Chiral Resolution with and without Resolving Agents

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The vast majority of new small-molecule drug candidates are chiral, and methods must be identified for obtaining the desired enantiomer in high purity. A common approach is to separate the enantiomers of a racemate. Certain chemo- and biocatalysts mediate transformations that provide chiral chemicals with high enantioselectivities; however, in most, both the desired and undesired enantiomers are obtained. Diastereomeric resolutions via salt formation, kinetic resolution, and chiral chromatography are techniques commonly used at commercial scale. All of these methods have drawbacks, and pharmaceutical companies are always seeking alternatives that carry lower costs and can be completed in shorter times

Why resolution?
Many compounds with bioactivity exist in two forms; enantiomers only differ by the way in which they rotate light. These enantiomers have identical chemical and physical properties, but interact differently with other chiral molecules, including many important biochemicals. Therefore, when a new candidate API is identified, a synthetic route that produces only the correct enantiomer must be developed.

Finding efficient methods for the production of chirally pure APIs is a continually important topic in the pharmaceutical industry, according to Robert Hof, chief operating officer of contract research organization Syncom, based in Groningen, The Netherlands. Chiral racemates remain one of the major sources for the single enantiomers required for chiral APIs and drugs. “Resolution is absolutely necessary for separating enantiomers,” asserts R.M. Kellogg, a cofounder of Syncom. Fortunately, he notes that striking advances have been made in diastereomeric resolutions, kinetic resolutions (stoichiometric, catalytic, and parallel), and chromatographic techniques. “The availability of many different methodologies allows the selection of the best methodology for a particular compound,” Kellogg says.

Chiral resolution by either diastereomeric salt formation or biocatalysis remains one of the most applied technologies due to its relative simplicity and scalability, according to Hof. “In particular, when racemization loops or-even better-dynamic kinetic approaches can be implemented for compounds with racemization-prone functionalities, 100% yield with 100% enantiomeric excess (pure enantiomer) can be achieved,” Hof adds.

Resolution by diastereomeric salt formation is often applied to amines and acids because they have reactive groups that can form salts with chiral resolving agents. Alcohols are often resolved using biocatalysts. For apolar compounds including alcohols, many amines, thiols, and sulfides, biocatalysis, sometimes combined with organometallic catalysts, can be the method of choice, according to Kellogg.

The challenge of resolving agents
When a racemate is separated via diastereomeric salt formation, the two enantiomers react with a single enantiomer of a chiral compound that will form a salt with both enantiomers. Because each component of the salt is chiral, the product has two chiral centers and is considered to be a diastereomer. Unlike enantiomers, diastereomers have different physical properties and often can be readily separated from one another via crystallization.

While there are well-established, inexpensive basic (e.g., cinchonidine and phenethylamine) and acidic (e.g., tartaric and camphor-1-sulfonic acids) resolving agents that are available on a large scale, efforts are continually being invested in the development of novel resolving agents that can be produced cost-effectively in quantities required for the production of commercial APIs. Syncom, for example, has developed strongly acidic chiral phosphoric and sulfonic acids. Others have also developed more economical, scalable routes for the production of older resolving agents that previously were not cost-effective for commercial API synthesis.

One challenge with the diastereomeric salt approach, however, is that the resolving agent must be relatively cheap or readily recyclable, according to Hof. “Large-scale manufacturing is all about process simplicity and a low cost of goods. For chiral resolution, therefore, the key is identifying a relatively cheap or easily recyclable resolving agent and, ideally, racemization conditions for the unwanted isomer,” he says.

Attrition-enhanced chiral resolution
An alternative approach is to find a way to separate enantiomers without the need to form diastereomeric salts. In fact, there are many types of chiral compounds that cannot form salts or for which it is difficult to do so due to steric hindrance, reduced reactivity, or other reasons.

For these compounds, Syncom has focused on attrition-enhanced deracemization, or Viedma ripening, in which chiral compounds that are conglomerates (i.e., the enantiomers crystallize separately) are resolved efficiently without the need to use a chiral resolving agent. The separation is achieved by grinding the solid racemate of a conglomerate under racemizing conditions. Separation of the enantiomers occurs because in a conglomerate, each enantiomer has a greater affinity for the same enantiomer and ultimately the two enantiomers will crystallize separately to form a mechanical mixture. This process is accelerated by creating very small crystals for more rapid dissolution and when performed under racemizing conditions provides a pure, single enantiomer in high yield.

Because few organic compounds are conglomerates, it is necessary to form derivatives of most substances to obtain conglomerates suitable for attrition-enhanced deracemization. A demonstration of this technique is an alternative synthesis of clopidogrel developed in the Syncom laboratories (1). “This need has stimulated search techniques for conglomerates (second harmonic generation techniques) and also more profound studies of crystallization behavior in order to predict the formation of conglomerates,” notes Kellogg.

In addition, Syncom is working with various partners to develop standard pharmaceutical multipurpose manufacturing equipment for performing commercial-scale, attrition-enhanced deracemization. The company is also working to develop attrition-induced resolutions of non-racemisable conglomerates, and recently reported the resolution of a salt of omeprazole, a drug for treatment of the symptoms associated with gastroesophageal reflux disease (GERD) and other conditions caused by excess stomach acid (2). 

Enantiospecific cocrystallization in solution
Researchers at Université Catholique de Louvain (UCL) in Belgium have developed another new resolution technology that does not require the use of chiral resolving agents. This approach is based on the fact that one enantiomer of a chiral compound will form a cocrystal with only one of the two enantiomers of a chiral coformer, allowing the resolution of a racemic mixture via enantiospecific cocrystallization in solution (3). 

The technique was demonstrated using an enantiomeric mixture of 2-(2-oxopyrrolidin-1-yl)butanamide. The S-isomer is an API (levetiracetam) for the treatment of epilepsy that must be separated by chiral chromatography because no effective chiral resolving agent has been found. A structure-based cocrystal screen was performed to identify coformer candidates, and then ternary-phase diagrams were constructed for different temperatures to identify the ideal coformer and conditions for enantioenrichment. Notably, after just one cocrystallization step, 70% of the S-enantiomer was separated. 

This approach to chiral resolution is attractive because it does not require the use of resolving agents and can be applied to compounds that do not readily or cannot form diastereomeric salts, many of which would require separation via chiral chromatography. The coformer technique is more cost-effective than chiral chromatography and can be performed using typical crystallization equipment (3). Syncom also participates in a project funded by the Dutch National Science Foundation in which co-crystallization together with attrition-induced deracemization is used to resolve commercial products, such as Naproxen.

Looking to nature
Even with these attractive alternative technologies, the search for improved chiral resolution methods is far from over. “Nature sometimes separates enantiomers for us. Roughly 10----15% of chiral organic compounds are conglomerates for which the enantiomers crystallize separately from one another. Could we imitate nature and learn how to crystallize compounds in a single step as single enantiomers and at the same time separate them? This scientific challenge remains as yet unanswered and is waiting to be met,” Kellogg asserts.

References
1. M.W. van der Meijden, et al., Org. Process Res. Dev. 13 (6), 1195-1198 (2009).
2. R.M. Kellogg, et al., Org. Proc. Res. Dev. 17 (6), 946-950 (2013).
3. G. Springuel and T. Leyssens, Cryst. Growth Des. 12 (7) 3374-3378 (2012).

Article DetailsPharmaceutical Technology
Vol. 39, Issue 2
Pages 40–41

Citation
When referring to this article, please cite it as:
C. Challener, " Chiral Resolution with and without Resolving Agents," Pharmaceutical Technology 39 (2) 2015.