Stereoselective Conjugate Addition of Alkyl Groups to (S)-4-(tert-Butyldimethylsilyloxy)-2-cyclopentenone Derivatives

Reaction of (S)-2-benzenesulfonyl-4-(tert-butyldimethylsilyloxy)-2-cyclopentenone (2) and a 2-methoxycarbonyl congener (3) with R2CuLi or RMgBr-CuI stereoselectively gave (2S,3R,4S)-3-alkyl-2-benzenesulfonyl-4-(tert-butyldimethylsilyloxy)cyclopentanone (4) and its (2S,3S,4S)-2-methoxycarbonyl derivative (5) as a major diastereoisomer. On the other hand, reaction with R3Al in toluene exclusively gave the corresponding 3,4-cis-adducts 6 and 7.