Synergistic Catalysis for Asymmetric [3 + 2] Cycloadditions of 2-Indolylmethanols with α,β-Unsaturated Aldehydes

A catalytic asymmetric [3 + 2] cycloaddition of 2-indolylmethanols with α,β-unsaturated aldehydes was developed for the first time. This transformation was achieved by a synergistic catalytic system consisting of a palladium complex, a Brønsted acid, and a chiral secondary amine to synthesize biologically active cyclopenta[b]indole derivatives with excellent diastereo- and enantioselectivities (up to >20:1 dr, up to 99% ee).