Multicomponent reactions for facile access to coumarin-fused dihydroquinolines and quinolines: synthesis and photophysical studies

Abstract

A simple and straightforward method for facile access to coumarin-fused dihydroquinolines (4) has been developed using the microwave-assisted multicomponent reactions of 4-hydroxycoumarin, aldehydes and aromatic amines in water catalyzed by bismuth triflate. Under solvent-free and conventional heating conditions, the same combination provided the corresponding coumarin-fused quinolines (5). An alternative and rapid method for the conversion of (4) to (5) with very good yields was also developed using N-bromosuccinamide. Single-crystal X-ray crystallographic analysis of one of the products (4q) showed that the products are regioselective and that the reactions proceed via 1,2-addition followed by 6π-electrocyclization instead of a Skraup–Doebner–von Miller type reaction. Substituted quinoline carboxylic acid derivatives (7) were synthesized selectively from (4) by ring opening of the coumarin moiety followed by aromatization using NaOH/DMSO under reflux conditions. Considering the presence of the polycyclic conjugated structure of the synthesized compounds 4 and 5 with the coumarin moiety, preliminary photophysical studies were carried out and promising quantum yields were observed along with a maximum quantum yield (∅_f = 0.65) for 4j.