Studies of the Selective<i>O</i>-Alkylation and Dealkylation of Flavonoids. XVIII. A Convenient Method for Synthesizing 3,5,6,7-Tetrahydroxyflavones
作者:Tokunaru Horie、Takashi Kobayashi、Yasuhiko Kawamura、Isao Yoshida、Hideaki Tominaga、Kazuyo Yamashita
DOI:10.1246/bcsj.68.2033
日期:1995.7
In the demethylation of 6-hydroxy-3,4′,7-trimethoxy-5-(tosyloxy)flavone with anhydrous aluminum bromide, the 5-tosyloxyl group was eliminated with bromination to give 8-bromo-3,6,7-trihydroxy-4′-methoxyflavone as the main product. When anhydrous aluminum chloride was used in the demethylation of the acetate, the 5-tosyloxyl group was cleaved prior to the demethylation to give 5,6,7-trihydroxy-3,4′-dimethoxyflavone. Demethylation of 6-hydroxy-4′,5,7-trimethoxy-3-(tosyloxy)flavone and its acetate with the bromide or chloride afforded the 5,6,7-trihydroxyflavone without the cleavage of the 3-tosyloxyl group, but was not suitable for the general synthesis of the 3,5,6,7-tetrahydroxyflavones because of the difficulty in removing the protecting group. Consequently, it was found that the direct demethylation of 3,6-dihydroxy-5,7-dimethoxyflavones with anhydrous aluminum chloride–sodium iodide in acetonitrile was the most useful general method for synthesizing 3,5,6,7-tetrahydroxyflavones. Additionally, the reported structures of two natural flavones were revised.
在无水溴化铝的脱甲基反应中,6-羟基-3,4′,7-三甲氧基-5-(托烯氧基)黄酮的5-托烯氧基基团在溴化作用下被消除,生成8-溴-3,6,7-三羟基-4′-甲氧基黄酮作为主要产物。当使用无水氯化铝进行醋酸酯的脱甲基反应时,5-托烯氧基基团在脱甲基前被切断,生成5,6,7-三羟基-3,4′-二甲氧基黄酮。6-羟基-4′,5,7-三甲氧基-3-(托烯氧基)黄酮及其醋酸酯与溴化物或氯化物的脱甲基反应生成5,6,7-三羟基黄酮,而不切断3-托烯氧基基团,但由于去除保护基团的困难,不适合于3,5,6,7-四羟基黄酮的一般合成。因此,发现采用无水氯化铝-碘化钠在乙腈中对3,6-二羟基-5,7-二甲氧基黄酮进行直接脱甲基反应是合成3,5,6,7-四羟基黄酮的最有效的一般方法。此外,修订了两种天然黄酮的已报告结构。