Collect. Czech. Chem. Commun. 1983, 48, 3629-3642

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

Pavel Kočovský

Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6


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 (IXXXV and XIXXXIV). Two acetoxy groups in X compete successfully in 6(O)π,n processes (XXXXVII + XLI).