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

Abstract

Lithiated 4-(t- butylsulfinyl)- and 4-(t-butylsulfonyl )-6-methylhept-2-ene, and [(E)-1-isobutylbut-2-enylldiphenylphosphine oxide, undergo conjugate addition with cyclopent-2-enone and 2-methylcyclopent-2-enone to deliver mixtures of diastereomers arising from reaction through C3 of the allyl system. The relative configuration of the major product obtained from the lithiated phosphine oxide and 2-methylcyclopent-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.