Stereoselective [3+2] Carbocyclization of Indole-Derived Imines and Electron-Rich Alkenes: A Divergent Synthesis of Cyclopenta[b]indole or Tetrahydroquinoline Derivatives.

An unprecedented stereoselective [3+2] carbocyclization reaction of indole-2-carboxaldehydes, anilines, and electron-rich alkenes to obtain cyclopenta[b]indoles is disclosed. This pathway is different from the well-established Povarov reaction: the formal [4+2] cycloaddition involving the same components, which affords tetrahydroquinolines. Moreover, by simply changing the Brønsted acid catalyst, this multicomponent coupling process could be divergently directed towards the conventional Povarov pathway to produce tetrahydroquinolines or to the new pathway (anti-Povarov) to generate cyclopenta[b]indoles. Supported by computational studies, a stepwise Mannich/Friedel-Crafts cascade is proposed for the new anti-Povarov reaction, whereas a concerted [4+2] cycloaddition mechanism is proposed for the Povarov reaction.