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 (sp(3)-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.

Cyclopenta[c]quinolines. -The method is not applicable to N-unprotected indole derivatives. The products exhibit strong fluorescence. -(GANGADHARARAO, G.; URUVAKILLI, A.; SWAMY*, K. C. K.; Org. Lett. 16 (2014) 23, 6060-6063, http://dx.