Brønsted Acid Mediated Alkenylation and Copper-Catalyzed Aerobic Oxidative Ring Expansion/Intramolecular Electrophilic Substitution of Indoles with Propargyl Alcohols: A Novel One-Pot Approach to Cyclopenta[<i>c</i>]quinolines

Abstract

Copper-catalyzed oxidative ring-expansion/intramolecular electrophilic substitution of 3-dienylindoles leading to cyclo­penta­[<i>c</i>]­quinolines and 3-indenyl­indoles under aerobic conditions is described. The precursors, 3-dienylindoles, are formed via Friedel–Crafts alkenylation followed by isomerization reactions of 2-substituted indoles with tertiary propargyl alcohols under Brønsted acid mediation. The methyl (sp³-C) group present in the propargyl alcohol becomes a part of a six-membered ring in the final ring-expansion products, the cyclo­penta­[<i>c</i>]­quinolines which are fluorescence active. Based on these observations, a novel one-pot strategy for ring expansion from indole to cyclo­penta­[<i>c</i>]­quinoline is discovered