Stereoselective N-Heterocyclic Carbene Catalyzed Formal [4+2] Cycloaddition: Access to Chiral Heterocyclic Cyclohexenones

The present study reports an asymmetric NHC-catalyzed formal [4+2] cycloaddition of heterocyclic alkenes containing polarized double bond with azolium-dienolate intermediate generated from α-bromo-α,β-unsaturated aldehydes without external oxidation of Breslow intermediate. Heterocyclic cyclohexenones were produced in good isolated yields (typically about 90%) with good stereochemical outcomes (typically dr > 20/1, and 70-99% ee). The synthetic utility of the protocol was exemplified by the scope of heterocyclic alkenes.