Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. XXIV. A Convenient Method for Synthesizing 6- and 8-Methoxylated 5,7-Dihydroxyisoflavones

摘要

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 methanol and the resultant products were converted into 7-hydroxy-5,8-dimethoxyisoflavones (8) by hydrogenolysis, followed by cyclization. The isoflavones were quantitatively demethylated to 5,7-dihydroxy-8-methoxyisoflavones (2) via their acetates. The isomeric 5,7-dihydroxy-6-methoxyisoflavones (1) were also synthesized from the chalcones, obtained from 2,3-dimethoxy- (16) or 2-isopropoxy-3-methoxy-4,6-bis(benzyloxy)acetophenones (21), by a similar method. On the other hand, the isoflavones with two hydroxy groups at the 2'- and 4'-positions were easily synthesized by the following method. Treatment of the rearranged product from 2,2',4,4'-tetrakis(benzyloxy)-3'6'-dimethoxychalcone (5f) with hydrochloric acid (HCl) in acetic acid afforded 2',4',7-tris(benzyloxy)-5,8-dimethoxyisoflavone (10f). The 5-methoxy group in the isoflavone was quantitatively cleaved to give the corresponding 5-hydroxyisoflavone (11f), which was isomerized to 2',4',7-tris(benzyloxy)-5-hydroxy-6-methoxyisoflavone (25f) in the presence of anhydrous potassium carbonate. Hydrogenolysis of the two 5-hydroxyisoflavones proceeded smoothly to give 2',4',5,7-tetrahydroxy-8-(2f) and 6-methoxyisoflavones (1f), respectively. The ~(13)C-NMR spectra of these isoflavones supported the proposed structures of polyhydroxyisoflavones. The proposed structures of two natural isoflavones were revised.