Evidence for [1,5] sigmatropic rearrangements of CLA in heated oils

Linoleic acid was heated at 200°C under helium. Analysis of degradation products by GC on a long polar open tubular capillary column showed the presence of CLA isomers. The identified mono trans CLA isomers were cis-9, trans-11, trans-9, cis-11, trans-10, cis-12, cis-10, trans-12, trans-8, cis-10, and cis-11, trans-13 18:2 acids. Oils containing different levels of linoleic acid (peanut, sesame seed, and safflower seed oils) were also heat treated, resulting in similar CLA distributions. Elution order was confirmed using cis-9, trans-11 and trans-10, cis-12 acid methyl esters standards and their respective configuration isomers (trans-9, cis-11, cis-10, trans-12), obtained after mild selenium-catalyzed isomerization. These results indicated that two conjugated mono trans isomers of 18:2 acid, cis-8, trans-10 and trans-11, cis-13 18:2 were absent from the series, thus strongly suggesting that some constraints were preventing their formation. By heating pure methyl rumenate (cis-9, trans-11 18:2) under similar conditions, isomerization resulted principally in a nearly equimolar mixture of methyl rumenate and trans-8, cis-10 18:2. Similarly, the methyl ester of trans-10, cis-12 18:2 acid was partially transformed into cis-11, trans-13 18:2 acid. Respective geometrical isomers were also formed in trace amounts. A concerted pericyclic isomerization mechanism, a 1,5] sigmatropic rearrangement, is proposed that limits the conjugated system to isomerization from a cis-trans acid to a trans-cis acid, and vice versa. This mechanism is consistent with undetected cis-8, trans-10 and trans-11, cis-13 18:2 isomers in heated oils containing linoleic acid.