Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. XI. A New Convenient Method for Synthesizing 3,5,7-Trihydroxy-8-methoxyflavones from 7-Hydroxy-3,5,8-trimethoxyflavones

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Abstract
The 3- or 5-methoxyl group in the ditosylates of 5,7-dihydroxy-3,4′,8-trimethoxy- and 3,7-dihydroxy-4′,5,8-trimethoxyflavones was quantitatively demethylated with anhydrous aluminum bromide in acetonitrile to give the corresponding monohydroxyflavones, which were easily hydrolyzed into 3,5,7-trihydroxy-4′,8-dimethoxyflavone. On the basis of the results, the following convenient method for synthesizing 3,5,7-trihydroxy-8-methoxyflavones from 7-hydroxy-3,5,8-trimethoxyflavones was established. 7-Hydroxy-3,5,8-trimethoxyflavones or their tosylates were demethylated with 5% w/v anhydrous aluminum bromide in acetonitrile to give a mixture of the corresponding 3- and 5-hydroxyflavones. The mixture was tosylated and the resultant mixture was demethylated under the same conditions. The demethylated products were hydrolyzed with anhydrous potassium carbonate in methanol to give the desired 3,5,7-trihydroxy-8-methoxyflavones in high yields.

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