Diastereo- and Enantioselective Construction of the Hexahydrocoumarin Scaffold via an Organocatalytic Asymmetric [3 + 3] Cyclization.

The first catalytic asymmetric construction of the biologically important hexahydrocoumarin scaffold has been established, which takes advantage of chiral thiourea-tertiary amine-catalyzed enantioselective transformations. Besides, this reaction also realized the first catalytic asymmetric [3 + 3] cyclization of 4-arylidene-2-aryloxazol-5(4H)-ones with cyclohexane-1,3-diones, which afforded structurally diverse 3-aminohexahydrocoumarin derivatives in excellent diastereoselectivities and high enantioselectivities (all >95:5 dr, up to 96:4 er). The investigation on the activation mode suggested that the chiral thiourea-tertiary amine catalyst simultaneously activated the two substrates via hydrogen-bonding interaction. Moreover, this reaction could be applied to a large scale synthesis of enantioenriched hexahydrocoumarin. This approach will not only provide an efficient method for the construction of the chiral hexahydrocoumarin scaffold but also enrich the research areas of asymmetric organocatalysis and catalytic enantioselective [3 + 3] cyclizations.