Racemate resolution via crystallization of diastereomeric salts: thermodynamic considerations and molecular mechanics calculations

2-Mercaptoimidazolium halides: structural diversity, stability and spontaneous racemisation

The complementarity of experiment and theory unravelled structural features, potential polymorphism, moisture and temperature dependent stability and the racemisation behaviour of 2-mercaptoimidazolium salts.

Improved Resolution of 4-Chloromandelic Acid and the Effect of Chlorine Interactions Using (R)-(+)-Benzyl-1-Phenylethylamine as a Resolving Agent

In order to avoid the disadvantage of commonly used resolving agent 1-phenylethylamine (hereafter: PEA), which is soluble in water, (R)-(+)-benzyl-1-phenylethylamine ((R)-(+)-BPA) was used to resolve 4-chloromandelic acid (4-ClMA) in this study. The optimal resolution conditions were determined: absolute ethanol as a solvent, the molar ratio of 4-ClMA to (R)-(+)-BPA as 1:1, the filtration temperature as 15 °C, and the amount of solvent as 1.6 mL/1 mmol 4-ClMA. Thermophysical properties, such as melting point, heat of fusion, and solubility, exhibited significant differences between the less and more soluble salts. The single crystals for the pair of diastereomeric salts were cultivated and their crystal structures were examined thoroughly. In addition to commonly observed interactions like hydrogen bonding and CH/π interactions. The chlorine…chlorine interaction was observed in the less soluble salt presenting as Cl…Cl between adjacent hydrogen network columns, while the Cl/π interaction was observed in the more soluble salt. It was found that halogen interactions played an important role in chiral recognition of 4-ClMA by (R)-(+)-BPA.

Diastereomeric resolution of p-chloromandelic acid with (R)-phenylethylamine

The optical resolution of p-chloromandelic acid using (R)-alpha-phenylethylamine as resolving agent was presented. The effect of solvents, molar ratio of racemate to the resolving agent, filtration temperature as well as the amount of solvent on resolution was investigated by orthogonal experimentation. The binary melting point phase diagram and crystal structure analysis of diastereomeric salts rationalized the success of the resolution.

Structure, Solubility, Screening, and Synthesis of Molecular Salts

The preparation of molecular salts as potential delivery vehicles for pharmaceutically active compounds is more common than current appreciation of the phenomena governing the solubility and isolation of salts suggests. In addition, it would appear that there are no reported measurements on a large enough data set for a serious structure-property relationship analysis to have been performed for this class of material. This means that at present, the ability to predict which salt forms will have desirable physical properties is essentially nonexistent. The work reported here sets out to explore these issues using new data on 17 salts obtained from a screen performed on the basic pharmaceutical ephedrine. The importance of solvent choice in salt formation, of salt selection in the control of bioavailability and of ternary phase equilibria in salt isolation and the relationship between a number of measured and calculated crystal properties are illustrated and discussed. The consequences of these relations for the general design, implementation, interpretation, and scale-up of salts screens are also explored.

Prediction of Solid Solution Formation in a Family of Diastereomeric Salts. A Molecular Modeling Study

The possibility of solid solution behavior of diastereomeric salts, containing multiple resolving agents of the same family (Dutch Resolution), is predicted by molecular modeling. Super-cells containing different ratios of resolving agents in the diastereomeric salt are constructed and optimized, and their lattice energy is computed. The energy difference between these "simulated solid solutions" and the native structures is related in an understandable fashion to the probability of solid solution formation. This procedure is applied to a family of diastereomeric salts of ephedrine and cyclic phosphoric acids, for which the ternary diagrams have been determined experimentally at 25 degrees C in ethanol. Good agreement between experimental and computational results indicates that this relatively simple and fast method could predict the stable character of solid solution behavior in binary systems.