is noted that in the case of terminal alkynes for intramolecular alkyne–azide cycloaddition step, no cu-catalyst was used and this reaction proceeded in situ using execs K₂CO₃ at room temperature.³⁵

Scheme 14 Synthesis of triazolo-imidazole fused benzodiazepanes 41 by sequential van Leusen/alkyne–azide cycloaddition reactions.

A one-pot, four-component reaction between o-azidobenzaldehydes 42, α-diketones 43, N-propargylamines 44, and ammonium acetate provided an efficient entry into 9H-benzo[f]imidazo[1,2-d][1,2,3]triazolo[1,5-a][1,4]diazepines 45 as shown by Kurth and coworkers. This transformation was carried out in the presence of InCl₃ in methanol at 60–100 °C, and afforded 1,4-diazepane systems 45 in moderate yields (Scheme 15). A mechanistic proposal was provided by the authors, which involves the coordination of InCl₃ to the oxygen of the aldehyde and activate it for nucleophilic attack. Subsequently, the generated imine B reacts with N-propargylamine to provide intermediate C. This intermediate undergoes an intermolecular cyclization with activated α-diketone to form intermediate D. That after intramolecular [3 + 2] Huisgen cycloaddition affords final product 45. In another possibility, intramolecular [3 + 2] Huisgen cycloaddition of intermediate C affords intermediate E that transforms to the observed product 45 by condensation with activated α-diketone (Scheme 16).³⁶

Scheme 15 Four-component synthesis of imidazotriazolobenzodiazepanes 45 starting from o-azidobenzaldehydes, α-diketones, N-propargylamines, and ammonium acetate.

Scheme 16 Mechanism that accounts for the formation of 45.

More recently, Kundu and co-workers developed the synthesis of benzimidazotriazolobenzodiazepanes 48 by iodine catalyzed cascade reaction of o-(propargylamino)anilines 46 and o-azidobenzaldehydes 47. The results demonstrated that the electronic character of the substituents in the both benzaldehyde and propargylamine had little effect on the rate of the reaction. However, replacing the arene ring in propargylaniline with a pyridine ring resulted in the formation of desired products with reduced yield (Scheme 17a). The results also showed that the use of a 4-methylated N-propargylaniline gave a mixture of regioisomeric products. According to the proposed mechanism, the reaction starts from the reaction of the carbonyl group with molecular iodine to form the intermediates A, followed by cyclocondensation with amine groups to give intermediate C. Subsequently, a sequential [3 + 2] cycloaddition and air O₂ oxidation to afford corresponding benzimidazotriazolobenzodiazepanes 48 (Scheme 17b).³⁷

Scheme 17 (a) Iodine catalyzed synthesis of benzimidazotriazolobenzodiazepanes 48 from 46 and 47; (b) mechanistic explanation of the synthesis of 48.

3.4. Pyrazolo-pyrrolo and pyrazolo-indolo 1,4-diazepanes

Recently, Balci's research team reported a beautiful protocol for synthesis of hitherto unknown pyrazolo-pyrrolo-diazepane 50 and pyrazolo-indolo-diazepane 52 skeletons by intramolecular cyclization of corresponding N-propargylated pyrazole-substituted pyrroles 49 and pyrazolo-substituted-indoles 51 respectively. The reactions were carried out in acetonitrile at room temperature with 0.25 equiv. of AuCl₃ as catalyst and gave the corresponding products in good to high yields (Scheme 18). However, the synthesis of requisite substrates 49 and 51 required many tedious steps and the use of a substrate with terminal alkyne failed to form the desired products. The authors probed the mechanism of the reaction and found that the reaction proceeded via a 7-endo-dig cyclization process. They showed that the same N-propargylamines 49 and 51 led to the selective formation of pyrazine derivatives under basic conditions via a 6-exo-dig cyclization strategy.^38,39

Scheme 18 Synthesis of hitherto unknown pyrazolo-pyrrolo-diazepane 50 and pyrazolo-indolo-diazepane 52 from corresponding N-propargylated pyrazole-substituted pyrroles 49 and pyrazolo-substituted-indoles 51.

4. Conclusion

In this review, we presented numerous atom and step economic processes for the synthesis of biologically interesting 1,4-oxazepane and 1,4-diazepane derivatives via inter- and intramolecular cyclization reactions of simple and easy available N-propargylamines. These processes do not require any isolation of intermediates or addition of further reagents to those initially mixed, and generally provided titled compounds in good yields. Almost, all of these successes were achieved in the past 10 years and further developments in this field can therefore be anticipated. We hope that this review will serve to stimulate research in this fascinating and very useful area of organic synthesis.

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