Acetoxyl group as control element in electrophilic addition: Participation by acetoxy group and its competition with other participating groups in hypobromous acid addition to some 5-cholestene derivatives

Abstract

The 3β-acetoxy cholestene II (with nonparticipating group at the position 19) is known to be attacked with hypobromous acid predominantly from α-site which results in formation of the diaxial bromohydrin XIV. By contrast, inversion of configuration of the 3-acetoxy group leads to a dramatic change in the reaction course: the 3α-acetoxycholestene derivative VIII is preferentially approached by the electrophile from β-site to give the corresponding 5β,6β-bromonium ion XXVII which on cleavage with 6(O)π,n participation by the 3α-acetoxyl yields two products XXIX and XXXI When hydroxy and acetoxy groups can compete in 5(O)n or 6(O)π,n processes, only hydroxyl group participation takes place (IX → XXV and XI → XXXIV). Two acetoxy groups in X compete successfully in 6(O)π,n processes (X → XXXVII + XLI).