Green Synthesis of Biscoumarins Using Phthalimide-N-sulfonic Acid

A simple one-pot reaction was carried out using phthalimide-N-sulfonic acid (PISA) as a solid catalyst for the synthesis of a variety of biscoumarins. An ethanol-water system was found to be the most effective reaction solvent for the reaction between 4-hydroxycoumarin and some (hetero)aromatic or aliphatic aldehydes. PISA effectively catalysed the synthesis of 3,3 - (arylmethylene)-bis-(4-hydroxycoumarins) in the range of 25-60 minutes. The mild conditions applied as well as the high yields obtained are significant features of the reaction. This method avoids the use of tedious work-up and column chromatographic purification of products, making the method expedient and superior. The efficient, cost-effective and eco-friendly catalyst could be recovered and reused several times.