STUDIES OF THE SELECTIVE O-ALKYLATION AND DEALKYLATION OF FLAVONOIDS .13. AN IMPROVED METHOD FOR SYNTHESIZING 5,6,7-TRIHYDROXYFLAVONES FROM 6-HYDROXY-5,7-DIMETHOXYFLAVONES

The demethylation of five 6-hydroxy-5,7-dimethoxyflavones 1 and their acetates with 30% w/v anhydrous aluminum chloride in acetonitrile was studied, and the following results were found. In the demethylation of 6-hydroxy-4',5,7-trimethoxyflavone (1a), 5,6-dihydroxy-4',7-dimethoxyflavone (2a) and 5,6,7-trihydroxy-4'-methoxyflavone (3a) were produced. Although the ratio of the two products varied according to the amount of aluminum chloride used, it became constant after 12-24 h because the cleavage of the 7-methoxy group in 2a was suppressed by iminoesterification of the 6-hydroxy group. In contrast, the demethylation of the 5- and 7-methoxy groups of acetate 4a proceeded smoothly by the process shown in Scheme II. The amount of 3a increased with increasing reaction time to give 3a as the main product after 36-48 h. The same phenomena were observed in the demethylation of the other 6-hydroxyflavones 1b-1e and their acetates 4b-4e. The demethylation of the acetates is widely applicable as a general method for synthesizing 5,6,7-trihydroxyflavones because the protection of hydroxy groups also suppresses the further cleavage of the methoxy group adjacent to the acetoxy group on the B ring of compounds such as 4d and 4e.