The lithium enolates arising by aprotic conjugate addition of lithiated but-2-enyl sulfoxides and a phosphine oxide to cyclopent-2-enone react efficiently with methyl cyanoformate to give the corresponding methyl 5-oxocyclopentanecarboxylates in good yields. The sulfoxides are smoothly deoxygenated with tributylphosphine diiodide in ether at 0º, and the resulting sulfides undergo intramolecular cyclization in the presence of tin(IV)chloride to give methyl oxobicyclo [3.3.0] octanecarboxylates , again in good to excellent yields. The cyclizations are highly stereoselective in that the syn- and anti-stereochemistries of the enolate trapped products obtained from (E)-and (Z)- sulfoxides are quantitatively reflected in the products obtained from the cyclization reactions.