Chiral acyl anion equivalents: asymmetric synthesis of 11-deoxy-ent-prostaglandin intermediates

Conjugate addition of the chiral formyl anion equivalent p-tolyl p-tolylthiomethyl sulphoxide (+)-(S)-(1) to the cyclopentenone derivative (2a) leads to the adduct (3a) in 45% yield with high diastereoselectivity. Reduction of (3a) gives the dithioacetal (4a), an interesting optically pure intermediate in the synthesis of 11-deoxy-ent-prostanoids. The high selectivity, contrasting with a lower selectivity observed using cyclopentenone, is to be ascribed to more severe steric demands in the transition state.