Add time:08/07/2019         Source:sciencedirect.com

A simple, efficient, green and solvent-free procedure for the synthesis of pyrano[3,2-c]chromen-5-ones has been developed by multi-component condensation of 4-hydroxycoumarin, aldehydes and (E)-N-methyl-1-(methylthio)-2-nitroethenamine in the presence of a catalytic amount of silica-supported tungstic acid (STA). The reaction transformation presumably occurs via Knoevenagel condensation, Michael addition, imine-enamine tautomerism intramolecular, O-cyclization, elimination of MeSH. The present environmentally benign protocol offers several advantages such as shorter reaction times, a wide range of functional group tolerance, use of an inexpensive heterogeneous catalyst, and high yield of products via a simple experimental and work-up procedure. The catalyst can be recovered and reused for at least four runs without any significant impact on the product yields.

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