N-Heterocyclic Carbene-Catalyzed Domino Ring-Opening/Redox Amidation/Cyclization Reactions of Formylcyclopropane 1,1-Diesters: Direct Construction of a 6−5−6 Tricyclic Hydropyrido[1,2-<i>a</i>]indole Skeleton

Catalyzed by N-heterocyclic carbenes (NHCs), domino ring-opening/redox amidation/cyclization reactions of the readily available formylcyclopropane 1,1-diesters with 2-chloro-1H-indole-3-carboaldehydes were reported. This methodology provides an efficient and direct construction of a 6−5−6 tricyclic hydropyrido[1,2-a]indole skeleton, which can be potentially applied for the synthesis of several types of polycyclic indole alkaloids.