Electrochemical [3 + 2]/[4 + 2] cyclization to indole-fused polycyclics†

Abstract

The preparation of highly complex indole-fused polycyclics has been in high demand because of their many biological activities. The electrochemical coupling of the radical intermediates with adjacent indole rings has been recognized as a green, straightforward, and powerful methodology for constructing these compounds. Electrochemical coupling is characterized by high atom and step economy under mild and exogenous oxidant-free conditions I. However, this methodology is also challenging and underdeveloped because of the low oxidation potential of indole rings, which, therefore, easily decompose under electrolysis conditions. Herein, we report straightforward electrosynthesis of indole-fused polycyclics achieved by carrying out radical domino couplings between N-methacryloylindoles and α-allyl-substituted methylenes. This electrolysis proved to be efficient in open air with good functional group tolerance, which meant that stringent removal of H₂O and oxygen was not necessary.