NHC-Catalyzed Conjugate Addition Involving α,β-Unsaturated Acyl Azoliums

Significance

Lupton and co-workers report a conjugate addition involving α,β-unsaturated acyl azoliums catalyzed by N-heterocyclic carbenes (NHCs). A variety of conjugate acceptors including those having β-disubstitution reacted easily with a range of enolates affording the corresponding ­dihydropyranones in good yields. The proposed mechanistic pathway suggests a fragmentation of the α,β-unsaturated enol ester to enolate I and α,β-unsaturated carboxylate II. An intermolecular version of this transformation is also possible.