Investigation of the Binary System of Proxyphylline Enantiomers: Structural Resolution and Phase Diagram Determination.

The solid-state landscape of proxyphylline (PXL), a chiral derivative of theophylline crystallizing as a racemic compound, was extensively investigated by means of thermal analyses and diffraction techniques. This study revealed the presence of five distinct polymorphic forms that were characterized: two polymorphs of the racemic mixture and three polymorphs of the pure enantiomer. The nature of each solid phase was confirmed by combining the different analytical techniques, revealing the presence of a thermodynamically stable racemic compound, RI (TFus= 134 °C), in equilibrium with the stable enantiopure crystal form, EI (TFus = 148.3 °C). Additionally, other crystal forms could be evidenced: a polymorph of the racemic compound, RII (TFus= 111.5 °C), as well as two metastable conglomerates, cEI and cEII, and two other polymorphs of the pure enantiomer, EII and EIII. The crystal structures of RI and EI are reported and discussed, highlighting the diversity of molecular conformations that can be adopted by the PXL molecule, which accounts for the versatility of the crystallization behaviors observed in this system. These findings enhance our understanding of the crystallization behavior of chiral pharmaceutical compounds and have implications for optimizing their crystallization processes in the pharmaceutical industry.

Use of Conglomerate Mixed Crystals to Deracemize a Stable Racemic-Compound-Forming System.

Deracemization extended to racemic-compound-forming systems is demonstrated. We present here the first results of an alternative for the resolution of systems that exhibit a stable racemic compound but also a closely related conglomerate-forming system. If the couples of enantiomers forming the racemic compound and the enantiomers of the stable conglomerate can syncrystallize in mirror-related partial solid solutions, it is possible to deracemize the racemic mixture of mixed crystals to access to a single handedness. The evidence for this possibility is given in three examples by using temperature-cycling-induced deracemization.