Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. XXIV. A Convenient Method for Synthesizing 6- and 8-Methoxylated 5,7-Dihydroxyisoflavones.
作者:Tokunaru HORIE、Kenichi SHIBATA、Kazuyo YAMASHITA、Kenichi FUJII、Masao TSUKAYAMA、Yoshizumi OHTSURU
DOI:10.1248/cpb.46.222
日期:——
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(bezyloxy)-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 13C-NMR spectra of these isoflavones supported the proposed structures of polyhydroxyisoflavones. The proposed structures of two natural isoflavones were revised.
自2,4-二羟基-3,6-二甲氧基苯乙酮二苄基醚(3)得到的2',4'-双(苄氧基)-3',6'-二甲氧基查耳酮(5),在甲醇中和硝酸铊(III)氧化重排,产物用氢解还原,闭环得到7-羟基-5,8-二甲氧基异黄酮(8)。这些异黄酮以它们的乙酸盐的接触方法定量地脱甲基,得到5,7-二羟基-8-甲氧基异黄酮(2)。5,7-二羟基-6-甲氧基异黄酮(1)以从2,3-二甲氧基或2-异丙氧基-3-甲氧基-4,6-双(苄氧基)苯乙酮(21)得到的查耳酮为原料,通过相似的方法制得。巯豆另一方面,2'和4'位带有二羟基的异黄酮很易通过下述方法合成。2,2',4,4'-四(苄氧基)-3',6'-二甲氧基查耳酮(5f)的重排产物,在醋酸中用盐酸处理,得到2',4',7-三(苄氧基)-5,8-二甲氧基异黄酮(10f).异黄酮(10f)中的5-甲氧基很快裂解得到相应的5-羟基异黄酮(11f),在无水碳酸钾存在下异构成2',4',7-三(苄氧基)-5-羟基-6-甲氧基异黄酮(25f)。两个5-羟基异黄酮中的苄基很容易氢解,分别得到2',4',5,7-四羟基-8-(2f)和6-甲氧基异黄酮(1f)。这些异黄酮的'C-NMR谱支持了多羟基异黄酮的分子结构的推定。两个天然异黄酮的分子结构进行了修订。