Organocatalyzed enantioselective [3 + 3] annulation for the direct synthesis of conformationally constrained cyclic tryptophan derivatives

Abstract

An enantioselective formal [3 + 3] annulation of 1-methylindoline-2-thiones and 4-arylmethylideneoxazolin-5(4H)-ones has been developed by the use of an L-tert-leucine-derived bifunctional tertiary amine-squaramide catalyst, which furnished a series of optically active conformationally strained β-branched cyclic tryptophan derivatives in acceptable yields with good to excellent diastereo- and enantioselectivities.