Neighbouring group effects in the addition of hypobromous acid to unsaturated steroids

Abstract

The reaction of the stereoisomeric 3-hydroxy(or acetoxy)-5α-cholest-1-enes and 1-hydroxy(or acetoxy)-5α-cholest-2-enes with N-bromoacetamide was investigated. With one exception [formation of the bromo-diol (15a)], the reactions take place by initial formation of an α-oriented bromonium ion; the 2α,3α-ions are preferentially attacked at C(3)(β with respect to the neighbouring group), leading to trans-diequatorial products, whereas the 1α,2α-bromonium ions are invariably attacked at C(2)(α with respect to the neighbouring group), leading to diaxial products. In two instances, internal nucleophilic displacement by appropriately oriented hydroxy-groups leads to bromo-epoxides [compounds (3) and (16)].