Review of the synthesis of acyclic and cyclic oxime ethers

Abstract

Oxime ethers have attracted much attention due to their potential biological activities and wide variety of synthetic applications. Developing more efficient methods for the synthesis of oxime ethers has been the subject of numerous of papers in recent years. This review surveys literature methods for the synthesis of acyclic and cyclic oxime ethers.

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Zohreh Mirjafary

Zohreh Mirjafary was born in 1982 in Isfahan, Iran. She graduated from Isfahan University of Technology before moving to Sharif University of Technology where she became a Ph.D. student in the Professor F. Matloubi Moghaddam research group in 2006. She spent nine months in research group of Professor D. Enders at RWTH Aachen University financed by a research grant from the German Academic Exchange Service (DAAD) in 2008. Now she is working at Azad University as Assistant Professor. Her research focused on the heterocyclic chemistry, organic methodology and catalysis.

Morteza Abdoli

Morteza Abdoli was born in Miyandoab, Iran, in 1987. He received his B.Sc. from the Payame Noor University in 2010. He pursued his postgraduate study at the same university under the supervision of Dr H. Saeidian and obtained his M.Sc. (1st class honor) degree in 2013. Currently he is doing his doctoral research on synthesis and reactions of sulfur-containing compounds under the supervision of Prof. Dr A. Kakanejadfard and Dr H. Saeidian, at Lorestan University.

Hamid Saeidian

Hamid Saeidian was born in Tarom, Zanjan Iran, in 1981. He received his B.S. degree in applied Chemistry from K. N. Toosi University of Technology (K. N. Toosi), Tehran, Iran, and his M.S. degree in organic chemistry from Sharif University of Technology, Tehran, Iran, in 2005. He completed his Ph.D. degree in 2009 under the supervision of Professor F. Matloubi Moghaddam. Now he is working at Payame Noor University as Assistant Professor. His research interests include heterocyclic chemistry, new methodologies in organic synthesis and mass spectral studies of organic compounds.

Ali Kakanejadifard

Ali Kakanejadifard was born in 1957 in Khorramabad District of Lorestan, Iran. He received his M.Sc. degree in 1990 and Ph.D. degree in 1997 from Tehran University. He became a Lecturer at Lorestan University and subsequently became a Reader in 2003 and Professor in 2007. His research interests include synthesis of heterocycles, macromolecules, Dioximes and Schiff Bases.

S. Morteza F. Farnia

S. Morteza F. Farnia received his Ph.D. in physical organic chemistry in 1989 from University of Southern California under the supervision of Prof. George Olah. He continued as Post-Doctoral fellow at Hydrocarbon Research Institute. His research interests are in the fields of nanofunctional materials with application in green chemistry and photocatalysis, environmentally friendly applications and characterizations.

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1. Introduction

The name oxime ether is an abbreviation of oxy-imine ether. As one of the prominent medicinal motifs, the oxime ether group is featured in a large number of pharmaceutically important compounds and is widely applied in a variety of pesticides.^1,2 For example, oxiconazole 1 (Fig. 1), with the brand name oxistat, is an antifungal drug marketed worldwide for the treatment of skin infections.^3–5 Fluvoxamine maleate 2 is used for treating obsessive compulsive disorder.^6–9 A series of novel thioaryl naphthylmethanone oxime ether analogs 3 exhibit excellent anticancer activities towards various cancer cells.¹⁰ Roxithromycin 4 is a semi-synthetic macrolide antibiotic which was introduced in the 1980s, and is used to treat infections caused by bacteria.^11–14 Fenpyroximate 5 is a pesticide with oxime ether motif, this compound is very active against acaricide and widely used around the world.^15–17 Wang's group showed that a series of benzoylphenylureas 6 have excellent larvicidal activities against oriental armyworm.¹⁸ These representative examples show that the oxime ether group offers very attractive options for drug design of various pharmacological agents, due to their relative ease of synthesis and their impressive medicinal chemistry applications.

Fig. 1 (a) Some medicine containing oxime ether functional group; (b) two pesticides containing oxime ether functional group.

Oxime ethers are important and versatile intermediates in organic synthesis. These compounds were successfully transformed into amines,^19–24 1,2-aminoalcohols,²⁵ α- and β-amino acids,^26,27 hydroxylamines,^28–41 nitriles,^42–44 pyridines,^45–50 benzofuranes,^51–53 indoles,^54,55 pyrroles,^56,57 pyrazines,⁵⁸ isoquinolines,^59,60 isoxazolesm,^61–64 8-hydroxytetrahydroquinolines,⁶⁵ aminocyclopentitols,⁶⁶ aziridines,⁶⁷ fluorenones,⁶⁸ diarylmethylidenefluorene and phenanthrene.⁶⁹ Furthermore, oxime ether is an elegant directing-group for activation of aromatic or vinylic C–H bonds for construction of new C–O, C–X and C–N bonds by metal-catalyzed cross-coupling reactions.^70–75 Considering the widespread synthetic applications and biological activities of oxime ethers and extensive attention on these compounds in recent years, especially in the field of metal-catalyzed cross-coupling reactions, there is an urgent need for a review article on the synthesis of titled compounds. In this review, we describe variety of methods for the synthesis of oxime ethers. We have classified these synthetic reactions based on the type (acyclic and cyclic), the starting materials (e.g. synthesis from oxime and alkyl halides, oxime and aryl halides, and oxime and epoxides) and the reactions type (e.g. cross-coupling reactions between oxime and arylboronic acids, and 1,3-dipolar cycloaddition of nitrile oxides to carbon–carbon double bonds). The most detailed discussion is focused on the synthesis of acyclic oxime ethers. It should be noted that we have not discussed synthesis of six membered cyclic oxime ethers, since it has recently been described in another publication.⁷⁶ To summarize, the main methods for the synthesis of acyclic and cyclic (four and five membered cycles) oxime ethers is depicted in Fig. 2.

Fig. 2 The main methods for synthesis of acyclic and cyclic oxime ethers.

2. Synthesis of acyclic oxime ethers

2.1. From oximes and alkyl halides

The best-known method for synthesis of acyclic oxime ethers is the reaction of oximes with alkyl- and aryl halides (Scheme 1).^76–95

Scheme 1 Synthesis of oxime ethers from oximes and alkyl(aryl) halides.

A safe method for the preparation of oximes involves reaction of carbonyl compounds (aldehydes and ketones) with hydroxyl amines (Scheme 2).^96–101 This type of reaction was introduced by Schiff¹⁰² in 1864 and nowadays is the best choice for the synthesis of titled compounds.^103–105

Scheme 2 Synthesis of oximes via condensation of carbonyl compound with hydroxyl amine.

The alkylation of the oxygen atom of oxime moiety with alkyl halides has been performed using various base, such as sodium hydride,^{76,77,80,82,90,91} sodium hydroxide,⁸⁶ potassium hydroxide,⁸¹ and potassium carbonate.^87,93,94 As well as, the system Na/alcohol has also been utilized.^78,85,88 Using potassium carbonate as base and acetonitrile as solvent clearly accelerated the alkylation of the oxime moiety compared to other bases/solvents, and the desired products were synthesized in good yields (Table 1).⁹³

Table 1 Synthesis of oxime ether 8 from oxime 7 and methyl 2-chloroacetate in the presence of K₂CO₃ in MeCN

Entry	Solvent	Base	Time (h)	Yield (%)
1	CH2Cl2	K2CO3	24	—
2	1,4-Dioxan	K2CO3	24	15
3	THF	K2CO3	18	30
4	DMF	K2CO3	18	40
5	Acetonitrile	K2CO3	8	70

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Recently, an excellent method for generation of oxime ethers 10a,b from oximes 9a,b and epichlorohydrin have been reported by Cerra and co-workers using acetone/water/K₂CO₃ system (Scheme 3).⁹⁴

Scheme 3 Synthesis of oxime ethers 10a,b from oximes 9a,b and epichlorohydrin.

Synthesis of oxime ethers from oximes and halides in the presence of phase transfer catalysis has been the subject of a number of papers. However, preparation of oxime ethers using this method resulted in poor to moderate yields of desired products.^80,81 In 2009, Li and co-workers reported the benzylation of oximes by combination of phase transfer catalysis and ultrasound irradiation. They tested several catalysts and solvents, and the system NaOH/benzyldimethyltetradecylammonium chloride/H₂O was found to be superior. Under optimized conditions, the reaction tolerates both electron-donating and electron-withdrawing groups in the phenyl ring of oxime and gave corresponding products in good to excellent yields (Table 2).⁹¹ To compare the yields of product 3a (78%) to the same reaction which was reported under toluene/H₂O/NaOH/TBAB by Wang et al. (76.6%),⁸⁶ it can be concluded that the latter system is superior, due to the former method which was carried out under ultrasound irradiation.

Table 2 The reaction of oxime 11 with benzyl bromide in aqueous media catalyzed by benzyldimethyltetradecylammonium chloride in combination with ultrasound irradiation

Entry	R^1	R^2	Time (min)	Product	Isolated yield (%)
1	2-OMe–Ph	H	30	13a	94
2	4-OMe–Ph	H	35	13b	86
3	4-NO2–Ph	H	30	13c	78
4	2,4-Di-Cl–Ph	H	30	13d	96
5	2-Cl–Ph	H	20	13e	68
6	3-Cl–Ph	H	50	13f	89
7	4-Cl–Ph	H	20	13g	73
8	Ph	H	30	13h	78
9	4-Cl–Ph	CH3	60	13i	90
10	4-OMe–Ph	CH3	60	13j	60

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The attempts to synthesis of O-propargylated oximes 16a,b with treatment of oximes 14 with propargyl bromide 15 in the presence of KOH in DMSO/H₂O 9 : 1 resulted in products with higher than 86% yield (Scheme 4).¹⁰⁶

Scheme 4 O-Propargylation of oximes 14a,b.

2.2. From oximes and aryl halides

In 2007, the successful metal catalyzed cross-coupling of aryl halides with oximes have been reported by Maitra et al.¹⁰⁷ Oximes 17a,b were found to undergo O–H arylation with various iodo- and bromoarenes 18 in the presence of CuI as catalyst, Cs₂CO₃ as base, Na- or K-tartrate as chelating agent, and 1,10-phenanthroline as a ligand in toluene or DMSO and gave corresponding O-aryl oximes 19a,b in moderate to good yields (Scheme 5). Some important information of the reactions are listed below: (1) the reactions will not work with 1-iodo-4-methoxybenzene; (2) aldoximes compare to ketoximes gave lower yield of desired products; (3) the protocol is efficient for intramolecular cross-coupling reactions but not for intermolecular version; and (4) haloarenes bearing electron-withdrawing substituents gave higher yield of products than haloarenes with electron-donating substituents.

Scheme 5 (a) Cross-coupling of ketoximes with haloarenes/catalytic CuI in refluxing toluene; (b) cross-coupling of aldoximes with haloarenes/catalytic CuI in DMSO at 30 °C.

Following this work, Buchwald research team in 2010, has investigated the O-arylation of ethyl acetohydroximate 20 with aryl chlorides, bromides, and iodides using (allylPdCl)₂ as catalyst, t-BuBrettPhos 22 or t-BuXPhos 23 as ligand in toluene at 65 °C. This method has several advantages such as good to excellent yields, short reaction time and broad substrate scope (Scheme 6). Key to the success of this reaction was the use of bulky biarylphosphine ligands 22 and 23, which promote C–O reductive elimination under relatively mild conditions.¹⁰⁸

Scheme 6 Palladium-catalyzed O-arylation of ethyl acetohydroximate.

With the objective of designing a comprehensive protocol to O-arylation of oximes, the scope of electrophilic partners was extended to diaryliodonium salts under transition-metal-free cross-coupling conditions. Several bases and solvents were tested and the system t-BuOK/DMF at room temperature was found to be superior. Under optimized conditions, both electron-donating and electron-withdrawing groups on either coupling partners were well tolerated and gave desired products in good to high yields (40–96% for 26 examples).¹⁰⁹ The use of Cs₂CO₃ as base and acetonitrile as solvent provided the same products in comparable yields (Scheme 7).¹¹⁰

Scheme 7 The O-arylation of oximes 1a–e with diaryliodonium salts.

2.3. From oximes and arylboronic acids

In 2009, Meyer and Feng research teams independently reported the copper catalyzed O-arylation of oximes with arylboronic acids.^111,112 The reaction was undertaken at room temperature using Cu(OAc)₂ as catalyst and pyridine as base in DCE. This method afforded O-arylated acetophenone oximes 29 in moderate yields with various meta and para-substituent arylboronic acids 28 (Scheme 8). It should be noted that using polystyrene supported copper catalyst 30 (Fig. 3) in aforementioned reaction gave relatively better results.¹¹³

Scheme 8 Cu(OAc)₂-mediated O-arylation of oximes 27 with phenylboronic acids 28.

Fig. 3 Chemical structure of polymer supported copper 30.

Recently, Bora and co-workers investigated the efficiency of different bases in titled reaction and showed the system Cu(OAc)₂/Cs₂CO₃ in DMF gave corresponding O-arylated acetophenone oximes 33 in higher yields than previous methods.¹¹⁴ Coumarins 31 were found to undergo efficient O-arylation with various arylboronic acids 32 in the presence of CuCl₂ as catalyst and NEt₃ as base (Scheme 9). This system shows good reactivity for a range of arylboronic acids. Para electron-rich aryl boronic acids and phenylboronic acid worked well under these reaction conditions. Meta- and para electron-deficient arylboronic acids gave coupling products in moderate to good yields.¹¹⁵

Scheme 9 CuCl₂-promoted O-arylation of (hydroxyimino)ethylcoumarin 31 with arylboronic acids 32.

To develop an efficient protocol for the synthesis of O-aryl oximes via cross-coupling reaction, Mulla and co-workers have investigated the O-arylation of acetophenone oximes with arylboronic acids in the presence of recyclable and heterogeneous copper fluorapatite (CuFAP) catalyst in methanol, and good to high yields of desired products was observed (61–96% for 30 examples). A plausible catalytic cycle is depicted in Fig. 4.¹¹⁶

Fig. 4 Plausible catalytic cycle for O-arylation of acetophenone oximes with arylboronic acids.

2.4. From oximes and olefins

Preparation of oxime ethers from oximes and olefins has been the subject of a number of papers. One of the earliest report of the successful formation of allylic oxime ethers via Michael addition of oximes have been published by Akcamur and Kollenz in 1987 (Scheme 10).¹¹⁷ This reaction showed an attractive route for the conversion of oximes into oxime ethers in good to high yields at mild reaction conditions and short reaction times.

Scheme 10 Formation of allylic oxime ethers 36 via Michael addition of oximes 35.

Meshram et al. expanded the efficiency of this method by using the Triton B as a nonmetallic organic base. All of aliphatic and aromatic oximes 37 with both electron-donating and electron-withdrawing substituents in treatment with α,β-unsaturated nitriles 38 or α,β-unsaturated esters 40 gave corresponding oxime ethers in good to high yields (Scheme 11).¹¹⁸

Scheme 11 Triton B-catalyzed Michael addition of oximes to electron-deficient alkenes.

A robust process for the synthesis of allylic oxime ethers involves the reaction of oximes with π-allyl metal complexes.^119–121 Treatment of oximes 42 with α,β-unsaturated acetates 43 in the presence of Pd(PPh₃)₄ as catalyst gave allylic oxime ethers 44 in good to high yields (Table 3). Interestingly, when the reaction was carried out under the [IrCl(cod)]₂/Et₂Zn/THF system, instead of oxime ethers 44, the branched oxime ethers 45 was observed as desired products in good to high yields (Scheme 12).¹¹⁹ Previously, this result has been reported by Takeuchi in allylic amination.¹²²

Table 3 Palladium-catalyzed reaction of 42A–D with acetates 43a–e^a

Entry	Oxime	Acetate	Product	Yield (%)
a Reactions were carried out with 42A–D (1 equiv.) and 43a–e (1.5 equiv.) in the presence of Pd(PPh3)4 (6 mol%) and K2CO3 (1 equiv.) in CH2Cl2.
1	42A	43b	44Ab	80
2	42A	43c	45Ac	71
3	42A	43d	45Ad	81
4	42A	43e	45Ae	73
5	42B	43a	45Ba	67
6	42C	43a	45Ca	51
7	42D	43a	45Da	75

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Scheme 12 Iridium-catalyzed reaction of 42A–D with acetates 43a–e.

With the objective of designing a comprehensive protocol to high regio- and enantioselective synthesis of the branched oxime ethers, the scope of electrophilic partners were extended to α,β-unsaturated phosphates (Scheme 13). The [IrCl(cod)]₂/pybox 48/Ba(OH)₂·H₂O/CH₂Cl₂ system was found to be optimal for this reaction. Notably, the system works well for the allylic substitution of phosphates with amines.^120,123

Scheme 13 Iridium-pybox-catalyzed allylic substitution with oximes.

Oximes 50 underwent Baylis–Hillman reaction with allyl bromides 51 in the presence of sodium hydride and triethyl amine, and the regioselective products 52 were formed in good yields. The mechanism of the reaction involves the deprotonation of oxime by NaH to generate oxime anion B and subsequent reaction of B with intermediate A (derived from the allyl bromide 51 and NEt₃) via path a and path b to produce regioselective oxime ethers 52 and 53 (Scheme 14).¹²⁴

Scheme 14 The proposed mechanism for the formation of 52 via Baylis–Hillman reaction of 50 with 51.

Jia and co-workers established an efficient protocol for radical cation promoted O-alkylation of oximes with N-vinyllactams. They showed treatment of oximes 54 with N-vinyllactam 55 in the presence of tris(4-bromophenyl)aminium hexachloroantimonate (TBPA⁺˙SbCl₆⁻) as an initiator and 2,6-di-tert-butyl-pyridine 56 as base afforded corresponding O-alkylated oxime ethers 57 in high to excellent yields at ambient temperature (Scheme 15). Generally, both electron-donating and electron withdrawing groups in the phenyl ring periphery of oximes were well tolerated.¹²⁵ The use of cerium(IV) ammonium nitrate (10 mol%) in acetonitrile provided the same products in comparable yields (73–95% for 15 examples).¹²⁶

Scheme 15 TBPA⁺˙SbCl₆⁻ initiated addition of oxime 54 with N-vinyllactam 55.

Direct generation of oxime ethers from allylic C(sp³)–H bonds and oxime without a metal catalyst was reported by Bao and co-workers. They tested several oxidants and solvents, and the 2,3-dichloro-5,6-dicyanoquinone (DDQ)/CH₂Cl₂ system was found to be superior. Mechanistically, the reaction involves hydride transfer from the allylic position to DDQ. Good yields were achieved in reaction with both oximes involving electron-donating and electron-withdrawing substituents (Scheme 16).¹²⁷ Following this work, the same group in 2013, extended their methodology to C–O bond formation between oximes and isochroman.¹²⁸

Scheme 16 Formation of oxime ether with oxime 58 and 1,3-diphenylpropene 59.

2.5. From oxime and epoxides

The nucleophilic substitution reaction of oxygen atom of oximes with epoxides for preparation of oxime ethers has been the subject of a number of papers. However, in primary reports a mixture of oxime ethers and nitrones have been examined for this reaction in various conditions, such as base and solvent types, etc.^129–133

In 2008, Soltani reported a highly efficient regio- and diastereoselective synthetic methodology for preparation of β-hydroxy oxime O-ethers 63 via the O-alkylation of oxime anions 61 with epoxides 62. This aqueous-mediated reaction carried out in the presence of KOH as base and gave corresponding E-oxime ethers 63 in good to high yields (Scheme 17). Interestingly, in contrast to previous methods, using this methodology gave no products derived from reaction of the nitrogen atom of oximes on epoxides, even in trace amounts.¹³⁴ Recently, Crich's research team performed the same reaction in DMF with a series of epoxides and acetophenone oximes.¹³⁵

Scheme 17 β-Hydroxy oxime O-ethers synthesized by the ring opening of epoxides with oximes.

2.6. From oximes and alcohols

One-pot O-alkylation of oximes with alcohols employing Ph₃P/CCl₄/DBU/TBAI catalyst system in refluxing acetonitrile was reported in 2010. A wide range of alcohols were efficiently transformed into oximes in good yields (Scheme 18). It is worth to note that the methodology showed excellent regioselectivity for generation of Z-isomers. The selectivity of this method was demonstrated via a competitive reaction of a mixture consisting of primary and secondary alcohols. The results showed high selectivity for the O-alkylation of oximes using the primary alcohols rather than the secondary analogues.¹³⁶

Scheme 18 One-pot O-alkylation of oximes via alcohols in refluxing acetonitrile.

2.7. From oximes and aryl nitro compounds

Baumann demonstrated that oximes can be converted to oxime ethers by treatment with 4-substituted aryl nitro compounds 68 in sodium methoxide at room temperature. The reaction tolerates aryl oximes and gave corresponding O-aryl oximes in moderate yields, but extension of the reaction to aldoximes and alkyl ketoximes bearing a hydrogen at α-position was failed (Table 4).¹³⁷

Table 4 Synthesis of oxime ethers 69 via treatment of oximes 67 with 68

Entry	R^1	R^2	R^3	Yield (%)	Entry	R^1	R^2	R^3	Yield (%)
1	–(CH2)5–		CO2Me	7	7	Ph	Ph	PhCO	66
2	–(CH2)5–		CN	22	8	Ph	Ph	CHO	59
3	Me	Me	CO2Me	12	9			CO2Me	31
4	Me	Me	CN	24	10			CN	73
5	Ph	Ph	CO2Me	28	11			PhCO	37
6	Ph	Ph	CN	61	12			CHO	48

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2.8. From oximes and Morita–Baylis–Hillman (MBH) carbonates

An excellent method for generation of oxime ethers is the reaction of oximes with Morita–Baylis–Hillman (MBH) carbonates. Chen and co-workers showed acetophenone oxime 70 and MBH carbonates 71 in the presence of commercially available hydroquinidine 1,4-phthalazinediyl diether 72 as a chiral catalyst at 50 °C gave O-allylic alkylated acetophenone oxime 73 in moderate to excellent yields with high enantiomeric excess (Scheme 19). However, the MBH carbonates bearing alkyl-substitution were incompatible in this type of reaction.¹³⁸

Scheme 19 Asymmetric O-allylic alkylation of acetophenone oxime 70 with MBH carbonates 71.

2.9. From condensation of carbonyl compounds with aminooxy groups

As it is mentioned in Section 2.2, oximes are synthesizes from the reaction of carbonyl compounds (ketones or aldehydes) with hydroxyl amines, in a two-step reaction (Scheme 20, route a). The reaction of carbonyl compounds with aminooxy groups is a one-step route for the synthesis of titled compounds (Scheme 20, route b).^139,140 However the synthesis of aminooxy groups requires another step.^141–144

Scheme 20 General route for synthesis of oxime ethers.

Synthesis of oxime ethers via condensation of carbonyl compounds with aminooxy groups are well described in the literature.^145–156 This reaction is usually conducted in various solvents and in the presence of an acid, which influence the yield of oxime ethers. Usual solvents which have been used are water,^145–147 methanol,^148,149 ethanol,¹⁵⁰ aqueous tetrahydrofuran,^146,147 chloroform¹⁵¹ and common acids include hydrochloric acid,^147,148 acetic acid,^148,150 piperazine-N,N′-bis(2-ethanesulfonic acid),¹⁵² pyridinium para-toluenesulfonate.¹⁵¹

The system MeOH/aqueous HCl gave excellent yield for the synthesis of O-aryl oximes at room temperature (Scheme 21).¹⁴⁸ The reaction of carbonyl compounds with benzyl hydroxyamines in absolute ethanol without catalyst gave superior result for preparation of O-benzyl oximes (Scheme 22).¹⁵³

Scheme 21 Preparation of oxime ethers 7Aa–7He in the presence of aqueous HCl as catalyst in MeOH.

Scheme 22 Preparation of oxime ethers 78 and 79 in EtOH without catalyst.

Indeed, the reaction rate is accelerated by using acid catalyst in aforementioned reactions. The acidic conditions are not compatible with biological systems and can damage biomolecules.¹⁵⁷ In 2008, to overcome this difficulty, Dirksen and Dawson introduced aniline as an efficient catalyst for condensation of carbonyl group with aminooxy group at neutral pH.¹⁵⁸ Crisalli and Kool¹⁵⁹ reported anthranilic acids and 3,5-diaminobenzoic acid (Fig. 5) as superior catalysts for oxime ether formation under neutral pH conditions. Using the same catalysts, Palandoken and co-workers synthesized sugar oxime ether surfactant 85 in moderate to excellent yields (Scheme 23).¹⁴⁹

Fig. 5 Chemical structure of anthranilic acids and 3,5-diaminobenzoic acid.

Scheme 23 Sugar oxime ether surfactant (SOESurf) synthesis.

2.10. Miscellaneous

Reaction of oximes with acetylenes is a potential route for synthesis of novel oxime ethers. The example for this type of reaction have been reported by Tigchelaar-Lutjeboer et al. It is shown that, the reaction of oximes 86 with ethoxyethyne 87 at 75–90 °C rise to the formation of di-oxime ethers 88 in moderate yields (Scheme 24). However the products are unstable compounds and in the case of aldoximes, corresponding di-oxime ethers were decomposed immediately.¹⁶⁰

Scheme 24 Addition of oximes to ethoxyethyne.

Reaction of oximes with cyclic peroxides is an efficient route for the synthesis of oxime ethers. 1-Methoxy-2,3,7-trioxa-bicyclo[2.2.1]hept-5-enes 90, derived from photooxygenation of 2-methoxyfurans 89, were converted into oxime ether hydroperoxides 93 by treatment of 4-nitrobenzaldehyde oxime 92. The reaction proceed via the oxygen nucleophilic trapping reaction of intermediate carbonyl oxides 91. However, in the most cases the products are unstable and rearrange into N-hydroperoxy alkylnitrones (Table 5).¹⁶¹

Table 5 Reaction of oximes with cyclic peroxides

Entry	R^1	R^2	R^3	Yield (%)
1	CO2Me	H	Ph	26
2	CO2Me	H	4-Me–Ph	10
3	CO2Me	H	4-OMe–Ph	38
4	CO2Me	H	4-Br–Ph	35
5	CO2Me	H	Ph	12
6	CO2Me	Me	Ph	51
7	CO2Et	Ph	Ph	80
8	H	CO2Me	Ph	—
9	CO2Me	CO2Me	Ph	—

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Treatment of methylglyoximes 94 with trialkyl orthoformate 95 resulted in the formation of a mixture of corresponding bis-O-alkylated oximes 96a in moderate yields and mono-O-alkylated analogues 96b in poor yields (Scheme 25).¹⁶²

Scheme 25 Synthesis of O-dialkoxymethyloximes from oximes and trialkyl orthoformate.

3. Synthesis of four membered cyclic oxime ethers (4H-1,2-oxazete)

Generally, 4H-1,2-oxazetes and their highly strained derivatives are known as reactive intermediates in thermal and photochemical reactions. These compounds undergo facile fragmentation to carbonyl compounds and nitrile oxides (Scheme 26).^163–168

Scheme 26 Fragmentation of 4H-1,2-oxazetes to carbonyl compounds and nitrile oxides.

However, there are some reports for preparation of relatively stable derivatives of 4H-1,2-oxazetes. Wieser and Berndt reported two different route for generation of two family of stable 4H-1,2-oxazete derivatives:^169,170 (1) treatment of 3-tert-butyl-1-chloro-4,4-dimethylpenta-1,2-diene 97 with N₂O₄ readily gave crystalline oxazete N-oxide 99. This compound undergoes partial transformation to 100 and stable 101 (Scheme 27); (2) elimination of H–X from oximes 102 and 103, followed by intramolecular cyclization of unsaturated nitro intermediates 104 and 105, gave corresponding stable oxazete 106 and 107 in good to excellent yields (Scheme 28).

Scheme 27 Synthesis of stable oxazetes from 3-t-butyl-1-chloro-4,4-dimethylpenta-1,2-diene.

Scheme 28 Synthesis of stable oxazetes from oximes.

Corkins and co-workers reported an efficient protocol for the synthesis of stable 3-tert-butyl-4,4-bis-(methylthio)-4H-1,2-oxazete 109 (Scheme 29). Addition of m-chloroperbenzoic acid to oxime 108 in CH₂Cl₂ leads directly to oxazete 109 after 16 hours at 0 °C in 90% yield.¹⁷¹

Scheme 29 Synthesis of stable oxazete 109 from oxime 108.

4. Synthesis of five membered cyclic oxime ethers

4.1. 1,3-Dipolar cycloaddition of nitrile oxides to carbon–carbon double bonds

The 1,3-dipolar cycloaddition of nitrile oxides to C=C bond is a fundamental tool and straightforward route to isoxazoline rings.^172–228 The same reaction with carbon–carbon triple bonds is one of the most efficient protocols for generation of isoxazoles.^229–232 Hydroxamic acid chlorides 110 are the commonly used precursors for generation of nitrile oxides 112. Nitro compounds can also serve as convenient nitrile oxides precoursors.^211–220 However, nitro compounds are commonly used for the synthesis of isoxazoline N-oxides.^233–246 The conversion of 110 to 112 and 1,3-dipolar cycloaddition of 112 to alkenes 111 usually carried out in a one-pot reaction sequence (Scheme 30a). This reaction is conducted with various bases, such as NaHCO₃,^172,173 KHCO₃,¹⁷⁴ AgNO₃ (ref. 173) and NEt₃.^174–180 The sequence in Scheme 30b shows how hydroxamic acid chlorides can be converted into nitrile oxides.

Scheme 30 (a) 1,3-Dipolar cycloaddition of nitrile oxides to C=C bond; (b) the mechanism of generation of nitrile oxides from hydroxamic acid chlorides.

It is noteworthy that nitrile oxide can be generated in situ from the corresponding aldoxime by chlorination with bleach and then dehydrochlorination.^221–228

The impressive methods have been developed for direct use of aldoximes instead of hydroxamic acid chlorides as nitrile oxides precursor. In these methods, generation of nitrile oxides and then a 1,3-dipolar cycloaddition to alkenes performed in single step using hypervalent iodine as oxidants.^247–253 Recently, a more robust protocol for the synthesis of 4,5-dihydroisoxazoles was introduced by Yoshimura et al. They have exemplified the direct reaction of aldoximes with variety of alkenes in the presence of iodoarenes as catalyst and oxone as a terminal oxidant (Scheme 31). The similar reaction with alkynes gave the corresponding isoxazoles.²⁵¹ In 2014, Yan improved the efficiency of Yoshimura protocol by using potassium chloride/oxone as oxidation system in water.²⁵²

Scheme 31 Iodine catalyzed generation of nitrile oxides from oximes and their cycloaddition with alkenes.

Following Yan, Bharate and co-workers have investigated the same reaction using DBU/NCS/DMF system and achieved better results.²⁵³

Recently, Wang and co-workers reported a beautiful three component reaction for the synthesis of 4,5-dihydroisoxazole rings with secondary amine at C-5 position (Scheme 32). Most of the applied secondary amines failed to participate in the reaction, whereas, pyrrolidine was well tolerated. Mechanistically, the reaction involves: (1) the formation of nitrile oxide by hydroxamic acid chloride 110 and dialkylamine 114; (2) condensation of aldehyde 115 with pyrrolidine to give enamine; (3) 1,3-dipolar cycloaddition of nitrile oxides to carbon–carbon double bonds of enamine to produce 4,5-dihydroisoxazole rings 116 containing dialkylamino moiety at C-5 position in good to excellent yields (77–99% for 18 examples).²⁵⁴

Scheme 32 Tri-component reaction for the synthesis of 4,5-dihydroisoxazole rings.

4.2. Intramolecular metal catalyzed cross-coupling reactions

Metal catalyzed cross-coupling reaction is a straightforward route for formation of =N–O–R linkage via reaction of oximes (=N–OH) with an electrophilic partner (X–R). The intramolecular version of this reaction is a highly effective protocol for generation of isoxazolines. The first example, was reported by Coffen and co-workers in 1984.²⁵⁵ Coupling of iodooxime 117 with propargyl alcohol using Pd(PPh₃)₂Cl₂/CuI/Et₃N/CH₂Cl₂ system, resulted in isoxazoline 118 in 95% yield (Scheme 33).

Scheme 33 Synthesis of isoxazoline 118 from iodo oxime 117 via metal-catalyzed cross-coupling reaction.

Subsequently Wailes reported the same cyclization using CuI/Et₃N/CH₂Cl₂ system and obtained isoxazoline 120 in 80% yield. Two other examples utilizing this protocol is depicted in Scheme 34.¹⁰⁷

Scheme 34 Intramolecular cross-coupling of ketoximes 119a,b with catalytic CuI in refluxing toluene.

4.3. Miscellaneous

In 2010, Knight and co-workers were able to take advantage of a new route for the synthesis of isoxazolines in their efforts to develop novel cyclization of O-propargylic hydroxylamines. It was shown that hydroxylamines 121 in treatment with 10% w/w silver nitrate/silica gel as a catalyst in CH₂Cl₂ at 20 °C underwent intramolecular hydroamination to give corresponding isoxazolines 122 in good to excellent yields (Scheme 35).²⁵⁶

Scheme 35 Synthesis of isoxazolines 122 from hydroxylamines 121.

Regioselective intramolecular carbon–hydrogen bond oxygenation at β-position of oxime moiety with activation of hydroxyl group is the newest route for generation of isoxazolines which have been introduced by Chiba et al. in 2013. The cyclization has been conducted using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) in DMF and gave desired products in moderate to high yields (Scheme 36). Mechanistically, it involves: (1) the reaction of oxime moiety with TEMPO to give iminoxyl radical; and (2) 1,5-H radical shift of iminoxyl radical result in the formation of corresponding isoxazoline.²⁵⁷

Scheme 36 TEMPO-mediated C–H oxygenation of oximes 123.

5. Conclusion

This review provides concise overview on the synthesis of acyclic and cyclic oxime ethers. The new strategies in this area such as synthesis of oxime ethers via metal-catalyzed cross-coupling reactions and intramolecular carbon–hydrogen bond oxygenation has further potential for development. We believed that the highly versatile and novel procedures for the synthesis of oxime ethers will be attainable in the near future.

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