During cyclization one of the double bonds is lost, so that the cyclic fatty acids isolated from sunflower oil are monoenoic while those from linseed oil are mostly dienoic acids, reflecting the sources as linoleic and linolenic acids, respectively. These fatty acids are now known to contain five and six membered rings, with two substituents one containing the carboxyl group and the other a hydrocarbon chain. Double bonds can be located within the rings or in either of the alkyl chains. While there may be a limited range of basic carbon skeletons with five- and six-membered rings in a finite number of positions, stereomutation of double bonds may occur giving products of the cis and trans configuration. Also, there is an approximately equal probability of formation of cis and trans isomers of substituents about the rings. Each basic component gives rise to four stereoisomers, and these can appear as a complex array of overlapping peaks on analysis by gas chromatography, for example.
In brief, our successful approach to a comprehensive determination of the structures and relative proportions of each of the cyclic fatty acids involved a preliminary separation of the cyclic fatty acid mixture (as phenacyl ester derivatives) by silver ion high-performance liquid chromatography (HPLC) into several fractions according to degree of unsaturation, the size of the ring and the position and configuration of the double bonds. Then the fractions were each examined by gas chromatography-mass spectrometry as their picolinyl ester and 4,4-dimethyloxazoline (DMOX) derivatives. These methods have enabled us to determine the nature and relative amounts of the cyclic dienoic fatty acids from linseed oil, cyclic monoenoic fatty acids from sunflower oil, bicyclic saturated fatty acids from linoleate and saturated cyclic fatty acids from oleate.
Cyclic Dienes Formed from α-Linolenate
Mass spectrometry of DMOX derivatives was especially helpful in analysing the dienoic fatty acids produced by cyclization of α-linolenate (in linseed oil, heated at 275°C under nitrogen). Perhaps surprisingly, these were simpler in composition than were the monoenes derived from linoleate, at least in relation to the essential molecular skeletons. Four basic structures only were present in significant concentrations. Each has a ring structure with one double bond in the ring and one in the aliphatic chain. Two of these isomers (50% of the total) contain cyclohexene rings and two have cyclopentene rings with the double bond in the ring being between carbons 12 and 13 of the original fatty acid chain in every instance. Determination of the position of the double bond in the ring is not easily accomplished by mass spectrometry alone, but that in position 12 of the cyclohexene ring gives a specific ion that is diagnostic.
The six-membered ring was formed in each case by cyclization between carbons 10 and 15 of the original fatty acid chain, i.e. from the first to the third double bond or vice versa. The five-membered rings were formed in a similar way by cyclization from carbons 10 and 15 to carbons 14 and 11, respectively. No cyclization occurred in a direction away from the double bond system. There was a slight preponderance of products formed by cyclization from carbon 15 in comparison with those from carbon 10. In geometrical isomerization induced by heating, the double bond in position 15 of linolenic acid appears to be more reactive than that in position 12 also.
With the cyclohexenyl fatty acids, the double bond in the aliphatic chain moved from position 9 or 15 to 8 or 16, respectively, and cis-trans isomerization occurred. Of these isomers, 83% had a trans-double bond, i.e. close to the point of thermodynamic equilibrium, suggesting that isomerization occurred as part of the cyclization process. In contrast, the double bonds in the aliphatic chains in the cyclopentenyl fatty acids remained in their original positions, i.e. 9 or 15. Although stereomutation occurred here also, there were similar proportions of the cis and trans isomers. Much more stereoisomerization occurred during the formation of cyclic dienes than with cyclic monoenes. Similarly, cis and trans isomers about the rings were present in approximately equal amounts. Thus, the four basic structures result in sixteen isomers-two configurational isomers about the double bond and two about the ring structure for each.
Working with identical samples, similar results were obtained by Mossoba and colleagues in the USA, although we do not agree with all their proposed structures. In particular, we have not been able to detect cyclic fatty acids with conjugated double bond systems. An analogous range of cyclic fatty acids were formed from γ-linolenic acid, when evening primrose oil was heated under simulated frying conditions.
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