Oxaziridine isomerization

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

In the discussion of some mass spectra of nitrones (41), intermediate isomerization to oxaziridines was concluded from the occurrence of aldehyde fragments. 

The isomerization of oxaziridines (1) to acid amides with migration of a substituent from C to N is a general reaction and is always observed when no other reactions predominate under the relatively harsh conditions (heating to above 150 °C or photolysis). Even then one can make acid amide formation the main reaction by working at 300 °C (57JA5739) and by dilution techniques. For example, caprolactam (63) is formed in 88% yield by flash pyrolysis of oxaziridine (52) at about 300 °C, whereas decomposition of (52) at lower temperatures gives almost no (63) (77JPR274). 

Like isomerization of oxaziridines to nitrones, acid hydrolysis of oxaziridines proceeds with conservation of the N—O bond. Both reactions are related mechanistically and are... 

Homolytic oxaziridine decomposition can be easily initiated by iron(II) ion in acidic media. Catalytic amounts are sufficient because chain reactions proceed. The reaction proceeds obviously in the case of 2-r-alkyloxaziridines like (56), where it yields the isomeric acid amide (111) (57JA5739). 

There are differences in the high temperature behavior. While oxaziridines almost always isomerize to acid amides, a similar reaction of diaziridines, which should lead to amidines, has not been observed. Sensitivity towards bases, often encountered in oxaziridines, is observed only in some special substituted diaziridines. The tendency of some classes of oxaziridines to transfer the nitrogen function also lacks in the diaziridine field. On homolytic reactions of diaziridines there are only a few observations. 

Oxaziridine syntheses were broadly Investigated from a stereochemical point of view. The formation of two isomeric bis-oxaziridines (249) from glyoxal pointed to a tetrahedral... 

Irradiation of the unsymmetrically substituted 2,3,4,6-tetraphenylpyridine 1-oxide (5) under these conditions gives 2,4,6,7-tetraphenyl-l, 3-oxazepine (7) in 30 % yield, together with 2,4,5,6-tetraphenylpyridin-3-ol (10) in 37% yield and 2,3,4,6-tetraphenylpyridine (30%).11 It has been suggested that the reaction proceeds by way of the oxaziridines 6 (which yields 7) and 8 (which yields the isomeric oxazepine 9) the latter rearranges to the pyridinol 10. 

Nitrones in general undergo a four-electron photocyclization to afford the corresponding oxaziridines. The process is stereospecific,47 proceeds via the excited singlet state, and is in certain instances photochemically reversible.48 Theoretical studies support this proposed pathway49 The nitrones 52, on direct irradiation, afford the oxaziridines 53, which on further irradiation are converted into the isomeric amides 54.so In contrast, triplet... 

For the first time, the primary nitrone (formaldonitrone) generation and the comparative quantum chemical analysis of its relative stability by comparison with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described (357). Both, experimental and theoretical data clearly show that the formal-donitrones, formed in the course of collision by electronic transfer, can hardly be molecularly isomerized into other [C,H3,N,0] molecules. Methods of quantum chemistry and molecular dynamics have made it possible to study the reactions of nitrone rearrangement into amides through the formation of oxaziridines (358). 

Other experiments suggest that nitrone deoxygenation proceeds through intermediate formation of nitrone RC, rather than the product of its isomerization, a oxaziridine (441). 

Photochemical Rearrangement Isomerization of nitrones to oxaziri-dines is a general reaction of various cyclic and acyclic nitrones (447-449). When this reaction is reversible, many transformations of nitrone to oxaziridine and back to nitrone can be carried out without decomposition. This reaction is of special interest in view of light energy accumulation (450, 451). 

Similarly, oxaziridines (244) are not likely to undergo reverse isomerization to the initial nitrones (225) (Scheme 2.85) (453). 

Oxaziridines, 6-oxa-l,4-diazabicyclo[3.1.0]hex-3-enes (245) and their 4-oxides (246), obtained in the photolysis of 4H -imidazole-3-oxides (223) and 1,3-dioxides (219), underwent a quick thermal isomerization to the starting nitrones. Further... 

Oxaziridines, the products of photochemical isomerization of polymeric nitrones, are characterized by high stability. The formation of an intramolecular hydrogen bond stabilizes the nitrone group with respect to UV-irradiation... 

Photochemical isomerization of nitrones to oxaziridines can occur in the crystal phase (458). 

Pyrroline-A-oxide (258) is isomerized into y-lactam (259) in the presence of lithium diisopropylamine (LDA) (470) and sodium trityl (471). In these reactions, deprotonation at C3 occurs, leading to carbanion (260). Then oxygen migration from Ni to C2 takes place via intermediate formation of oxaziridine... 

As a final example of regioselective control (Scheme 25), sulfonium ylides usually give nitrones when reacted with nitroso groups via oxaziridine intermediates. However, 7-aryl-2-dimethylamino-3,4,5,6-tetrahydropteridine-4,6-diones 120 were directly formed when nitrosopyrimidine 121 reacted with dimethylphenacylsulfonium bromides 122 instead of the isomeric 5-oxides <1996H(43)437>. It was also reported that the resulting pteridines were reduced to give 7,8-dihydro derivatives 123 with sodium dithionite (Table 7). 

Many nitrones in this series are converted to oxaziridines by UV irradiation for example, a solution of (248) in 2-propanol gave (249) on exposure to daylight. This isomerization was reversed on heating. UV irradiation of (248) in ethanol/benzene induced transformations to (250) and (251), probably via (249). 

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