[4+2] Cycloaddition of α,β-Unsaturated Aldehydes with Chalcones Using NHC (Bode Catalyst 2) and Acid Co-catalyst

Chi et al. have reported a [4+2] cycloaddition of α,β-unsaturated aldehydes and chalcones with the NHC catalyst (Bode catalyst 2) to give the corresponding lactones selectively. In previous reports, reaction of α,β-unsaturated aldehydes with NHC catalysts afforded homoenolate, which cyclized with chalcones to give five membered cyclic compounds. In this report, they demonstrated the addition of an acid co-catalyst such as acetic acid to homoenolates since it gives less electrophilic enol intermediates. The enols react with chalcones through [4+2] cycloaddition to form the corresponding lactones with high enantioselectivity, which cannot easily be prepared using other approaches with NHCs.