2‐alkenyl‐4,4‐dimethyloxazolines as derivatives for the structural elucidation of iosmeric unsaturated fatty acids

Abstract

Several types of unsaturated fatty acid methyl esters were converted into 4,4-dimethyloxazoline (DMOX) derivatives and analyzed by mass spectrometry to further evaluate the feasibility of using this derivative for locating the positions of double bonds in isomeric fatty acids. Five isomeric 20-carbon tetraenoic acids were analyzed in which the four cis double bonds were systematically moved from the 4,7,10,13- to the 8,11,14,17-positions. It was possible to locate the positions of all four double bonds in the 7,10,13,16- and 8,11,14,17-isomers by appropriate ions differing by 12 atomic mass units. In a similar way the three terminal double bonds in the 4,7,10,13-, 5,8,11,14- and 6,9,12,15-isomers could be assigned. Odd-numbered ions at m/z 139, 153 and 167 which are accompanied by an even mass ion at 138, 152 and 166, respectively, are diagnostic for DMOX derivatives of acids with their first double bond, respectively, at positions 4, 5 and 6. It was thus possible to assign the location of all four double bonds in these three isomers. A comparison of the spectra of the DMOX derivatives of 17,17,18,18-d4 vs. 9,10,12,13-d4 linoleic acid suggests that double bonds preferentially migrate toward the polar end of the molecule prior to fragmentation. The merit of using DMOX derivatives to locate double-bond positions in mono- and dicarboxylic acids, produced during beta-oxidation of polyunsaturated fatty acids, was evaluated. The spectra of 3-cis- and 4-cis-decenoic acids differ as do the spectra of 8-carbon dicarboxylic acids with their double bonds at positions 3 and 4.