Favorsky Rearrangements of α-Halogenated Acetylcycloalkanes. 3

Article abstract of DOI:10.1021/jo00144a017

The rearrangement of 21-bromopregnenolone acetate with sodium methoxide in dimethoxyethane leads to an epimeric mixture of 17-methylated ethio esters in which the 17α-methyl derivative prevails, in contradistinction to the rearrangement under identical conditions of 17-bromopregnenolone acetate which affords predominantly the 17β-methyl 17α-etio ester.This excludes the possibility that in every cyclopropanonic Favorsky rearrangement a dipolar species is formed as the primary intermediate from the originally produced enolate ion.All the resuls of cyclopropanonic Favorsky rearrangements may be explained by the assumption of a competition between a concerted and a nonconcerted cyclopropanone formation, the protic and polar character of the medium exerting an important influenceon the concertedness and the nonconcertedness of the mechanism.As a possible alternative, a gradient of mechanisms could be considered.The competition between reactions leading to rearrangement and substitution products and the dependence of their relative importance on the medium are also discussed.