Conjugate addition reactions of dimethylhydrazone anions and β-keto ester dianions to a chiral sterically hindered vinyl sulfone are reported. Further functionalization of the resulting α-sulfonyl-stabilized anion by alkylation with allyl bromide followed by reductive desulfonylation, ozonolysis, and intramolecular aldol reaction provides an enolized α-carboalkoxy enone, the net result being a new annulation sequence for the synthesis of seven-membered rings. Determination of the relative stereochemistry of conformationally mobile cis- and trans-fused bicyclo[5.4.0] systems by combined Karplus/molecular mechanics calculations is described. Cyclization of a pair (40, 3) of chiral Z dienes via an intramolecular Diels-Alder reaction stereospecifically affords chiral polycyclic adducts (41, 4) that are potential precursors for cytochalasin C. © 1982, American Chemical Society. All rights reserved.