Photosensitized oxidation of methyl linoleate: Secondary and volatile thermal decomposition products

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Abstract
Studies of photosensitized oxidation of methyl linoleate show that the greater relative concentration of 9‐ and 13‐hydroperoxides than 10‐ and 12‐hydroperoxides is characteristic of singlet oxygenation and not due to either simultaneous autoxidation or type 1 photosensitized oxidation. Cyclization of the internal 10‐ and 12‐hydroperoxides accounts for their lower relative concentrations. Secondary products separated by silicic acid and high pressure liquid chromatography were characterized spectrally (IR, UV,1H‐NMR,13C‐NMR, GC‐MS). Major secondary products included diastereomeric pairs of 13‐hydroperoxy‐10,12‐epidioxy‐trans‐8‐octadecenote (I and III) and 9‐hydroperoxy‐10,12‐epidioxy‐trans‐13‐octadecenoate (II and IV); minor secondary products included hydroperoxy oxy genated and epoxy esters. Thermal decomposition of the hydroperoxy cyclic peroxides produced hexanal and methyl 10‐oxo‐8‐decenoate as major volatiles from I and III and methyl 9‐oxo‐nonanoate and 2‐heptenal from II and IV. Hydroperoxy cyclic peroxides may be important sources of volatile decomposition products of photooxidized fats.