Stereoselection in the aprotic conjugate addition reactions of lithiated 1-Isobutylbut-2-enyl sulfoxide, sulfone and phosphine oxide with cyclopent-2-enone and Methylcyclopent-2-enone

Lithiated 4-(t-butylsulfinyl)- and 4-(t-butylsulfonyl)-6-methylhept-2-ene, and [(E)-l-isobutylbut- 2-enyl]diphenylphosphine oxide, undergo conjugate addition with cyclopent-2-enone and 2-methylcyclopent-2-enone to deliver mixtures of diastereomers arising from reaction through C 3 of the allyl system. The relative configuration of the major product obtained from the lithiated phosphine oxide and 2-methy!cyclopent-2-enone has been determined by an X-ray crystallographic study. The proportions of diastereomers formed are rationalized in terms of carbanions which have a low barrier to rotation about the C1-C2 bond of the allyl system, and steric interactions prevailing in the trans-decalyl and related transition states experienced by the reactions. The reaction of the lithiated phosphine oxide with 2- methylcyclopent-2-enone has a diastereoselection useful for synthetic approaches towards vitamin D derivatives.