Catalytic Asymmetric Vinylogous Conjugate Addition of Butenolide to 2-Ester-Substituted Chromones: Access to Chiral Chromanone Lactones via Trapping of a Copper(I) Enolate by Trimethyl Borate.

A copper-catalyzed asymmetric vinylogous conjugate addition of butenolide to 2-ester-substituted chromones is described, and it delivers syn - or anti -chromanone lactones with high stereoselectivities. The enantioselectivity-determining step varied with the use of B(OMe) 3 as an additive, resulting in enhanced stereoselectivities, as revealed by density functional theory calculations, which also provided theoretical insight into the origin of the ligand-dependent diastereodivergence.