Control of the stereochemistry of C14 hydroxyl during the total synthesis of withanolide E and physachenolide C.

The stereochemical outcome of the epoxidation of Δ 14-15 cholestanes with mCPBA is controlled by the steric bulk of a C17 substituent. When the C17 is in the β configuration, the epoxide is formed in the α face, whereas if the C17 is trigonal (flat) or the substituent is in the α configuration, the epoxide is formed in the β face. The presence of a hydroxyl substituent at C20 does not influence the stereochemical outcome of the epoxidation.