Access to Polycyclic Indole-3,4-Fused Nine-Membered Ring via Cascade 1,6-Hydride Transfer/Cyclization.

A cascade aldimine condensation/1,6-hydride transfer/Mannich-type cyclization of indole-derived phenylenediamine with aldehydes was developed for one-step construction of a polycyclic indole-3,4-fused skeleton. Aldehyde serves as a key to start the whole process, including 1,6-hydride transfer enabled δ-C(sp3)-H activation of the secondary amine. The challenges of construction of medium-sized rings are addressed via hydride transfer chemistry.