Silica Supported Zr(HSO4)4 Catalyzed Solvent-free Synthesis of [1]Benzopyrano[ 4,3-b][1]benzopyran-6-ones and Xanthenones

A clean and efficient procedure is described for the synthesis of 7-(4-hydroxy-2-oxo-2H-1-benzopyran-3-yl)- 6H,7H-[1]benzopyrano[4,3-b][1]benzopyran-6-ones and 2-(2,3,4,9-tetrahydro-3,3-dimethyl-1-oxo-1H-xanthen-9-yl)-3- hydroxy-5,5-dimethyl-2-cyclohexen-1-ones from a quasi-Knoevenagel-Michael reaction sequence of salicylaldehydes with compounds having active methylene groups [4-hydroxycoumarin (4-hydroxy-2H-1-benzopyran-2-one) and dimedone (5,5-dimethyl-1,3-cyclohexanedione)] in a ratio of 1:2, in the presence of a catalytic amount of 60% Zr(HSO4)4/SiO2 as an efficient heterogeneous catalyst at 60 °C and under solvent-free conditions.