Photoinduced transformations. Part 36. Stereochemical integrity of the terminus of the migrating carbon in the photo-Beckmann rearrangements of some cholestanone oximes

Abstract

Analysis of the products of the photo-Beckmann rearrangement of four isomeric Cholestanone oximes and an A-nor-cholestanone oxime, in all of which the hydroxyimino-group is in the terminal ring A, has shown that in each case only two structurally isomeric lactams are formed, in combined yields of 25–53%, together with a small amount of the parent and an isomeric ketone. In all the lactams formed, the chirality of the migrating groups is retained. Although the differences in the amounts of the two isomeric lactams formed in each photorearrangement were very small, the lactams obtained by migration of the more substituted carbon centre were always produced in slightly larger amount, and the difference was greater in the case of cholestan-1-one oximes than with cholestan-4- and -6-one oximes. On the basis of these and previous results, the pathway of the photo-Beckmann rearrangement of most alicyclic ketone oximes may be understood in terms of a simple scheme involving transformation of excited singlet oxime into an oxaziridine intermediate followed by reorganization of the singlet excited intermediate to the lactams in a fully concerted manner. In all the present photorearrangements, no products of photo-Beckmann fission were formed; this shows the synthetic utility of this photorearrangement. ¹H N.m.r. spectra of the oximes are discussed.