The GLC and TLC resolution of diastereoisomeric polyhydroxystearates and assignment of configurations

Abstract

Trifluoroacetate (TFA) derivatives of methyl 12-hydroxystearate, methyl ricinoleate, five positional isomers of methylthreo- anderythro-dihydroxystearate, four diastereoisomeric methyl 9,10-12-trihydroxystearates, and four racemic diastereoisomeric methyl 9,10-12,13-tetrahydroxystearates were prepared and analyzed by gas-liquid chromatography (GLC). The isomericthreo- anderythro-dihydroxystearates that had not previously been resolved by GLC were separated. Excellent resolution of the diastereoisomeric pairs of methylthreo- anderythro-9,10-12-triand and methylerythro, erythro- andthreo, threo-9,10-12,13-tetrahydroxystearates was obtained by GLC of their TFA derivatives. Analyses of these high-molecular-weight compounds were carried out on polar and nonpolar packed columns used routinely for methyl ester analysis.The various methyl mono-, di-, tri-, and tetrahydroxysterate esters were also analyzed by thin-layer chromatography (TLC) on Silica Gel G adsorbent layers and on Silica Gel G impregnated with sodium arsenite.Relative and absolute configurations were assigned to the various diastereoisomeric tri- and tetrahydroxysterates based on information obtained from GLC, TLC, synthetic ratios, and molecular-models.A micro hydroxylation method that gives quantitative yields ofthreo- anderythro-dihydroxy acids from various concentrations of C(18) monoene geometrical isomers was developed. Subsequent GLC analysis of the isomeric methyl dihydroxy TFA derivatives allows the quantitative determination of double-bond configuration on small samples without expensive or specialized equipment.