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Azepinoindole Synthesis via a N-Bromosuccinimide-Induced Cycloisomerization of Enaminoester/Enaminone.

A protocol based on a newly developed N-bromosuccinimide (NBS)-induced cycloisomerization was described to prepare tricyclic azepino[4,5-b]indoles from simple β-enaminoesters or β-enaminones containing an indole unit. A mechanism involving a Pictet-Spengler cyclization, an aziridine ring formation, and a regioselective C-N bond cleavage was proposed to account for the medium-sized ring formation and the migration of electron-withdrawing group (ester, ketone).