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

The present work was undertaken from the stand-point of radical-capturing a bility with regard to the antioxidative ability of flavonoids, especially f lavonols distributed widely in woody plants. In regard to the flavonols, si x methyl derivatives were initially prepared from quercetin and its litinos ide. Their radical-capturing constants were determined strictly by the stop ped-flow spectroscopic method. It was proved that the radical-capturing abi lity of quercetin mainly involves the vicinal C-3, and C-4, hydroxyl groups and the C-3 hydroxyl group. To clarify the reaction mechanism begun at the C-3 hydroxyl group of quercetin, 5,7,3',4'-tetramethylquercetin (TMQ), fla von-3-ol (F30) and so on were treated with 2,2'-azo-bis-(2,4-dimethylvalero nitrile) (AMVN). Six products (1-6) containing one depside and its two hydr olytic products, two valeronitrile adducts, and others were isolated from t he reaction mixture of TMQ and their structures determined by instrumental analyses. Similarly, F30 gave four products, 7-10, which corresponded to th e above products 1-3 and 5 tone depside, its two hydrolytic products, and o ne adduct), respectively. 3,5,7,3 ',4'-Pentamethylquercetin (PMQ) and flavo n-3-O-methylate (F3M) gave no products. The quantitative change of the prod ucts with reaction time was determined spectroscopically. An initial reacti on pathway for the radical-capturing reaction of flavon-3-ols with AMVN was proposed based on the products and their amounts. The main route - formati on of depside and its hydrolytic products via ketohydroperoxide (3 ") or ke tohydroperoxy radical (4 ") - was similar to that of the oxidation reaction of quercetin with quercetinase and light.