Photochemical rearrangement oxaziridines

En gros, the N insertion reactions can be subdivided into a Beckmann type and a Schmidt-type rearrangement part. Furthermore, some photochemical rearrangements of chiral oxaziridines are known to generate a range of optically active lactams. 

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). 

A transformation of this type also occurs in the photochemical rearrangement of the acyclic nitrone (177). to the amide (178) and an intermediate oxaziridine may. be, involved in the photochemical conversion149 of aryl oximes into the corresponding amides [Eq. (42)]. 

Considering that the present paper does not intend to analyze the chemical reactions of oxaziridines, we refer only to some publications in this field published in recent years. Certain oxaziridines undergo stereoelectronically controlled photochemical rearrangement into lactams (equation 49)204-2i9... 

The photochemical rearrangement of nitrones to amides often involves initial formation of oxaziridine intermediates. These oxaziridines can be isolated in good to excellent yields (Table 31). 

Thiaziridines have been postulated as reactive intermediates in the photochemical rearrangement of thiazoles, isothiazoles, -and 4-aryl-l,3,2-oxathiazolium S-oxide," and in the conversion of oxaziridines into imines and elemental sulphur on treatment with thiourea, potassium thiocyanate, or potassium ethyl xanthate. Irradiation of a mixture of 7V-sulphinyl-aniline and diphenyldiazomethane gave iV-diphenylmethyleneaniline, which was probably formed by extrusion of sulphur monoxide from 2,3,3-triphenyl-thiaziridine 1-oxide formed as an intermediate. ... 

Lattes and co-workers (23) have studied the photochemical and thermal rearrangement of oxaziridines into amides (79 80). 

Other relevant described reactions, which affect the stability [72] and are related to some adverse effects of these compounds (see below) [73], have been the photochemical transformations. In fact, QDO undergoes rearrangements in the presence of UV-irradiation, passing through an oxaziridine intermediate, like an acyclic nitrone (Fig. 8). The final products depend on the QDO 2,3-substitutions and reaction conditions [72,74,75]. 

Both pyrazine iV-oxides and 2,3-dihydropyrazines rearrange photochemically to imidazoles. Irradiation of the 2,5-disubstituted oxides (187) gives mixtures of the imidazole products (188, 189), probably through the intermediacy of oxaziridine intermediates (Scheme 109). 

Other bicyclic oxaziridines give rise to some interesting products either through ring contraction or more complex cleavage. For example, when oxaziridine 68 is heated to 150°C, it rearranges to an azetidine 69 in 44% yield.(This reaction is also observed photochemically and appears to be more general under these conditions.)... 

The remarkable stability reportedfor 2-phenyl-3,3-dibenzoyloxaziridine in the face of Padwa s inability to isolate oxaziridines but only amides upon oxidation of jV-phenylacylimine derivatives and the lability of nitrones 80 and 82 has recently been shown to be in error.The product from the thermal or photochemical isomerization of A -phenyl-C,C-dibenzoylnitrone is the expected rearrangement product, A -benzoylphenylglyoxanilide. 

The reported synthesis and properties of an oxaziridine by the thermal isomerization of the C7-trityl ester of 3-ac/nitrocamphor bears reinvestigation, particularly in view of the recent report that irradiation of nitronate salts yields hydrox-amic acids but that oxaziridine intermediates could not be detected.) A -Sulfonyl-oxaziridines also rearrange, thermally to nitrones that further decompose to products, but photochemically they yield amides. ... 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

The first photochemical Beckmann rearrangement of aromatic aldoximes was reported in 1963. Subsequently, cyclohexanone oxime was shown to rearrange, upon photolysis, to caprolactam. Although the presence of oxaziridines in the solutions of photolyzed oximes was demonstrated, no oxaziridines have been isolated from these reaction mixtures presumably because of the general instability of oxaziridines that have no substituents on the ring nitrogen. The qualitative results are consistent with the intermediacy of oxaziridines in the photolysis of oximes to amides, yet the possibility of the reactions following other pathways cannot be ruled out. ... 

Oxaziridines are converted to ring-expanded lactams under photochemical conditions. A-Tosyl aziridines with an a-hydroxyalkyl substituent give a pinacol rearrangement product in the presence of Lewis acids, such as Sml2> in this case a keto-A-tosyl amide. ... 

Pyrazine Al-oxides and 2,3-dihydropyrazines also rearrange photochem-ically into imidazoles. From 2,5-disubstituted pyrazine 1-oxides (73) two products, 2-acyl(or aroyl) 4-substituted imidazole (74) and 2,4-disubstituted imidazole (75), result. " " The reaction (Scheme 16) presumably takes place through two isomeric oxaziridine derivatives. 

Evidence that the photochemical Beckmann rearrangement of oximes proceeds through an oxaziridine intermediate ccmes fran the observation that the relative migratory tendencies of H and aryl groups in the rearrangement of RCH NCH (R=Ph, 4-MeOCgH, 4- 1 ) are similar to those of (352)25. ... 

Although further examples of photochemically induced rearrangement of nitrones and /V-oxides have been described this year, little novelty can be attached to most of these reports. The N-oxide (76), on irradiation in the presence of a proton source such as persulphate or dilute sulphuric acid, is converted into the corresponding 4-aryl-2 -l,4-benzoxazin-3(4iZ>one (77) 5 evidence for the pathway outlined in Scheme 3 involving the oxaziridine (78) as an intermediate is presented. 

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