Studies of the Selective O-Alkylation and Dealkylation of Flavonoids. IX. A New Method for Synthesizing 3, 5-Dihydroxy-7, 8-dimethoxyflavones from 3-Hydroxy-5, 7, 8-trimethoxyflavones

2-Hydroxy-3, 4, 6-trimethoxy-ω- (ρ-methoxybenzoyloxy) acetophenone was prepared from 2, 3, 4, 6-tetramethoxyacetophenone via the corresponding ω-bromoacetophenone. Cyclization of the acetophenone with ρ-methoxybenzoic anhydride by application of the Allan-Robinson reaction afforded 3-hydroxy-4', 5, 7, 8-tetramethoxyflavone along with 2- (α-hydroxy-4-methoxybenzylidene) -4, 6, 7-trimethoxycoumaranone. The 5-methoxy group in the tosylate or mesylate of the 3-hydroxyflavone was selectively cleaved with anhydrous aluminum bromide in acetonitrile to give the corresponding 5-hydroxyflavone, which was converted into 3, 5-dihydroxy-4', 7, 8-trimethoxyflavone with anhydrous potassium carbonate in methanol. 3, 3', 4', 5-Tetrahydroxy-7, 8-dimethoxyflavone was also synthesized by a similar method. The demethylation of the 5-methoxy group in flavones with a 3-hydroxy group is applicable to the synthesis of 3, 5-dihydroxyflavones.