13C-NMR spectra of positional isomers of long-chain conjugated diacetylenic fatty esters

Abstract

The 13C-NMR spectra of a set of 14 positional isomers of conjugated diacetylenic fatty esters were studied. Full interpretation of the spectroscopic results was only possible after sub-sets of symmetrical dialkyl and dialkanoate substituted conjugated diacetylenes were prepared and the effects of the ω-methyl and carbomethoxy group on the chemical shifts of the acetylenic carbon nuclei were assessed. The chemical shift of the outer and inner acetylenic carbon atoms of the methyl alkadiynoates appeared at about 75–79 and 65–66 ppm, respectively. Strong shielding α-effects of the diacetylenic system on the immediately adjacent carbon nuclei were observed. In general, the carboxylic ester group caused a strong shielding effect on the outer carbon nuclei of the diacetylenic systems, but a strong deshielding effect on the inner carbon nuclei. The position of the conjugated diacetylenic system in the alkyl chain could be determined for isomers, where the number of methylene groups between the diacetylenic system and the ester group was less than 7; and in isomers where the number of methylene groups between the ω-methyl group and the diacetylenic system was less than 4. Twelve of the 14 conjugated diacetylenic fatty ester isomers were identified by this technique.