Free-Radical Scavenging Reactions of .alpha.-Tocopherol during the Autoxidation of Methyl Linoleate in Bulk Phase

Abstract

Methyl linoleate containing alpha-tocopherol was autoxidized at 37 and 60 degrees C in bulk phase. The reaction products of alpha-tocopherol at 37 degrees C were 4a,5-epoxy-8a-hydroperoxy-alpha-tocopherones (1), a mixture of methyl 9-(8a-dioxy-alpha-tocopherone)-10(E),12(Z)-octadecadienoates and methyl 13-(8a-dioxy-alpha-tocopherone)-9(Z),11(E)-octadecadienoates (2), a mixture of methyl 9-(alpha-tocopheroxy)-10(E),12(Z) -octadecadienoates and methyl 13-(alpha-tocopheroxy)-9(Z),11(E)-octadecadienoates (3), alpha-tocopherol spirodiene dimer (4), and alpha-tocopherol trimer (5). Compound 3 was the major product at 60 degrees C, and the E,E-isomers of 3 were produced in addition to the E,Z-isomers. When methyl linoleate containing a low concentration of alpha-tocopherol was autoxidized at 37 degrees C, alpha-tocopherol suppressed the formation of methyl linoleate hydroperoxides, and the main products of alpha-tocopherol were 1 and 2. In contrast, (alpha-tocopherol at a high concentration induced the accumulation of methyl linoleate hydroperoxides, and the reaction products were 1-5. The autoxidation at 60 degrees C proceeded rapidly, and the main product of alpha-tocopherol was 1. In addition, the accumulation of 3 was observed under air-insufficient conditions. The results indicate that the alpha-tocopheroxyl radical produced can react with both alkyl and alkylperoxyl radicals of unsaturated lipids during the autoxidation. Competing with these trapping reactions, 1 appears to be formed primarily by autoxidation reactions of alpha-tocopherol.