Photo-Beckmann rearrangement oxaziridine intermediate

Mechanisms of the photo-Beckmann rearrangement have been advanced to involve the transformation of an excited singlet state of the oxime into an oxaziridine followed by the reorganization of the singlet excited intermediate to a lactam or amide. ... 

Oxaziridines have been proposed as intermediates in the photo-Beckmann rearrangement of oximes. A reinvestigation of the photochemistry of (+)-camphor oxime (60) has revealed that the... 

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). 

While the factor that controlled the regioselectivity of the photo-Beckmann rearrangement of steroidal a,P-unsaturated oximes remains obscure, the different regioselectivity in the lactam formation may originate from a stereoelectronic effect — the relative geometry between a lone pair on the nitrogen and the migrating CC bond in the oxaziridine intermediate. 

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