A procedure for preparing oxazolines of highly unsaturated fatty acids to determine double bond positions by mass spectrometry

A convenient, mild, reliable method has been developed for preparing oxazolines of fatty acids and for using these derivatives to determine double bond locations in long-chain polyunsaturated and polyconjugated fatty acids. Fatty acyl mixed anhydrides are prepared using isobutylchloroformate and then converted to their ethanolamides by treatment with ethanolamine. Ethanolamides are subsequently cyclized to the corresponding oxazolines in ≥85% yields by treatment with trifluoroacetic anhydride under mild conditions (>50° for 30-60 min). This general protocol can also be used to synthesize 4,4-dimethyloxazoline and benzoxazole derivatives of fatty acids. Gas chromatography-mass spectrometry of oxazoline derivatives of fatty acids yields prominent ions diagnostic of the structures of the parent fatty acids and, in the case of unsaturated fatty acids, indicating the positions of the double bonds. The utility of the method is illustrated with several fatty acids, including the conjugated 4E,6E,8E,10E,13Z,16Z,19Z-docosaheptaenoic acid.