Stereoselective synthesis of γ-butyrolactones via organocatalytic annulations of enals and keto esters

4,5,5-Trisubstituted gamma-butyrolactones bearing two stereocenters including one quaternary carbon center have been synthesized in excellent yields via N-heterocyclic carbene-catalyzed annulations of enals and ketoesters. Chiral N-heterocyclic carbenes were used to tune the diastereoselectivity up to an 83/17 cis/trans ratio and the enantioselectivity up to 78% ee (trans isomer).