Modeling of chiral gas chromatographic separation of alkyl and cycloalkyl 2-bromopropionates using cyclodextrin derivatives as stationary phases

Abstract

Chiral 2-bromopropionates are widely used as raw materials or intermediates for synthesis of chiral pesticides and medicines with high biological activities. Enantioseparation of 2-bromopropionates is crucial for evaluating the optical purity of chiral 2-bromopropionates. In this study, the enantioseparation of 8 pairs of alkyl 2-bromopropionates and 5 pairs of cycloalkyl 2-bromopropionates were examined using 7 different cyclodextrin derivatives (CDs). For the enantiomers of methyl, ethyl, bromoethyl and prop-2-yn-1-yl 2-bromopropionates, three different kinds of enantioseparation results including baseline separation, partial separation and no separation, are obtained on the 7 different CDs. Besides the baseline enantioseparation of methyl and ethyl 2-bromopropionates on some CDs, baseline enantioseparation of 2-bromoethyl 2-bromopropionate on 2,3,6-tri-O-methyl-β-CD with resolution as 1.78, prop-2-yn-1-yl 2-bromopropionate on 2,3,6-tri-O-methyl-β-CD and 2,3-di-O-pentyl-6-O-propyl-β-CD with the resolution as 3.45 and 1.77, respectively, are also obtained, and can be used for determination of the optical purity. However, for the enantiomers of isopropyl, isobutyl, tert-butyl and 3-methylbut-2-en-1-yl 2-bromopropionates, only partial enantioseparation are obtained on 5 CDs, meaning that these methods cannot be used for determination of the optical purity. To 5 pairs of cycloalkyl 2-bromopropionates, the 7 CDs exhibit low enantioseparation abilities. While the investigated 5 cycloalkyl-2-bromopropionates don't result in baseline enantioseparation when they are analyzed on the 7 CDs, partial enantioseparation of 4 cycloalkyl 2-bromopropionates, cyclopentyl, cyclohexyl, cyclohexylmethyl and benzyl 2-bromopropionates, are obtained on some CDs, and no any enantioseparation of phenyl 2-bromopropionates are observed on the 7 CDs. The baseline enantioseparation methods for methyl, ethyl, bromoethyl, and prop-2-yn-1-yl 2-bromopropionates have been validated for the parameters linearity, limit of detection, limit of quantification, recovery, intra-day and inter-day precision. Moreover, the validated baseline enantioseparation methods for methyl 2-bromopropionate using columns coated with 4 different CDs have been successfully applied on the determination of optical purity of chiral methyl 2-bromopropionate sample. Furthermore, molecular docking study is performed to investigate the 2,3,6-tri-O-methyl-β-cyclodextrin (PM-β-CD) inclusion complexes with various 2-bromopropionates. The docking results show the contribution of differences of geometry and interaction energy between the inclusion complexes of two enantiomers with PM-β-CD to enantioseparation of some 2-bromopropionates, however the docking results cannot explain the enantioseparation mechanism of other 2-bromopropionates, especially cycloalkyl 2-bromopropionates. This study provides information on the selection of chiral stationary phase CDs for the optimal GC enantioseparation conditions of some new 2-bromopropionates. The methodology of this study can be applied to evaluate the optical purity of some chiral 2-bromopropionates.

Keywords: Alkyl 2-bromopropionate; CGC method validation; Cycloalkyl 2-bromopropionate; Cyclodextrin derivative; Enantioseparation; Molecular modeling.