1983 Volume 56 Issue 12 Pages 3773-3780
The demethylation of seven 8-hydroxy-5,7-dimethoxyflavones with anhydrous aluminum chloride or bromide in acetonitrile was studied and the following results were obtained. (1) Anhydrous aluminum chloride in acetonitrile selectively split the 5-methoxyl group without cleavage of the 7-methoxyl group, and seven 5,8-dihydroxy-7-methoxy-flavones were quantitatively synthesized by the demethylation. (2) In the demethylation with anhydrous aluminum bromide, the 5- and 7-methoxyl groups on 4′,5,7-trimethoxy- and 3′,4′,5,7-tetramethoxy-8-hydroxyflavone were selectively split to give the corresponding 5,7,8-trihydroxyflavones in good yield. However, the demethylation was not accessible to the synthesis of 5,7,8-trihydroxy-3′,4′,5′-trimethoxyflavone, 4′,5,7,8-tetrahydroxy-3′-methoxy-, and 3′,5,7,8-tetrahydroxy-4′-methoxyflavone, because of the cleavage of the methoxyl groups on the B ring. The synthesized flavones were employed for the identification of the two natural flavones, which were proposed to be 4′,5,8-trihydroxy-7-methoxyflavone and 4′,5,8-trihydroxy-3′,7-dimethoxyflavone. The former, salvitin, was revised to 4′,5,7-trihydroxy-6-methoxyflavone and the latter seemed to be 4′,5,7-trihydroxy-3′,8-dimethoxyflavone.
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