Silica Supported Zr(HSO4)4 Catalyzed Solvent-free Synthesis of Benzopyrano[ 4,3-b]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-benzopyrano[4,3-b]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.
Keywords:  Benzopyrano[4, 3-b] benzopyran-6-ones, Quasi-Knoevenagel-Michael reaction sequence, silica supported
Zr(HSO4)4, solvent-free conditions, xanthenones.
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