This is done by adding an enantiopure acid or base to the mixture so that the resulting salts are not mirror images of each other. Instead, they are diastereomers with different chemical and physical properties that allow their separation.
Sometimes companies use diastereomeric crystallization as a rapid method before a permanent solution is found. For example, in the case of Pfizer’s nerve pain treatment Lyrica, the initial route for Lyrica used an equimolar quantity of S-Mandelic acid to resolve the desired enantiomer to obtain multi-kilo quantities of Lyrica; before the enzymatic route was developed [that took a few years].
In other cases, diastereomeric crystallization is used to obtain higher purity. In one case, using enzymatic resolution of alcohol, we were unable to increase enantiomeric purity beyond 90%. Finally, desirable “EE” was obtained when we used diastereomeric crystallization of corresponding acid phthalate. We feel this combination of methods has great utility, substantially reducing the quantities of material that needs to be handled in the conventional chiral salt fractionation.
But in most cases, diastereomeric crystallization is the prefered method of choice for manufacturing that can scale up easily for manufacturing purposes as shown below:
2-Amino-5-methoxytetralin is an intermediate to assemble N-0923 (Nagase & Co., Ltd., Japan), a potent dopamine D-2, effective against Parkinson’s disease (currently in clinical trial).This intermediate is resolved by diastereomeric salt formation with (S)-Mandelic acid. The undesired isomer is racemized to increase efficiency of the process.
In vivo study on Sulphostin, a diapeptidyl peptidase IV inhibitor, its large-scale method was investigated. During the synthesis the most difficult part was simplified by optical resolution of the end product by basic resolving agent (achieved 2-fold process improvement compared to previous method.)
Citalopram, a selective serotonin reuptake inhibitor introduced in 1989 for treatment of depression, is a racemic mixture whose entire inhibitory activity resides in the S (+)-enantiomer, also known as escitalopram. The key intermediate was difficult to purify due to resolution problems, which was addressed nicely by using (+)-di-p-toluoyl tartaric acid and recrystallizing salt twice in medium polar solvent.
AG-7404 (Pfizer), a key intermediate acid was resolved by (-)norephedrine. The acid was enriched with undesired isomer, which was then racemized completely. The final compound was obtained by enzymatic resolution. Thus the method was a combination of diastereomeric crystallization as well as enzymatic resolution.
High performance liquid chromatography (HPLC) is a popular method used by most pharmaceutical companies to get quick analytical quantities of pure enantiomers during the early drug-discovery stage. Using the chiral columns offered by many companies like Chiral Technologies, Regis, Millipore Sigma, Phenomenex, scientists can obtain small quantities of pure enantiomer in the matter of days. However, this technology is not feasible for scale-up manufacturing and requires large amount of solvents.
The technique is based on the following technology: A pair of enantiomers is considered to be resolvable if alpha > 1.1. Here one enantiomer is retarded in its passage through the column because of its preferential binding to the chiral stationary phase. Because of this, the two enantiomers of a racemate emerge from the column at different times and with different volume fractions of eluent. Unfortunately, most chiral resolutions involve only small differences in eluent fractions.
Although most preparative chiral separations have been performed in the conventional batch-mode process, interest in simulated moving-bed (SMB) chromatography is growing.
Simulated Moving Bed or SMB chromatography is used to separate one chemical compound or one class of chemical compounds from a mixture to provide significant quantities of the purified or enriched material at a lower cost than could be obtained using simple (batch) chromatography. It combines continuous countercurrent chromatographic separation with chemical reaction. By using multiple columns in series and a complex valve arrangement, which provides for flow of the feed mixture and solvent ("eluent" or "desorbent") at any column, its performance can be increased dramatically.
SMB technology is being used for the separation of the enantiomers of chiral molecules at production scale. For example, it was used during the separation of the enantiomers of the antiasthmatic agent formoterol and of the antitussive agent guaifenesin. Compared to the batch elution chromatography, a reduction of the mobile phase consumption of respectively 81% and 84% was achieved using SMB. The results show that the influence of the feed rate and extract rate on the separation affect the high purity and productivity and enable the production of both the enantiomers.
SMB offers lower production cost by requiring less column volume, less chromatographic separation media ("packing" or "stationary phase"), and less solvent. Although because of its complex assembly, its adoption requires higher initial cost compared to single column operations, and also leads to higher maintenance cost.
The supercritical fluid chromatography technique or SFC is an innovative technology used in the analysis as well as the separation of various racemates. The various enantiomers have been separated using SFC due to its several merits over conventional HPLC methods such as low cost, wide polarity compatibility, higher column efficiency, and fast analyzing capability, which are desirable features for the pharmaceutical industry.
SFC has been found to be a better technique than HPLC for the chiral separations on a preparative scale because of its requirement of a low volume of solvents and easier recovery of the pure enantiomers. However, it is not cost-effective or scalable in a production environment beyond kilo scale levels.
One of the compounds developed using SFC is Diniconazole. It is a broad-spectrum triazole fungicide that is used to prevent plant diseases such as powdery mildew, bunt, rust, smut, and Septoria leaf spot. Diniconazole is a chiral chemical with one chiral center and is sold commercially as a mixture of R- and S-enantiomers. It has been reported that R-(-)-diniconazole has higher bactericidal activity, while S-(+)-diniconazole shows higher plant growth regulator activity. It is important to know if environmental factors have altered the ratio of R- and S-diniconazole.
Chiral Supercritical Fluid Chromatography (SFC) and mass spectrometry detection were used to study the different ratios of diniconazole in various plant materials. A ChromegaChiral™ CCA column was used to provide baseline resolution of the two diniconazole enantiomers. However, they are closely separated. In this study 14 different chiral stationary phases were evaluated for the chiral SFC separation.
ChiroSolve offers Chiral Resolution Kits
Separation of chiral molecules (enantiomers) from racemic mixture using Diastereomeric Crystallization Technique can be achieved and enhanced by the addition of chiral acids or bases. The resulting formation of salts can be separated on the basis of their differing solubility.
Using this principle, ChiroSolve Inc. has developed Chiral Resolution kits that allow scientists to quickly identify most commercially feasible chiral separation conditions for a given racemate within a matter of days. It does this by offering 96-vial high-throughput Chiral Screen Kits that:
Are made from chemically inert polypropylene material that can withstand temperatures of -200C to 1200C, so that full resolution can be done inside the vials
Contain chirally pure resolving agents that are easily cheaply available in large quantities and that have historically offered chiral separation
Contain 384 to 576 resolution conditions that racemate is checked against in parallel
Offer results within 24 hours after simple heating and cooling efforts