Solvent-free synthesis of 3-carboxycoumarins

3-Carboxycoumarins, an important class of biologically active compounds, are synthesized in very high yield and in an energy efficient manner, without use of noxious reagents or volatile organic solvents, either by direct reaction of the solid reagents, or in an aqueous slurry. Remarkably, even though the first step in the reaction cascade is a condensation reaction, the reaction proceeds rapidly as a stirred aqueous slurry without application of heat. A reason for this phenomenon is advanced and recycling of the aqueous filtrate post product separation is described. Although the reaction is not 100% atom efficient (1 mole of acetone is evolved for each mole of coumarin produced) it is a significant improvement on previously described methods for the synthesis of such compounds.