OXIDATION OF VITAMIN-E - EVIDENCE FOR COMPETING AUTOXIDATION AND PEROXYL RADICAL TRAPPING REACTIONS OF THE TOCOPHEROXYL RADICAL

Peroxyl radicals generated by thermolysis of azobis(2,4-dimethylvaleronitrile) (AMVN) in oxygenated acetonitrile reacted with d-α-tocopherol (vitamin E, 1) to produce 8a-[(2,4-dimethyl-1-nitrilopent-2-yl)dioxy]tocopherone (3a), 4a,5-epoxy-8a-hydroperoxytocopherone (4a), and 7,8-epoxy-8a-hydroperoxytocopherone (5a). When formed in acetonitrile/water (6:4, v/v), 4a and 5a hydrolyzed quantitatively to 2,3-epoxy-α-tocopherylquinone (6) and 5,6-epoxy-α-tocopherylquinone (7), respectively, whereas 3a underwent limited hydrolysis to α-tocopherylquinone (8). Peroxyl radicals further oxidized 3a and its homolog, 8a-hydroperoxy-α-tocopherone (3b), which was formed in small amounts during the oxidation of 1. However, neither 3a nor 3b were consumed by peroxyl radicals when the reaction mixture contained 1. Formation of epoxytocopherones 4a/5a therefore does not proceed by oxidation of 3a or 3b. Epoxytocopherones 4a and 5a may be formed instead by epoxidation of the tocopheroxyl radical (2), followed by reaction of the epoxy 2 with oxygen and a H⋅ donor. Thus, 2 either may trap peroxyl radicals to yield 8a-(alkyldioxy)tocopherones or produce epoxytocopherones by peroxyl radical-dependent autoxidation. © 1990, American Chemical Society. All rights reserved.