STUDIES OF THE SELECTIVE O-ALKYLATION AND DEALKYLATION OF FLAVONOIDS .23. DEMETHYLATION BEHAVIORS OF 6-HYDROXY-4',7-DIMETHOXY-5-TOSYLOXYFLAVONES WITH ANHYDROUS ALUMINUM HALIDES IN ACETONITRILE

In the demethylation of 6-hydroxy-3,4',7-trimethoxy-5-tosyloxyflavone (1) with anhydrous aluminum bromide (AlBr3) or anhydrous aluminum chlo ride-sodium iodide (AlCl3-NaI) in acetonitrile, the elimination of the 5-tosyloxy group proceeded after demethylation to give 8-bromo-3,6,7- trihydroxy-4'-methoxyflavone (6) or 3,6,7-trihydroxy-4'-methoxyflavone (5) as a main product, The demethylation of 6-hydroxy-4',7-dimethoxy- 5-tosyloxyflavone (2) with AlCl3-NaI also afforded 6,7-dihydroxy-4'-me thoxyflavone (12), but that with AlBr3 afforded 8-bromo-5,6,7-trihydro xy-4'-methoxyflavone (13) as a main product. The demethylation of 1 wi th anhydrous aluminum chloride (AlCl3) was accompanied by migration of the tosyl group to give a mixture of 3,6-dihydroxy-7,4'-dimethoxy-5-t osyloxyflavone (3) and 5-hydroxy-3,4',7-trimethoxy-6-tosyloxyflavone ( 9), but that of 2 proceeded after the cleavage of the 5-tosyloxy group to give a mixture of 5,6-dihydroxy-7,4'-dimethoxy- (14) and 5,6,7-tri hydroxy-4'-methoxyflavones (15) other than the corresponding 6-tosylox yflavone (16), In the demethylation of the acetates of 1 and 2 with Al Cl3, the cleavage of the 5-tosyloxy group proceeded prior to the demet hylation to afford the corresponding 5,6,7-trihydroxyflavones (8 and 1 5), although the demethylation of the former acetate was accompanied b y formation of 3,5,7-trihydroxy-4'-methoxy-6-tosyloxyflavone (10). Mec hanisms are proposed for these reactions.