Resolution process of tofisopam has been re-evaluated now based on our new investigations. Originally, it was carried out in the water-chloroform system, where the intermediate salt of high diastereomeric excess was described as (R)-TOF·(R,R)-DBTA·(H2O)3. Opposed to previous assumptions, we have actually found that a different solvate composition, (R)-TOF‐(R,R)-DBTA-CHCl3, forms with chloroform, in which molecules of CHCl3 are captured and held with different strengths. Moreover, resolution of TOF with (R,R)-DBTA is possible (and favourable) in water-free solvent and solvent mixture. However, presence of chloroform is essential, and thus, chloroform is also a suitable solvent alone. Among the tested solvents, toluene-chloroform mixture results in the highest resolution efficiency, while the highest enantiomeric purity was achieved when acetonitrile was in the system too. Resolution efficiency can be also increased by using the quasi-racemic resolving agent and thermodynamic control. Purification of enantiomeric mixtures was examined, and recrystallization of the diastereomeric salt was found to be the most efficient solution. Instructive behaviour of the complex enantiomer-conformer system of tofisopam is emphasized.

Diastereomeric salt crystallization is a convenient method to resolve chiral drug substances when other separation methods like preferential crystallization and solid-state deracemization cannot be applied directly. This is the case...

Diastereomeric salt crystallization is a classical, widely applicable chiral resolution technique, which enables the separation of the enantiomers of both racemate and conglomerate-forming compounds. A resolution method for racemic pregabalin with l-tartaric acid was developed to obtain pure (S)-pregabalin l-tartrate monohydrate crystals with the yield ranging from 43 to 50%. A series of designed resolution experiments were executed at different cooling rates and temperature end points to estimate the crystallization kinetics using population balance modeling. Inline ATR-FTIR measurements of these experiments were used to calculate concentrations in the crystallization phase and to collect solid–liquid equilibrium data by the solubility trace method after product sampling. Since diastereomeric salts are dissociable ionic compounds, solubility product expressions were used in the model to express the relative supersaturation as the driving force of crystallization. As a result, a population balance model with secondary nucleation, growth, and agglomeration mechanisms was identified, and the simulated supersaturation profiles and product size distributions well reproduced the measured data.