Stereoselective Construction of Halogenated Quaternary Carbon Centers by Brønsted Base Catalyzed [4+2] Cycloaddition of α-Haloaldehydes.

Asymmetric construction of halogenated quaternary carbon centers under mild reaction conditions remains challenging. Reported here is an unprecedented and highly stereoselective Brønsted base catalyzed [4+2] cycloaddition between either α-chloro- or α-bromoaldehydes and cyclic enones. The key intermediate, an α-halogenated enolate, is susceptible to dehalogenation and can be stabilized and stereochemically controlled using bifunctional tertiary amines. This method provides facile access to a collection of optically pure bicyclic dihydropyrans having three contiguous stereocenters, including a halogen-bearing quaternary carbon center. Of note, the product can be transformed in situ into densely functionalized spirocyclopropanes in a highly efficient and stereoselective manner.