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

AbstractLinoleic 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.