Studies on the gas—liquid chromatographic separation of diastereoisomeric esters

Abstract

In continuation of our studies on the mechanism of separation of diastereoisomeric esters by gas chromatography, we have examined the gas—liquid partition behavior of a number of structurally related isomeric pairs. Differences in free energies of distribution, Δ(Δ G°), have been measured for a series of methylalkylcarbinyl a-haloalkanoates in order to investigate the effect of polar substitution of groups attached to the acidic asymmetric center. The importance of electronic effects on the acid side of the molecule on separation is further indicated through a study of meta- and para-substituted 3-methyl-2-butyl mandalates and methylalkylcarbinyl a-methylalkanoates. The influence on separation of the structure of groups attached to the alcohol side of the molecule has been studied through a series of alkylvinylcarbinyl a-acetoxypropionates. The importance of chain length of an alkyl group attached to the alcoholic asymmetric center is indicated through the chromatographic behavior of a series of a-acetoxy- and a-hydroxypropionates of secondary alcohols. Using a 150 ft. capillary column, Δ(Δ G°) values as small as −3 cal/mole have been measured. These results are interpreted in part in terms of the influence of preferred conformations of the groups attached to the asymmetric centers on the differences in gas—liquid partition behavior of the isomeric pairs.