N-Alkylation-Initiated Redox-Neutral [5 + 2] Annulation of 3-Alkylindoles with o-Aminobenzaldehydes: Access to Indole-1,2-Fused 1,4-Benzodiazepines

Described herein is an unprecedented N-alkylation-initiated redox-neutral [5 + 2] annulation of 3-alkylindoles with o-aminobenzaldehydes via a cascade N-alkylation/dehydration/[1,5]-hydride transfer/Friedel–Crafts alkylation sequence. A series of indole-1,2-fused 1,4-benzodiazepines are facilely constructed in moderate to good yields in one step. This protocol features excellent regioselectivity, metal-free conditions, high step economy, and wide substrate scope.