Stereoconvergent Conjugate Addition of Arylboronic Acids to α-Angelica Lactone Derivatives: Synthesis of Stereochemically Complex γ-Butyrolactones

Catalyzed stereoconvergent 1,4-additions to unsaturated carbonyls are rare but of high potential value. This letter details the development of enantioselective arylation reactions of boronic acids and beta,gamma-butenolides. These reactions are catalyzed by commercially available hydroxy[(S)-BINAP]-rhodium(I) dimer to afford stereochemically complex gamma-butyrolactone derivatives. The reaction products provide functionality amenable to further manipulation and can lead to products with up to three contiguous stereocenters. The reaction proceeds under a dynamic kinetic resolution manifold by isomerizing the achiral starting material into an interconverting mixture of enantiomeric conjugate acceptors, followed by catalyst -controlled, enantiomer-selective 1,4-addition. Base -promoted racemization of the intermediate alpha,beta-butenolide is possible due to the high kinetic and thermodynamic acidity of the gamma-proton.