Fundamental Research On Resolution Of Chiral Drug Intermediates By Chromatography

Future directions in modern drug discovery will be focused on the development of chiral drug candidates. To explore new technologies for resolution and preparation of optically pure enantiomers is also a critical challenge to pharmaceutical industry. As an increasing number of chiral stationary phases have been developed and some of them are commercially available, preparative chromatography based on chiral stationary phase, such as simulated moving bed chromatography, has received significant attention and is now considered as a powerful approach for preparation of single-enantiomer drugs or chiral intermediates. In this work, the key intermediates of paroxetine trans-(±)-4-(4'-fluorophenyl)-1-methylpiperidine (1) and trans-(±)-3-ethoxycarbonyl-4-(4'-fluorophenyl)-1-methylpiperidine-2,6-dione (2) were resolved by chiral liquid chromatography and supercritical fluid chromatography. Some fundamental issues were studied. The details are summarized as follows.1. The performance of different chiral stationary phases for the racemic compounds 1 and 2 were compared by HPLC. The effects of the alcohol modifier including ethanol, 1-propanol and 2-propanol on enantioseparation were investigated. It was found that compounds 1 and 2 were well separated on Chiralpak AD-H and that only compound 2 can be baseline resolved on Kromasil CHI-TBB. Retention factor and resolution for all enantiomers decreased with the content of modifier increased, while minor variation in enantioselectivity was observed. For compound 2, an unusual phenomena, i.e., the retention of enantiomer on Chiralpak AD-H increased when the alcohol modifier was changed from 2-propanol to ethanol, was found. This implied that the chiral discrimination mechanism did not lie in hydrogen bonding.2. The equilibrium constants and axial dispersion and mass transfer coefficients were determined by moment analysis based on the lumped kinetic model for linear chromatography. The results show that axial dispersion coefficient did not change obviously as the temperature varied. In case of Chiralpak AD-H, the axial dispersion coefficients for both enantiomers are about 10^-5 cm²/s, while it is about 10^-4 cm²/s for compound 2 on Kromasil CHI-TBB column. Fast kinetics of mass transfer in both chiral stationary phases was observed and more than 50% of height equivalent of theoretical plate (HETP) comes from axial dispersion. The model parameters obtained were utilized to simulate the elution profiles and the simulated and experimental results match well. 3. A flow-type apparatus has been established for solubility determination of solid in supercritical CO₂. The solubility of paroxetine intermediates in supercritical CO2 was measured in the pressure range from (8 to 24) MPa and at temperatures of (308.15, 318.15, 328.1) K. Results showed that under the present operation conditions, the solubility of compound 1 and 2 were in the range of (7.27×10^-5 to 1.184×10^-3) and (3.38×10^-4 to 4.57×10^-3) in mole fraction, respectively. The solubility increased with rising pressure or density at constant temperature as well with temperature increased at constant density. The crossover pressure for compound 1 and 2 can be found at about 13 and 14 MPa, respectively. The experimental solubility data were correlated with the Peng-Robinson equation of state with an AARD above 10%, besides empirical equations of Kumar-Johnston, Chrastil and Méndez-Santiago-Teja were also used to fit the data with an AARD of 6.59%, 10.12% and 10.70%, respectively.4. Separation of paroxetine intermediates by chiral supercritical fluid chromatography was studied. The effect of pressure, temperature and type of modifier in mobile phase on the retention factor, selectivity and resolution was investigated. It was found that retention for both enantiomers decreased markedly as modifier concentration rose up to 10% (vol%). In the case of racemate 2, better resolution was obtained when using 2-propanol as modifier in the place of methanol and ethanol, while for racemate 1, methanol provided the best performance. Both retention and resolution decreased with increase in pressure. When using 2-propanol as modifier, enantiomers were more retained as the temperature increased. Nevertheless, in the case of methanol as modifier, retention factor only increased with temperature increased at the pressure below 18 MPa. Increasing pressure had little significant effect on the selectivity, and lower temperature was favorable to the enantioseparation.5. The lumped kinetic model for liquid chromatography was adopted to describe the chromatographic process in supercritical fluid by introducing all band broadening effects to the overall mass transfer coefficient, which can be estimated from the equation of HETP. Adsorption isotherms of both enantiomers of compound 1 on Chiralpak AD from supercritical fluid were determined by a characteristic point (ECP) method. It was observed that adsorption capacity decreased with the increasing pressure, and minor variation was found when the temperature altered. With the adsorption isotherms and mass transfer coefficients obtained, the simulated elution profiles showed a good agreement with the experiments.