Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. XIV. A Convenient Method for Synthesizing 5,6,7-Trihydroxy-3-methoxyflavones from 6-Hydroxy-3,5,7-trimethoxyflavones

Demethylation of 6-hydroxy-3,5,7-trimethoxyflavones and their acetates was studied and the following results were found. Demethylation of 6-hydroxy-3,4′,5,7-tetramethoxyflavone with 30% anhydrous aluminum chloride in acetonitrile afforded a mixture of 5,6-dihydroxy-3,4′,7-trimethoxyflavone and 5,6,7-trihydroxy-3,4′-dimethoxyflavone, but demethylation of its acetate formed the 5,6,7-trihydroxyflavone as a main product. The latter reaction was applicable as a general method for synthesizing 5,6,7-trihydroxy-3-methoxyflavones. On the other hand, the demethylation of 6-hydroxy-3,4′,5,7-tetramethoxyflavone with 10% anhydrous aluminum bromide in acetonitrile afforded the 5,6,7-trihydroxyflavone as a main product without cleavage of the 4′-methoxyl group and the reaction was more conveniently applicable for the synthesis of the 5,6,7-trihydroxyflavones. The method, however, was not adapted to the synthesis of the flavones with methoxyl groups adjacent to the hydroxyl group on the B ring, since the benzyloxyl group and the methoxyl group adjacent to the hydroxyl group were simultaneously cleaved under demethylating conditions. Seven 5,6,7-trihydroxy-3-methoxyflavones were synthesized by these methods and their properties were clarified.