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

The cyclic fatty acids isolated from evening primrose oil that had been hea ted to 275 degrees C were fractionated by silver ion high-performance liqui d chromatography and analysed by gas chromatography-mass spectrometry as th e dimethyloxazoline derivatives, before and after deuteration. Sixteen cycl ic dienoic fatty acids derived from gamma-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 cycl ization and the remaining two were in their original positions (C-9 and C-1 2, and C-6 and C-9, respectively). There were eight cyclohexenyl fatty acid s 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 chai n were formed. The structures were analogous to those previously reported f rom alpha-linolenic acid in the sense that the trienoic unit reacted in a s imilar way irrespective of its position along the C-18 chain. The relative proportions of acids with different structural features were also similar f or both trienoic acids. Mechanisms involving allylic radicals could account for all products, and the limited number of acids formed compared to possi ble products is discussed. (C) 1999 Published by Elsevier Science Ireland L td. All rights reserved.