Optimisation of the derivatization in cellulose-type chiral selectors for enantioseparation by centrifugal partition chromatography.

Cellulose was chemically modified with hydrophobic dodecanoyl groups followed by 3,5-dimethylphenylcarbamoyl substituents forming mixed ester/carbamate derivatives in order to improve the solubility in lipophilic solvents compared to the corresponding homosubstituted cellulose tris(3,5-dimethylphenylcarbamate). Two mixed derivatives of different degree of substitution were prepared, tested as chiral selectors (CSs) in counter-current chromatography (CCC) and compared to the homo-substituted derivative. Alternatively, cellulose tris(3,5-dichlorophenylcarbamate), was synthesised and tested with the same purpose. The racemic drugs pindolol and warfarin were used as test compounds. Biphasic organic/aqueous solvent systems, composed of methyl isobutyl ketone, t-butyl methyl ether or ethyl acetate and aqueous ammonium acetate or sodium phosphate buffer, were used. Centrifugal partition chromatography, a variety of CCC, in the classical elution mode and the pH-zone-refining displacement mode was applied. The enantioseparation of pindolol and warfarin was achieved in the latter conditions. The presence of dodecanoyl chains on the CS increased solubility in the organic solvents used. The selectivity of the mixed dodecanoyl/3,5-dimethylphenylcarbamoyl cellulose derivative with low degree of dodecanoyl groups (degree of substitution 0.3/degree of substitution 2.6, respectively) was similar to that of the homo-substituted derivative. Furthermore, the loading capacity for pindolol was increased by a factor of three compared to cellulose tris(3,5-dimethylphenylcarbamate). Nevertheless, the increasing degree of substitution with dodecanoyl groups on the CS, although improved solubility in the stationary phase, contributed negatively to the enantioselectivity, where warfarin was more affected than pindolol.