Formal Enantioselective [4+3] Cycloaddition by a Tandem Diels–Alder Reaction/Ring Expansion

The tandem Diels–Alder reaction/ring expansion between cyclopentadiene and unsaturated aldehydes is a highly stereoselective process for the synthesis of bicyclo[3.2.1]octenones, the formal products of a [4+3] cycloaddition. When the initial cycloaddition was conducted in the presence of the chiral Lewis acid catalyst 14, the bicyclo[3.2.1]octenones were obtained in two steps in 72–85 % yields with up to 96 % ee and>98 % de.