Oxaziridine reactions photochemical

There is a scattered body of data in the literature on ordinary photochemical reactions in the pyrimidine and quinazoline series in most cases the mechanisms are unclear. For example, UV irradiation of 4-aminopyrimidine-5-carbonitrile (109 R=H) in methanolic hydrogen chloride gives the 2,6-dimethyl derivative (109 R = Me) in good yield the 5-aminomethyl analogue is made similarly (68T5861). Another random example is the irradiation of 4,6-diphenylpyrimidine 1-oxide in methanol to give 2-methoxy-4,6-diphenyl-pyrimidine, probably by addition of methanol to an intermediate oxaziridine (110) followed by dehydration (76JCS(P1)1202). 

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). 

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... 

They have obtained unambiguous evidence that the photochemical process is completely regiospecific. They have further concluded that this process must be controlled by stereoelectronic factors because the bond (R — C) which is anti to the nitrogen lone pair in oxaziridine 79 migrates more easily than the bond (R"— C) which is syn to the lone pair. Syn-anti isomerization by nitrogen inversion in oxaziridines 79 was rigorously excluded under the reaction conditions (r.t.). 

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]. 

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. 

The photochemical isomerization reaction has been shown" to be stereospecific following the disrotatory photochemically allowed cyclization path. Thus the stereochemistry of the nitrone is preserved in the oxaziridine. However, since some nitrones undergo trans-cis isomerization under the conditions of photolysis, mixtures of oxaziridines often result. 

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.)... 

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. ... 

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. 

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. 

The photochemical electrocyclic reaction of CH2N=0 to form oxaziridine has been explored on the basis of the potential-energy surfaces that are obtained by MINDO/3 Cl calculations. ... 

Further study of the photo-Beckmann rearrangement of oximes suggests that the reaction occurs by way of a concerted pathway from an excited singlet oxaziridine intermediate.84 Thus, in the rearrangement of syn- or anti-5jS-cholestan-6-one oximes (107), which are photochemically interconvertible, the original configuration at C-5 is retained in both lactam products (108) and (109). 

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