STUDIES OF THE SELECTIVE O-ALKYLATION AND DEALKYLATION OF FLAVONOIDS .20. A CONVENIENT METHOD FOR SYNTHESIZING 5,6,7-TRIHYDROXYISOFLAVONES AND 5,6-DIHYDROXY-7-METHOXYISOFLAVONES

3',6'-Bis(benzyloxy)-2',4'-dimethoxychalcones which were derived from dibenzyl ether of 3,6-dihydroxy-2,4-dimethoxyacetophenone, were oxidat ively rearranged with thallium(III) nitrate (TTN) in methanol to give )-2,4-dimethoxyphenyl]-3,3-dimethoxypropan-1-ones; these pro ducts wer e converted into 6-hydroxy-5,7-dimethoxyisoflavones by hydrogenolysis followed by cyclization. The 5-methoxy group in the acetates of the is oflavones was selectively cleaved with 5% (w/v) anhydrous aluminum bro mide in acetonitrile to give quantitatively the corresponding 5-hydrox yisoflavones, which were hydrolyzed to 5,6-dihydroxy-7-methoxyisoflavo nes. The acetates were also demethylated to 5,6,7-trihydroxyisoflavone s with 30% (w/v) anhydrous aluminum chloride in acetonitrile at 70 deg rees C for 36-48 h, The spectral properties of these isoflavones were examined and some natural isoflavones were identified.