Asymmetric synthesis of cyclopenta[b]indoles via organocatalytic formal (3 + 2) cyclization of β-keto ester with azonaphthalene

Abstract

Enantioselective formal (3 + 2) cyclization of cyclic β-keto esters with azonaphthalenes has been established. A range of cyclopenta[b]indoles have been synthesized in good yields (up to 99% yield) with high diastereo- and enantioselectivity (up to 96% ee, >19 : 1 dr) by using guanidine-amides as catalysts under mild reaction conditions. A bifunctional hydrogen-bond activation model was rationalized for the origin of enantioselectivity.