N-Heterocyclic Carbene Catalyzed Synthesis of δ-Sultones via α,β-Unsaturated Sulfonyl Azolium Intermediates

A limited array of reactive intermediates have enabled a wealth of discoveries in N-heterocyclic carbene organocatalysis. In this study, the viability of alpha,beta-unsaturated sulfonyl azoliums as double electrophiles in new reactions is examined. Specifically, the (3+3) annulation of such species with the trimethylsilyl enol ethers of various 1,3-dicarbonyl compounds has been developed. This reaction provides access to a range of novel unsaturated delta-sultones (18 examples) in good yields (40-88%) under mild reaction conditions. Mechanistic studies and the development of an enantioselective variant (55% yield, 73:27 e.r.) support the intermediacy of an alpha,beta-unsaturated sulfonyl azolium species.