Photosensitized oxidation of methyl linoleate monohydroperoxides: Hydroperoxy cyclic peroxides, dihydroperoxides, keto esters and volatile thermal decomposition products

Abstract

Abstract Previous studies of lipid secondary oxidation products have been extended to 6‐membered hydroperoxy cyclic peroxides from the singlet oxygenation of a mixture of 9‐ and 13‐hydroperoxides from autoxidized methyl linoleate. The oxidation product was fractionated by silicic acid chromatography with diethyl ether/hexane mixtures, and selected fractions were separated by polar phase high performance liquid chromatography. Products characterized by thin layer chromatography, gas liquid chromatography, ultraviolet, infrared, nuclear magnetic resonance and mass spectrometry included: 6‐membered cyclic peroxides (13‐hydroperoxy‐9,12‐epidioxy‐10‐ and 9‐hydroperoxy‐10,13‐epidioxy‐11‐octadecenoates), dihydroperoxides (8,13‐ and 9,14‐dihydroperoxyoctadecadienoates) and keto dienes (9‐ and 13‐oxooctadecadienoates). The 6‐membered hydroperoxy cyclic peroxides are apparently formed by 1,4‐addition of singlet oxygen to 9‐ and 13‐hydroperoxides with trans, trans ‐conjugated diene systems. Thermal decomposition of the 6‐membered hydroperoxy cyclic peroxides at 200 C produced methyl 9‐oxononanoate and hexanal as the major volatiles. Other volatiles included 2‐pentylfuran, pentane, 4‐oxo‐2‐nonenal, methyl furanoctanoate and methyl 9,12‐dioxo‐10‐dodecenoate.