A New Stereoselective Synthesis of (20R)- and (20S)-Steroidal Side Chains.

Reaction of (E)-ethyl 3β-t-butyldimethylsiloxypregna-5,17-dien-21-oate with lithium di-isopropylamide followed by alkyl halides results in the predominant formation of (20S)-alkylation products in >88% isolated yields; a synthetic application to both (20R)- and (20S)-steroidal side chains is also described.