32 フラボノール類の接触酸素酸化における置換水酸基の反応に及ぼす影響

概要

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Base catalyzed oxygenation of flavonols has been demonstrated to provide a model for the reaction of quercetinase, a dioxygenase from Aspergillus flavus. However, when hydroxy group is substituted at 7-(5-)position of the flavonols, the oxygenation is found to change the reaction path completely. The oxygenation of 4',7-dihydroxyflavonol in an aqueous alkaline solution gave p-hydroxyphenyl-glyoxylic acid and 2,4-dihydroxybenzoic acid and in an absolute methanol solution containing MeONa gave p-hydroxyphenylglyoxylic acid, methyl p-hydroxyphenylglyoxylate, and methyl 2,4-dihydroxybenzoate. The results are rationalized by assuming that intramolecular decomposition of hydroperoxide intermediate formed in the reaction course is depressed by conjugation of the hydroxy group at 7-position to carbonyl group at 4-position, and resulting triketone intermediate formed by intermolecular oxido-reduction undergoes further oxidative solvolysis. Co-salen catalyzed oxygenation of flavonols in DMF quantitatively gave the corresponding depsides even when hydroxy group is substituted at 7-position, where effect of the oxygen function on the catalysis is reflected only on the reaction rate, which is in order of 4'-OH>7-OH, OH>OMe>H. The present results indicate that in the oxygenation of flavonols the conditions where hydroxy group at 7-(5-)position is not dissociated is essentially needed to give the same product as that in the enzyme reaction.
1974-10-01