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

Abstract

AbstractGas 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.