Photosensitized oxidation of methyl linolenate monohydroperoxides: Hydroperoxy cyclic peroxides, dihydroperoxides and hydroperoxy bis‐cyclic peroxides

Abstract

AbstractTo elucidate the biological and flavor significance of lipid secondary oxidation products, cyclic peroxides were prepared by photosensitized oxidation of the hydroperoxides in autoxidized methyl linolenate. The oxidation product was fractionated by silicic acid chromatography, followed by high pressure liquid chromatography (HPLC) on a microporous silica column. Products characterized by thin layer chromatography (TLC), gas chromatography (GC), ultraviolet (UV), infrared (IR), nuclear magnetic resonance (NMR) and mass spectroscopy (MS) included 6‐membered cyclic peroxides (9‐hydroperoxy‐10,13‐epidioxy‐11, 15‐and 16‐hydroperoxy‐12,15‐epidioxy‐9, 13‐octadecadienoates), 15‐membered cyclic peroxides (9‐hydroperoxy‐10,12‐epidioxy‐13,15‐ and 16‐hydroperoxy‐13, 5‐epidioxy‐9,11‐octadecadienoates), dihydroperoxides (9,12‐; 9,16‐; 10,12‐; 10,15‐; 10,16‐; 13,15‐; 13,16‐dihydroperoxy octadecatrienoates) and hydroperoxy bis‐cyclic peroxides, each with one 5‐and one 6‐membered ring (9‐hydroperoxy‐10,12,13,16‐bis‐epidioxy‐14‐ and 16‐hydroperoxy‐9,12,13, 15‐bis‐epidioxy‐10‐octadecenoates). The 6‐membered cyclic peroxides are formed by 1,4‐addition of singlet oxygen to the conjugated diene system in 9‐ and 16‐linolenate hydroperoxide isomers after their isomerization to thetrans, trans configuration. The bis‐cyclic peroxides are formed by 1,4‐addition of singlet oxygen to the hydroperoxy 5‐membered cyclic peroxides derived from the 12‐ and 13‐linolenate hydroperoxide isomers. Secondary oxidation products similar to those identified in this study previously have been shown to be important precursors of volatile compounds that may contribute to flavor deterioration of fat‐containing foods.