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

Article abstract of DOI:10.1248/cpb.44.486

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-methoxyisoflavones. 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 isoflavones were examined and some natural isoflavones were identified.