Mandelic acid and α-methoxymandelic acid were purchased from TCI (Japan). 3-Phenyllactates were prepared according to the literature method.14 Ethyl 2-hydroxy-4-phenylbutyrate is the product of asymmetric hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al2O3 catalyst, 7 supplied by the State Key Laboratory of Catalysis (China). All other reagents were commercially available.All the α-hydroxycarboxylic acids were esterified with methanol or ethanol and 98% sulfuric acid, according to a previously published procedure.
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Instruments and proceduresChromatographic separations were performed with a modified GC-1121 capillary gas chromatograph (Shanghai Analytical Instrument Factory, China) equipped with a flame-ionization detector. Both the injector and detector temperatures were maintained at 250˚C. With H2 as the carrier gas, all the separations were performed isothermally with a split ratio of 1:100.Chiral columns were prepared as described earlier. 13 Briefly, fused silica capillary tubes (30 m × 0.25 mm i.d.) deactivated using PEG-20M were coated statically with mixture solutions of the CD derivatives and polysiloxane diluents dissolved in CH2Cl2, and then conditioned at 190˚C for over 4 h. Enantiomer separation of mandelates and their analogs, which are important intermediates in asymmetric synthetic and pharmaceutical chemistry, was investigated by capillary gas chromatography using different cyclodextrin derivative chiral stationary phases (CD CSPs). The used cyclodextrin derivatives included permethylated β-CD (PMBCD),, respectively. Among all the CSPs used, PMBCD and DNTBCD exhibited the broadest and best enantioselectivity for all the racemates investigated. Some thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed in enantiomer separation processes of mandelates and their analogs. Based on thermodynamic data and molecular mechanics calculations, the chiral recognition mechanism of mandelate derivatives on CD CSPs is discussed.