Studies of the selective O-alkylation and dealkylation of flavonoids. XXII. A convenient method for synthesizing 3,5,7-trihydroxy-6-methoxyflavones

Article abstract of DOI:10.1248/cpb.45.446

The demethylation of 3,4-dioxygenated 6-methoxy-2- isopropoxyacetophenones was studied and it was found that 4-benzyloxy-6- hydroxy-3-methoxy-2-isopropoxyacetophenone was easily obtained from 4- benzyloxy-3,6-dimethoxy-2-isopropoxyacetophenone by selective demethylation with anhydrous aluminum bromide-sodium iodide in acetonitrile. The acetophenone was converted into 7-benzyloxy-3-hydroxy-6-methoxy-5- isopropoxyflavones by cyclization followed by oxidation with dimethyldioxirane. The isopropoxy group in the flavones was selectively cleaved with anhydrous aluminum chloride via the corresponding tosylates to give 7-benzyloxy-3,5-dihydroxy-6-methoxyflavones, which were converted into the desired 3,5,7-trihydroxy-6-methoxyflavones by hydrogenolysis. The process was suitable as a general method for synthesizing 3,5,7-trihydroxy-6- methoxyflavones and five flavones were synthesized. We also examined the ¹³C-NMR spectra of twelve kinds of 3,5,6,7-tetraoxygenated 4'- methoxyflavones and revised the proposed structure of a natural flavone.

Full text of DOI:10.1248/cpb.45.446

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