Resolution by Preferential Crystallization of Proxyphylline by Using Its Salicylic Acid Monohydrate Co‐Crystal

Simultaneously resolving BINOL and proline using a stoichiometric cocrystal switch

We here highlight the importance of stoichiometry for simultaneous cocrystal resolution. Focusing on combining the racemates of binol and proline, we show that a 1 : 2 ratio leads to formation of a full racemic compound, whereas a 2 : 1 ratio, leads to conglomerate formation, with simultaneous resolution of both binol and proline. Playing on stoichiometry, one achieves a reversible switch between the racemic compound and conglomerate. This is the first investigation of such behavior combining two racemates.

Overcoming a solid solution system on chiral resolution: combining crystallization and enantioselective dissolution

Chiral resolution of rac-4-cyano-1-aminoindane, a key intermediate of ozanimod, was successfully achieved through a combination of crystallization and enantioselective dissolution with up to 96% ee. The disastereomeric salt with di-p-toluoyl-L-tartaric acid was characterized by the construction of a binary phase diagram and ternary isotherm. Enantioselective dissolution was then employed to further enrich the enantiomer.

Reinvestigating the Preferential Enrichment of DL-Arginine Fumarate: New Thoughts on the Mechanism of This Far from Equilibrium Crystallization Phenomenon

Preferential enrichment (PE) is a crystallization process, starting from either a racemic of slightly enantio-enriched solution (ca. +5%) that results in a high enantiomeric excess in the liquid phase (>+90%ee) and a slight opposite excess in the deposited crystals (−2 to −5%ee). The mechanism(s) of this symmetry-breaking phenomenon is (are) still a matter of debate since it eludes rationalization by phase diagram formalism. In this publication, we thoroughly reinvestigate the PE phenomenon of arginine fumarate by using a new approach: the process is monitored by introducing isotopically labeled arginine enantiomers into the crystallization medium to better understand the mass exchanges during crystallization. These experiments are supported by chiral HPLC-MS/MS. This study permits re-evaluating the criteria that were thought mandatory to perform PE. In particular, we show that PE occurs by a continuous exchange between the solution and the crystals and does not require the occurrence of a solvent-mediated solid−solid phase transition.

L-Proline, a resolution agent able to target both enantiomers of mandelic acid: an exciting case of stoichiometry controlled chiral resolution

We present a thought-provoking development in chiral resolution. Using a resolving agent of a given handedness, L-proline, we show that both R- and S-enantiomers of mandelic acid can be resolved from a racemic mixture simply by varying the stoichiometry. We are the first to report this specific feature, achieved by the existence of stoichiometrically diverse cocrystal systems between R- and S-mandelic acid and L-proline.

Steps towards a nature inspired inorganic crystal engineering

This Perspective outlines the results obtained at the University of Bologna by applying crystal engineering strategies to develop nature inspired organic-inorganic materials to tackle challenges in the health and environment sectors. It is shown by means of a number of examples that co-crystallization of inorganic salts, such as alkali and transition metal halides, with organic compounds, such as amino acids, urea, thiourea and quaternary ammonium salts, can be successfully used for (i) chiral resolution and conglomerate formation from racemic compounds, (ii) inhibition of soil enzyme activity in order to reduce urea decomposition and environmental pollution, and (iii) preparation of novel agents to tackle antimicrobial resistance. All materials described in this Perspective have been obtained by mechanochemical solvent-free or slurry methods and characterized by solid state techniques. The fundamental idea is that a crystal engineering approach based on the choice of intermolecular interactions (coordination and hydrogen bonds) between organic and inorganic compounds allows obtaining materials with collective properties that are different, and often very much superior to those of the separate components. It is also demonstrated that the success of this strategy depends crucially on cross-disciplinary synergistic exchange with expert scientists in the areas of bioinorganics, microbiology, and chirality application-oriented developments of these novel materials.

Simultaneous Chiral Resolution of Two Racemic Compounds by Preferential Cocrystallization*

We tap into an unexplored area of preferential crystallization, being the first to develop simultaneous chiral resolution of two racemic compounds by preferential cocrystallization. We highlight how the two racemic compounds RS-mandelic acid (MAN) and RS-etiracetam (ETI) can be combined together as enantiospecific R-MAN⋅R-ETI and S-MAN⋅S-ETI cocrystals forming a stable conglomerate system and subsequently develop a cyclic preferential crystallization allowing to simultaneous resolve both compounds. The developed process leads to excellent enantiopurity both for etiracetam (ee>98 %) and mandelic acid (ee≈95 %) enantiomers.

An industrial perspective on co-crystals: Screening, identification and development of the less utilised solid form in drug discovery and development

Active pharmaceutical ingredients are commonly marketed as a solid form due to ease of transport, storage and administration. In the design of a drug formulation, the selection of the solid form is incredibly important and is traditionally based on what polymorphs, hydrates or salts are available for that compound. Co-crystals, another potential solid form available, are currently not as readily considered as a viable solid form for the development process. Even though co-crystals are gaining an ever-increasing level of interest within the pharmaceutical community, their acceptance and application is still not as standard as other solid forms such as the ubiquitous pharmaceutical salt and stabilised amorphous formulations. Presented in this chapter is information that would allow for a co-crystal screen to be planned and conducted as well as scaled up using solution and mechanochemistry based methods commonly employed in both the literature and industry. Also presented are methods for identifying the formation of a co-crystal using a variety of analytical techniques as well as the importance of confirming the formation of co-crystals from a legal perspective and demonstrating the legal precedent by looking at co-crystalline products already on the market. The benefits of co-crystals have been well established, and presented in this chapter are a selection of examples which best exemplify their potential. The goal of this chapter is to increase the understanding of co-crystals and how they may be successfully exploited in early stage development.

Cocrystals of Praziquantel: Discovery by Network-Based Link Prediction

Cocrystallization has been promoted as an attractive early development tool as it can change the physicochemical properties of a target compound and possibly enable the purification of single enantiomers from racemic compounds. In general, the identification of adequate cocrystallization candidates (or coformers) is troublesome and hampers the exploration of the solid-state landscape. For this reason, several computational tools have been introduced over the last two decades. In this study, cocrystals of Praziquantel (PZQ), an anthelmintic drug used to treat schistosomiasis, are predicted with network-based link prediction and experimentally explored. Single crystals of 12 experimental cocrystal indications were grown and subjected to a structural analysis with single-crystal X-ray diffraction. This case study illustrates the power of the link-prediction approach and its ability to suggest a diverse set of new coformer candidates for a target compound when starting from only a limited number of known cocrystals.