Radical addition of linoleic hydroperoxides to α‐tocopherol or the analogous hydroxychroman

Abstract

Abstract Either linoleic acid hydroperoxide (LOOH) or methyl linoleate hydroperoxide react anaerobically with either α‐tocopherol (TOH) or its model compound‐2,2,5,7,8‐pentamethyl‐6‐hydroxychroman (COH)‐to form principally an addition compound of the two reactants. The reaction can be catalyzed either by 1.28 X 10 −5 M Fe(III) or by proflavin (0.01%) sensitized by visible light. The presence of air in the reaction terminates the addition, and quinones become the major products from TOH or its model compound. The addition compound synthesized from COH and LOOH (a 4.9∶1 ratio of 13‐hydroperoxy‐ cis ‐9, trans ‐12‐octadecadienoic acid and 9‐hydroperoxy‐ trans ‐10, cis ‐12‐octadecadienoic acid) was used to solve structural details of the bridging function. Three isomers of the addition compound (methyl esterified) were isolated and identified as methyl 11‐(2,2,5,7,8‐pentamethyl‐6‐oxychroman)‐ cis ‐12,13‐epoxy‐ trans ‐9‐octadecenoate; methyl 11‐(2,2,5,7,8‐pentamethyl‐6‐oxychroman)‐ trans ‐12,13‐epoxy‐ trans ‐9‐octadecenoate; and methyl 11‐(2,2,5,7,8‐pentamethyl‐6‐oxychroman)‐ cis ‐9,10‐epoxy‐ trans ‐12‐octadecenoate in order of decreasing abundance. The mechanism appears to be free radical addition brought about by the catalytic formation of alkoxy radicals from the hydroperoxide and chromanoxy radicals from TOH or its model.