Diesters of alkane diols and 2-hydroxy fatty acids: Identification and discrimination of isomers with the aid of NMR spectroscopy

Abstract

High resolution nuclear magnetic resonance spectroscopy has been shown to be extremely useful for the identification and discrimination of naturally occurring diesters of 1,2- and 2,3-alkanediols as well as for fatty alkyl esters of acylated 2-hydroxy fatty acids. A comparison of 220 MHz spectra of 1,2 and erythro- 2,3-alkanediol diesters exhibits the following distinguishing features: (1) two non-equivalent methylene protons from the glycol group of 1,2-alkanediol diesters resonate at 3.87 ppm and 4.17 ppm respectively while these resonances are completely absent in the spectrum of 2,3-isomer; (2) methylene protons adjacent to ester carbonyl groups appear as two overlapping triplets at 2.22 ppm in 1,2-alkanediol diesters while the corresponding protons in the 2,3-isomer are displayed as two partially overlapping triplets centered at 2.15 ppm and 2.2 ppm respectively; and (3) methyl protons adjacent to glycol group in 2,3-isomer appear as downfield doublet at 1.13 ppm; this downfield doublet is not shown by 1,2-alkanediol diesters. Erythro- and threo-2,3-alkanediol diesters have also been distinguished from each other; two α-methylenes in erythro isomers appear as partially overlapping triplets while these protons in threo isomer display an apparent quartet centered at 2.22 ppm. Fatty alkyl esters of acylated 2-hydroxy fatty acids display a triplet at 4.79 for 2-position methylene proton, a distinguishing feature not shown by diacyl alkanediols. A distinction between diester lipids and other classes of neutral lipids has also been achieved by the study of nuclear magnetic resonance spectra, particularly in the region of 3–6 ppm.