Oxaziridines photochemical transformations

Other examples of photochemical transformations of nitrones into alkaloids have been described (461). According to the view of the authors, some trans-oxaziridines give products derived from the trapping of aminyl radicals (AR) by the pendant alkenes (Scheme 2.90). 

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]. 

Alkynes react with the excited states of aromatic nitro compounds (equation 98) , perhaps in part by way of cycloaddition to give an unstable dioxazole by analogy with the photoaddition of nitrobenzene to alkenes. However, [2 + 2] cycloaddition is an attractive alternative, since photochemical transformation of nitrones (33) to amides via oxaziridines is well documented. 

Photochemical transformations of pyrimidines can also give rise to some imidazole products. For example, in benzene or methanol, photolysis of a number of substituted 1-oxides yielded small quantities of imidazoles " " (Eq. 20). These reactions have been explained by involving oxaziridines, 1,2,4-oxadiazepines, or zwitterions as possible intermediates. 

Theoretical ab initio SCF investigations have been reported for the photochemical transformations of oxaziridine into a nitrone, into an amide, or into methanal plus nitrene. ... 

Also, nitrones can be formed by photochemical oxidation (X350 nm) of aldimines in acetonitrile, in the presence of O2 over a TiC>2 suspension (20, 21). Air oxidation of imines into oxaziridines with their subsequent transformation into nitrones, using cobalt catalysts, provides good yields. Utilization of molecular oxygen in the oxidation process seems highly promising due to its cost-effectiveness, availability, and the possibility of industrial application (22). 

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)]. 

There are examples of photochemically induced transformations of pyrimidines into imidazoles. In benzene or methanol solution, substituted pyrimidine 1-oxides give low yields of imidazoles (Scheme 108). Such reactions have been explained by invoking oxaziridines, 1,2,4-oxadiazepines and zwitterionic species as intermediates. 

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