Singlet-oxygen ene reaction with 3 beta-substituted stigmastanes. An alternative pathway for the classical Schenck rearrangement

The course of the singlet-oxygen ene reaction with stigmasta-5,22-dienes ma y be controlled if in the substrate a good leaving group as substituent is present at 3-C. Thus when the 5 alpha -hydroperoxystigmasta-5,22-diene is f ormed instead of the well known allylic rearrangement to yield the 7 alpha -hydroperoxystigmasta-5,22-diene isomer an intramolecular nucleophilic subs titution can then occur yielding 5 alpha -hydroxystigmasta-6,22-dien-3-one. Various stigmasta-5,22-dienes were chosen to elucidate which feature of th e stigmastane is necessary to control the course of the reaction. Thus, 3 b eta -F, 3 beta -Cl, 3 beta -Br, 3 beta -I and 3 alpha -Br-stigmasta-5,22-di enes were firstly prepared and fully characterized in order to study their reaction with O-1(2) under different experimental conditions. 3 beta -Aceto xy- and 3 beta -mesyloxy-stigmasta-5,22-diene derivatives were also prepare d and studied.