A Study of the Reactivity of Polyhydroxylated Sterol Derivatives

A detailed study of regiospecific silylation, oxidation, and reduction of sterols is reported. The different reactivity of 2α and 3α hydroxy groups is demonstrated on a 2α,3α-dihydroxy-5α-pregnane-6,20-dione as a model substrate. A regiospecific silylation of the 2α-alcohol occurred when silyl chloride was used under mild reaction conditions. The selective oxidation of one of the 2α,3α-diols by dimethyldioxirane or the Dess–Martin periodinane led to a mixture of α-hydroxyketones with high prevalence of 3,6,20-trione over 2,6,20-trione in the ratio 10:1. A regiospecific reduction of a model substrate by lithium tri-tert-butoxyaluminium hydride gives the C6 alcohol.