Enantioselective N-Heterocyclic Carbene-Catalyzed Annulations of 2-Bromoenals with 1,3-Dicarbonyl Compounds and EnaminesviaChiral α,β-Unsaturated Acylazoliums

Abstract

The N-heterocyclic carbene (NHC)-catalyzed generation of chiral α,β-unsaturated acylazoliums from 2-bromoenals followed by their interception with 1,3-dicarbonyl compounds or enamines, the formal [3+3] annulation reaction, is reported. The reaction results in the enantioselective synthesis of synthetically and medicinally important dihydropyranones and dihydropyridinones, and tolerates a wide range of functional groups. It is noteworthy that the reaction takes place under mild reaction conditions utilizing relatively low catalyst loadings. In addition, based on DFT calculations, a mechanistic scenario involving the attack of the nucleophile from below the plane of the α,β-unsaturated acylazoliums, and the mode of enantioinduction is presented.