Synthesis of 16-dehydro-20-oxopregnanes from 17α,20-epoxy-23,24-dinorcholan-22-oic acids. Highly stereospecific oxirane → allyl alcohol isomerization of an epoxycarboxylic acid

A microbial degradation product of natural sterols was converted into traditional precursor of steroid syntheses by a simple sequence. The title isomerization, the key step, was investigated to demonstrate a concerted mechanism, in which a cyclic transition state, involving the oxirane oxygen, the β- and γ-carbon, the γ-proton to be removed and the catalyst coordinated by the carboxylate group, is postulated.