Screening Approach for Identifying Cocrystal Types and Resolution Opportunities in Complex Chiral Multicomponent Systems

Cocrystallization of racemic-compound-forming chiral molecules can result in conglomerate cocrystals or diastereomerically related cocrystals, which enable the application of chiral separation techniques such as preferential crystallization and classic resolution. Here, a systematic method to identify the types and phase diagrams of cocrystals formed by chiral target compounds and candidate coformers in a particular solvent system is presented, which allows the design of suitable chiral resolution processes. The method is based on saturation temperature measurements of specific solution compositions containing both enantiomers of chiral molecules and a coformer. This method is applied to analyze three different systems. For racemic phenylalanine (Phe) in water/ethanol mixtures one of the enantiomers selectively cocrystallizes with the opposite enantiomer of valine (Val), forming the more stable diastereomerically related cocrystal. The racemic compound ibuprofen crystallizes with the nonchiral coformer 1,2-bis(4-pyridyl)ethane (BPN) as racemic compound cocrystals. More interestingly, when it is combined with trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (BPE), the racemic compound ibuprofen cocrystallizes as a conglomerate, which in principle enables the application of preferential crystallization of this racemic compound. The systematic method shows the benefit of using pseudo-binary phase diagrams. Such pseudo-binary phase diagrams depict the saturation temperature on a very specific route through the quaternary phase diagram, allowing the identification of various cocrystal types as well as the corresponding cocrystallization conditions. The systematic method can be used to identify a suitable solid phase for chiral separation, and the obtained phase diagram information enables the performance of a crystallization-mediated chiral resolution process design. Such a guideline for a chiral resolution process design has never been reported for conglomerate cocrystal systems such as IBU:BPE, presented in this study.