Studies of the Selective O-Alkylation and Dealkylation of Flavonoids.XX. A Convenient Method for Synthesizing 5,6,7-Trihydroxyisoflavones and 5,6-Dihydroxy-7-methoxyisoflavones.

Abstract

3', 6'-Bis(benzyloxy)-2', 4'-dimethoxychalcones, which were derived from dibenzyl ether of 3, 6-dihydroxy-2, 4-dimethoxyacetophenone, were oxidatively rearranged with thallium(III) nitrate (TTN) in methanol to give 2-aryl-1-[3, 6-bis(benzyloxy)-2, 4-dimethoxyphenyl]-3, 3-dimethoxypropan-1-ones; these products were converted into 6-hydroxy-5, 7-dimethoxyisoflavones by hydrogenolysis followed by cyclization. The 5-methoxy group in the acetates of the isoflavones was selectively cleaved with 5% (w/v) anhydrous aluminum bromide in acetonitrile to give quantitatively the corresponding 5-hydroxyisoflavones, which were hydrolyzed to 5, 6-dihydroxy-7-methoxy-isoflavones. The acetates were also demethylated to 5, 6, 7-trihydroxyisoflavones with 30% (w/v) anhydrous aluminum chloride in acetonitrile at 70°C for 36-48 h. The spectral properties of these isoflanones were examined and some natural isoflavones were identified.