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[ c ]quinolines

Abstract

Copper-catalyzed oxidative ring-expansion/intramolecular electrophilic substitution of 3-dienylindoles leading to cyclopenta[c]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[c]quinolines which are fluorescence active. Based on these observations, a novel one-pot strategy for ring expansion from indole to cyclopenta[c]quinoline is discovered.