Enantioselective resolution of chiral aromatic acids by biphasic recognition chiral extraction

This paper reports a new chiral separation technology—biphasic recognition chiral extraction for the separation of aromatic acid enantiomers such as α-cyclohexyl-mandelic acid (CHMA) and naproxen (NAP). The biphasic recognition chiral extraction system was established by adding hydrophobic d(l)-isobutyl tartrate in 1,2-dichloroethane organic phase and hydrophilic β-cyclodextrin (β-CD) derivative in aqueous phase, which preferentially recognize the (R)-enantiomer and (S)-enantiomer, respectively. These studies involve an enantioselective extraction in a biphasic system, where aromatic acid enantiomers form complexes with the β-cyclodextrin derivative in the aqueous phase and d(l)-isobutyl tartrate in the organic phase, respectively. Factors affecting the extraction mechanism are analyzed, namely the influence of the concentrations of the extractants and aromatic acid enantiomers, the types of the extractants, pH, and temperature. The experimental results show that the biphasic recognition chiral extraction is of much stronger chiral separation ability than the monophasic recognition chiral extraction, which utilizes the cooperations of the forces of the tartrate and the β-CD derivative. Hydroxypropyl-β-cyclodextrin (HP-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (ME-β-CD) have stronger recognition abilities for the (S)-aromatic acid enantiomers than those for (R)-aromatic acid enantiomers, among which HP-β-CD has the strongest ability. d-Isobutyl tartrate preferentially recognizes (R)-CHMA and (S)-NAP, while l-isobutyl tartrate preferentially recognizes (S)-CHMA and (R)-NAP. The maximum enantioselectivities of CHMA and NAP are 2.49 and 1.65, under conditions at which the pH values of the aqueous phases are 2.7 and 2.5, at the ratio of 2:1 of isobutyl tartrate] to HP-β-CD].