Brønsted acidic ionic liquid-catalyzed tandem reaction: an efficient approach towards regioselective synthesis of pyrano[3,2-c]coumarins under solvent-free conditions bearing lower E-factors†
1-Butane sulfonic acid-3-methylimidazolium tosylate, [BSMIM]OTs, is found to be a remarkable catalyst for the tandem cyclization of 4-hydroxycoumarin with chalcones for the syntheses of pyrano[3,2-c]coumarins under solvent-free conditions. The developed protocol is applicable for the construction of biologically important pyranocoumarins from easily accessible chalcones having various substituents. This reaction possibly proceeds through Michael addition followed by cyclization. The feasibility of catalyst recycling is also demonstrated. This method produces only water as the byproduct and represents a green synthetic protocol. The catalytic reaction proceeded very smoothly under solvent-free conditions and showed high regioselectivity. Clean reaction, non-chromatographic purification technique, easily accessible reactants, and metal and solvent-free and environmentally friendly reaction conditions are the notable advantages of this procedure. In addition, this method shows lower E-factors.