Base-catalysed oxygenolysis of 3-hydroxyflavones

Abstract

Hydroxyflavones, except those containing a 7hydroxy-group, undergo base-catalysed oxygenolysis under mild conditions leading to oxidative cleavage of the heterocyclic ring to give the corresponding depsides and carbon monoxide in excellent yields. The same result is obtained in the reaction of quercetinase, a dioxygenase. The oxygenation of 3,4',7-trihydroxyflavone in aqueous solution gave p-hydroxyphenylglyoxylic acid and 2,4-dihydroxybenzoic acid ; in absolute methanol containing sodium methoxide methyl 4-hydroxyphenylglyoxylate and methyl 2,4-dihydroxybenzoate as well as 4-hydroxyphenylglyoxylic acid were obtained. The formation of these products is rationalized by assuming that 2-hydroperoxy-4',7-dihydroxyflavan-3,4-dione is formed first and is reduced with the intervention of the 7hydroxy-group to give 2,4',7-trihydroxyflavan-3,4-dione, which is solvated and oxidized under the reaction conditions. 2'-Substituted-3-hydroxyflavones were not susceptible to oxygenolysis.

Antiviral Activity of Flavones and Potentiation by Ascorbate

“…a direct virucidal effect as well as interference with plaque formation, has been shown recently (Kaul et al, 1985). However, these results are at variance with previous reports according to which quercetin was virucidal only to enveloped, but not to unenveloped viruses (B61adi et al, 1977;Veckenstedt & Pusztai, 1981).The discrepancies about the antiviral properties of quercetin may originate in the oxidative degradation of this flavone in aqueous solution (Nishinaga et aL, 1979;Rajananda & Brown, 1981). In this communication, the anti-poliovirus activity of quercetin was determined in the presence and absenceof the antioxidant ascorbate.…”

“…The discrepancies about the antiviral properties of quercetin may originate in the oxidative degradation of this flavone in aqueous solution (Nishinaga et aL, 1979;Rajananda & Brown, 1981). In this communication, the anti-poliovirus activity of quercetin was determined in the presence and absenceof the antioxidant ascorbate.…”

Hydroxyl Free Radical-Mediated Oxidative Degradation of Quercetin and Morin: A Preliminary Investigation

“…An end product that may arise from such a cascade of reactions is PA, as proposed by Matsuura et al (1970), and Nishinaga et al (1979), for photosensitized oxygenation and base-catalysed oxygenolysis of quercetin, respectively, since both pathways share common intermediate and, presumably, end products. However, the detection of PCA may suggest the formation of the depside protocatechuoyl phloroglucinol carboxylic acid, which could arise from the oxidation of the 3-hydroxyl group.…”

DPPH-initiated oxygenation of 3-hydroxyflavone to O-benzoylsalicylic acid

“…3-Hydroxyflavone [17] was prepared according to literature methods. Diazomethane was freshly prepared by known procedure in ether and immediately used for methylation reactions.…”