Studies of the Selective<i>O</i>-Alkylation and Dealkylation of Flavonoids. XXI. A Convenient Method for Synthesizing 3,5,7-Trihydroxy-6,8-dimethoxyflavones and 5,7-Dihydroxy-3,6,8-trimethoxyflavones
作者:Tokunaru Horie、Takeshi Kitou、Yasuhiko Kawamura、Kazuyo Yamashita
DOI:10.1246/bcsj.69.1033
日期:1996.4
The Friedel–Crafts acetylation with boron trifluoride was studied and it was found that 1-(2,4-dihydroxy-3,5,6-trimethoxyphenyl)ethanone was conveniently synthesized from 2,3,5,6-tetramethoxyphenyl acetate in a mixture of acetic anhydride and acetic acid. 1-[2-Hydroxy-3,5,6-trimethoxy-4-(methoxymethoxy)phenyl]ethanone was cyclized to 7-hydroxy-5,6,8-trimethoxyflavone by using the Baker–Venkataraman transformation. The 7-benzyl ether of the flavone was oxidized with dimethyldioxirane and then treated with a small amount of p-toluenesulfonic acid to give 7-benzyloxy-3-hydroxy-5,6,8-trimethoxyflavone, which was converted into the methyl ether and tosylate. The 5-methoxy group in the methyl ether or tosylate was selectively cleaved with anhydrous aluminum bromide in acetonitrile under mild conditions to give the corresponding 5-hydroxyflavones and the latter compound with a 3-tosyloxy group was smoothly converted into the 3,5-dihydroxyflavone by hydrolysis with potassium carbonate in methanol. The hydrogenolysis of the 7-benzyloxy-3,5-dihydroxyflavone and its 3-methyl ether afforded quantitatively 3,5,7-trihydroxy-6,8-dimethoxyflavone and its 3-methyl ether. The process was suitable as a general method for synthesizing 3,5,7-trihydroxy-6,8-dimethoxyflavones and their 3-methyl ethers and the six flavones were synthesized for the clarification of their 1H and 13C NMR, MS, and UV spectral properties.
研究了三氟化硼的 Friedel-Crafts 乙酰化反应,发现在乙酸酐和乙酸的混合物中,可以方便地从 2,3,5,6- 四甲氧基苯基乙酸酯合成 1-(2,4-二羟基-3,5,6-三甲氧基苯基)乙酮。1-[2-羟基-3,5,6-三甲氧基-4-(甲氧基甲氧基)苯基]乙酮通过贝克-文卡塔拉曼转化环化成 7-羟基-5,6,8-三甲氧基黄酮。黄酮的 7-苄基醚被二甲基二氧环己烷氧化,然后用少量对甲苯磺酸处理,得到 7-苄氧基-3-羟基-5,6,8-三甲氧基黄酮,并将其转化为甲醚和对甲苯磺酸盐。在温和的条件下,用无水溴化铝在乙腈中选择性地裂解甲醚或对甲苯磺酸盐中的 5-甲氧基,得到相应的 5-羟基黄酮,而后一种带有 3-对甲苯磺酸盐基团的化合物在甲醇中用碳酸钾水解,顺利地转化为 3,5-二羟基黄酮。通过氢解 7-苄氧基-3,5-二羟基黄酮及其 3-甲基醚,可定量得到 3,5,7-三羟基-6,8-二甲氧基黄酮及其 3-甲基醚。该方法适用于合成 3,5,7-三羟基-6,8-二甲氧基黄酮及其 3-甲基醚,合成的六种黄酮的 1H 和 13C NMR、MS 和 UV 光谱特性得到了澄清。