A highly efficient and recyclable silica-supported tungstic acid (STA) catalyst for the synthesis of pyrano[3,2-c]chromen-5-ones under solvent free conditions

摘要

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. (C) 2017 Elsevier Ltd. All rights reserved.