Antioxidative activity of tree phenolic constituents I: Radical-capturing reaction of flavon-3-ols with radical initiator

Abstract

The present work was undertaken from the standpoint of radical-capturing ability with regard to the antioxidative ability of flavonoids, especially flavonols distributed widely in woody plants. In regard to the flavonols, six methyl derivatives were initially prepared from quercetin and its litinoside. Their radical-capturing constants were determined strictly by the stopped-flow spectroscopic method. It was proved that the radical-capturing ability of quercetin mainly involves the vicinal C3, and C4, hydroxyl groups and the C3 hydroxyl group. To clarify the reaction mechanism begun at the C3 hydroxyl group of quercetin, 5,7,3′,4′-tetramethylquercetin (TMQ), flavon-3-ol (F3O) and so on were treated with 2,2′-azo-bis-(2,4-dimethylvaleronitrile) (AMVN). Six products (1-6) containing one depside and its two hydrolytic products, two valeronitrile adducts, and others were isolated from the reaction mixture of TMQ and their structures determined by instrumental analyses. Similarly, F3O gave four products, 7-10, which corresponded to the above products 1-3 and 5 (one depside, its two hydrolytic products, and one adduct), respectively. 3,5,7,3′,4′-Pentamethylquercetin (PMQ) and flavon-3-O-methylate (F3M) gave no products. The quantitative change of the products with reaction time was determined spectroscopically. An initial reaction pathway for the radical-capturing reaction of flavon-3-ols with AMVN was proposed based on the products and their amounts. The main route - formation of depside and its hydrolytic products via ketohydroperoxide (3″) or ketohydroperoxy radical (4″) - was similar to that of the oxidation reaction of quercetin with quercetinase and light.