Monocyclic dienoic fatty acids formed from γ-linolenic acid in heated evening primrose oil

Abstract

The cyclic fatty acids isolated from evening primrose oil that had been heated to 275°C were fractionated by silver ion high-performance liquid chromatography and analysed by gas chromatography-mass spectrometry as the dimethyloxazoline derivatives, before and after deuteration. Sixteen cyclic dienoic fatty acids derived from γ-linolenic acid were characterised. There were eight cyclopentenyl fatty acids with rings from either C-7 to C-11 or C-8 to C-12 of the original chain. One double bond was lost on cyclization and the remaining two were in their original positions (C-9 and C-12, and C-6 and C-9, respectively). There were eight cyclohexenyl fatty acids with rings from C-7 to C-12 and double bonds at either C-5 and C-9 or C-9 and C-13. All possible combinations of configurational isomers of both the straight chains about the ring and of the double bond in the straight chain were formed. The structures were analogous to those previously reported from α-linolenic acid in the sense that the trienoic unit reacted in a similar way irrespective of its position along the C 18 chain. The relative proportions of acids with different structural features were also similar for both trienoic acids. Mechanisms involving allylic radicals could account for all products, and the limited number of acids formed compared to possible products is discussed.