Elucidation of cyclic fatty acid monomer structures. Cyclic and bicyclic ring sizes and double bond position and configuration

Gas chromatography (GC)-electron ionization mass spectrometry of 2-alkenyl-4,4-dimethyl-oxazoline derivatives was used to confirm the identities of a complex mixture of C₁₈ diunsaturated cyclic fatty acid monomers (CFAMs) that were isolated from heated flaxseed (linseed) oil. The positions of double bonds and 1,2-disubstituted unsaturated 5- and 6-membered rings along the fatty acid hydrocarbon chains were established by this method. The oxazoline spectra exhibited a homologous ion series with a pattern of peaks that were 14 u (u=atomic mass unit) apart but interrupted when a double bond (12-u mass interval) or a ring was present along the fatty acid chain. The identity and location of a ring were indicated by a large interval of 68, 82, 66, 80, 78, or 120 u for a saturated 5- or 6-membered ring, monounsaturated 5- or 6-membered ring, diunsaturated 6-membered ring, or monounsaturated bicyclic ring system (fused 5- and 6-membered rings), respectively. The double bond configuration for the methyl ester derivatives of these CFAMs was established by GC-matrix isolation-Fourier transform infrared spectroscopy. The elucidated alkenyl structures at C₂ in diunsaturated 2-[alkenyl]-4,4-dimethyloxazolines were 8-(2-but-trans-1-enyl-cyclopentenyl)octyl, 9-(2-propyl-cyclopentenyl)non-rans-8-enyl, 9-(2-propyl-cyclopentenyl)non-cis-7-enyl, 8-(2-but-cis-1-enyl-cyclopentenyl)ocytl, 9-(2-propylcyclopentenyl)non-cis-8-enyl, 8-(2-propyl-cyclohex-cis-4-enyl)oct-trans-7-enyl, 8-(prop-trans-1-enyl-cyclohex-cis-4-enyl)ocytl, and 8-(2-propylcyclohexa-cis,cis-3,5-dienyl)octyl. Physical science aide, 1991–1992; currently a medical student at the University of Maryland, Baltimore, MD.