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

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