Synthesis and NMR properties of positional isomers of methyl allenic fatty esters

Abstract The synthesis of six C 14 and nine C 18 positional allenic fatty esters is described. Ethyl 2,3-allenic C 14 fatty ester was synthesized by reaction of dodecanoyl chloride with (carbethoxymethylene) triphenylphosphorane. Ethyl tetradeca-3,4-dienoate was prepared via the propargylic intermediate (3-hydroxydodec-1-yne) by reaction with triethyl orthoacetate in the presence of propionic acid. A general synthetic approach was adopted for the remaining positional isomers including the terminal ω-allenic, ω-1 and ω-2 positional isomers. Propargylic intermediates were condensed with ω-bromoalkanoic acids to yield the corresponding α-hydroxyacetylenic acid intermediates. The hydroxy group was converted to the bromide and elimination of the bromide function by chromium(II) chloride furnished the desired racemic allenic fatty ester isomers. The 2,3-; 3,4-; 4,5-; ω-, ω-1 and ω-2 positional isomers of allenic fatty esters were characterized by 1 H-NMR analysis from the chemical shifts of the allenic protons (δ 5.0–5.6) and the shifts of the protons of the methyl, methylene or methyl ester group adjacent to the allene system. 13 C-NMR analysis of the allenic fatty esters identified 11 of the 15 positional isomers studied. The chemical shift of the allene carbon atoms appeared at the region of 84–96 and 203–212 ppm for the outer and central allenic carbon nuclei of the allene, respectively. The ω-isomer furnished unique signals at 74.52, 208.52 and 90.11 ppm for the terminally located allene system in the alkyl chain.