STUDIES OF THE SELECTIVE O-ALKYLATION AND DEALKYLATION OF FLAVONOIDS - XXIV - A CONVENIENT METHOD FOR SYNTHESIZING 6-METHOXYLATED AND 8-METHOXYLATED 5,7-DIHYDROXYISOFLAVONES

2',4'-Bis(benzyloxy)-3',6'-dimethoxychalcones (5), which were obtained from the dibenzyl ether of 2, 4-dihydroxy-3,6-dimethoxyacetophenone ( 3), were oxidatively rearranged with thallium (III) nitrate In methano l and the resultant products were converted into 7-hydroxy-5,8-dimetho xyisoflavones (8) by hydrogenolysis, followed by cyclization. The isof lavones were quantitatively demethylated to 5,7-dihydroxy-8-methoxyiso flavones (2) via their acetates. The isomeric 5,7-dihydroxy-6-methoxyi soflavones (1) were also synthesized from the chalcones, obtained from 2,3-dimethoxy- (16) or opropoxy-3-methoxy-4,6-bis(benzyloxy)acetophen ones (21), by a similar method, On the other hand, the isoflavones wit h two hydroxy groups at the 2'- and 4'-positions were easily synthesiz ed by the following method, Treatment of the rearranged product from 2 ',4,4'-tetrakis(benzyloxy)-3'6'-dimethoxychalcone (5f) with hydrochlor ic acid (HCl) in acetic acid afforded 2',4',7-tris(benzyloxy)-5,8-dime thoxyisoflavone (10f). The 5-methoxy group in the isoflavone nas quant itatively cleaved to give the corresponding 5-hydroxyisoflavone (11f), which was isomerized to ,4',7-tris(benzyloxy)-5-hydroxy-6-methoxisofl avone (25f) in the presence of anhydrous potassium carbonate, Hydrogen olysis of the two 5-hydroxyisoflavones proceeded smoothly to give 2',4 ',5,7-tetrahydroxy-8-(2f) and 6-methoxyisoflavones (1f), respectively, The C-13-NMR spectra of these isoflavones supported the proposed stru ctures of polyhydroxyisoflavones. The proposed structures of two natur al isoflavones were revised.