Enantioseparation of alkyl tropate by high performance liquid chromatography using (2R, 3R)-di-n-propyl tartrate

Optically active tartaric acid and its diester derivatives are well known as effective chiral selectors. In the current studies, a self-prepared diester of tartaric acid (2R, 3R)-di-n-propyl tartrate (DPT) was used as a chiral mobile-phase additive (CMPA) for the enantioseparation of seven kinds of alkyl tropate on a silica gel column by high performance liquid chromatography (HPLC). Four kinds of alkyl tropate were successfully enantioseparated using this chiral system, indicating that DPT is a novel useful chiral selector. The influences of mobile-phase composition and solute structure on enantioseparation were extensively studied. Although different types of alcoholic additive or various concentrations of alcoholic additive, DPT and dichloromethane in the mobile phase had great influence on the retention factor (k′) and resolution (Rs), the mobile-phase composition essentially had no effect on the separation factor (α). In contrast, the structure of alkyl tropate greatly influenced the separation factor (α). The results revealed that an increase in the bulkiness of the O-alkyl groups of tropic acid esters reduced the separation factors (α), indicating that inhibition of the formation of effective hydrogen-bond interaction between DPT and solutes by the bulkiness of the O-alkyl group decreases the retentivity difference between the transient diasterieomeric complex pairs. Our observations suggested that the hydrogen-bond interactions between chiral additives and solutes might be the major chiral recognition mechanism for DPT as CMPA.