Preferential crystallization enantiomer separation

The separation of a racemic compound into its enantiomers is called resolution. Various methodologies have been used for the resolution of the enantiomers on both analytical and preparative scales. The different techniques may be categorized into two classes the classical approach, using enzymatic degradation of one of the enantiomers, and preferential crystallization. Modem technologies include spectroscopic, electrophoretic, and chromatographic methods. 

A more useful variation of the mechanical separation applied by Pasteur is the method of inoculation, originally discovered by Gemez in 1866.7 This is a method in which a supersaturated solution of a racemic mixture is inoculated with pure crystals of one of the enantiomers and let only the crystals of the same kind of enantiomer are allowed to grow selectively. This method is called preferential crystallization or entrainment. 

In case the racemate is a true racemic mixture, this cannot be separated by preferential crystallization, but can be resolved using the diastereomer crystallization developed by Pasteur in 1848. A solution of the racemic mixture in water or methanol is allowed to react with a pure enantiomer (resolving agent), thereby forming a mixture of diastereomers that can be separated by crystallization. 

Resolution by entrainment can sometimes be used to separate racemic mixtures when there are distinct differences in the rates of crystallization of the two optical isomers. This preferential crystallization is initiated by seeding with the crystals of one enantiomer. This technique has been shown to be effective in the production of thiamphenicol (21). 

Crystallization Method. Such methods as mechanical separation, preferential crystallization, and substitution crystallization procedures are included in this category. The preferential crystaUization method is the most popular. The general procedure is to inoculate a saturated solution of the racemic mixture with a seed of the desired enantiomer. Resolutions by this method have been reported for histidine (43), glutamic acid (44), DOPA (45), threonine (46), A/-acetyl phenjialanine (47), and others. In the case of glutamic acid, the method had been used for industrial manufacture (48). 

In order to prevent spontaneous crystallization of the other isomers, processes to remove concentration (supersaturation) of the other isomers are essential. In general this has been done by crystallization of the undesired enantiomer in parallel or in series with the crystallization of the desired enantiomer. Figure 13 (9) illustrates some parallel processes where undesired enantiomers are crystallized either in a separate crystallizer where mother liquors circulate or in a single crystallizer with separated space. The obtained undesired enantiomer is then converted into the desired enantiomer by the racemization reaction to improve the yield of the desired component. Our proposal is to combine the preferential crystallization of the desired isomer with the racemization reaction in a single crystallization vessel (10), The idea is not new and is outlined in a book by Jacques et al. where the use of aldehyde or ketone as the catalyst for the racemization reaction is suggested. 

By means of various analytical techniques, we can observe homochiral supramolecular architectures in a wide range of phases, from the crystalline state to a dilute solution, as described above. If we want to apply these systems for practical use. such as in optical resolution of racemates, the next crucial subject is how to manipulate the homochiral assemblies. In the case of separation of conglomerates, one can often sort the enantiomers by hand, as was performed by Pasteur. Although this method can be applied only when crystals have sufficient size to allow determination of their sign of rotation, an alternative technique named "preferential crystallization" was developed, which has its origin in 1866 in the work of Gernez. " He observed that a supersaturated solution of racemic dextrorotatory salt yielded only... 

Etodolac that belongs to a class of nonsteroidal antiinflammatory drugs, used for the treatment of mild-to-moderate pain, fever, and inflammation, can be resolved by preferential crystallization after a conglomerate formation of its derivatives. Using this method, etodolac enantiomers were recovered with an overall yield of more than 20% and the purities were greater than 99.9%. Methods for the separation of enantiomers dming crystallization are discussed in chapter 56 in more detail. 

Preferential crystallization is an attractive crystallization technique to separate pure enantiomers from conglomerate mixtures. It was discussed in great detail by Coquerel" and Saigo and Sakai. ... 

Polenske D, Lorenz H, Seidel-Morgenstern A. Separation of propranolol hydrochloride enantiomers by preferential crystallization thermodynamic basis and experimental verification. Cryst. Growth Des. 2007 7 (9) 1628-1634. 

These are all unusual. Thus, by collecting the enantiomerically enriched mother liquors with the same handedness, very efficient separation of the two enantiomers (>96% ee) has been easily achieved. Therefore, to probe if Preferential Enrichment occurs or not for a given compound, one only have to repeat recrystallization of the racemic sample several times at 25, 0, or -20°C and measure the ee value of the supernatant solution after each crystallization. 

Resolution The reagent 1 reacts with racemic alcohols, thiols, amines, and cyanohydrins to give diastcreomeric derivatives that can be separated by column chromatography or crystallization. Subsequent hydrolysis gives the enantiomers and 1 is recovered. With racemic alcohols, preferential acetalization of the (R)-cnantiomers is observed. 

Dilongifolylborane is simply prepared by admixture of borane-dimethyl sulfide and 2 equiv. (+)-lon-gifolene in THF, whereupon the product crystallizes out of solution as the dimer, and can readily be separated from the solvent. It is used as a suspension for hydroboration reactions. Diisopinocampheylborane can be prepared in a similar way from (+)- or (-)-a-pinene, and early work was carried out with reagent so prepared. However, ot-pinene is often available only in purities up to ca. 95%, so that Ipc2BH produced by direct hydroboration can also be somewhat impure. Fortunately, equilibration of the reagent (dimethyl sulfide must first be removed if borane-dimethyl sulfide is used for the hydroboration) with cn-pinene, at 0 C over several days, results in preferential incorporation of the major enantiomer of a-pinene into the Ipc2BH. This then becomes available in 98-99% enantiomeric purity. 

The title compound (107) [42] has been found to form inclusion complexes with a wide variety of solvent molecules [43]. In the complexation, racemates or conglomerates of 107 were formed, depending on the choice of solvent. In the latter case, the inclusion crystals consisting of one enantiomer of 107 were formed preferentially and the enantiomeric separation of 107 could be performed. For example, recrystallization of rac-107 from the solvents shown in Table 3.3-10 gave a 1 1 complex of the rac-107 with the solvent as yellow crystals. On the other hand, recrystal-lization of the rac-107 from the solvents shown in Table 3.3-11 gave a 1 1 complex... 

Occurrence of Spontaneous Crystallization of an Unseeded Enantiomer. When we consider the purity drop on the phase diagram, it is the prerequisite that the seeds added to cause the preferential resolution are completely pure. However if the seeds are contaminated by the undesired enantiomer, the phenomena could be completely different. Since in general seeds are taken from a previous batch, they are likely to have been contaminated by the adhered mother liquor due to incomplete phase separation or by some non-equilibrium crystallization phenomena such as inclusion of mother liquor or agglomeration of crystalline particles. For the case of the former, drying of the crystals generates small particles of the undesired enantiomer which will then be introduced to a racemic solution as seeds. 

One striking example mentioned in this final chapter requires us to bend the term racemate, to include very near racemates that contain a very small enantiomeric excess. Enrichment of such samples by direct crystallization-based methods would typically only be attempted by committed optimists. In such a situation, we could synthesize more of the excess enantiomer preferentially if we had an appropriately asymmetric autocatalytic reaction - our initial excess enantiomer could replicate at the expense of the other. Preparatively, this is the effective separation of the enantiomers we used at the outset. Such a system has its physical realization in the Soai autocatalysis in which a very small enantiomeric excess of a pyrimidyl alcohol is amplified over several cycles to give an almost enantiopure sample of the alcohol (Scheme 1.10) [19]. 

The lipase-catalysed resolution of alcohols or acids relies on the preferential reaction of one enantiomer (Scheme 4.3). The racemic alcohol (a) or add (b) can undergo transesterification with either an acylating agent or alcohol respectively. In this way, the products of the reaction can be easily separated by chromatography or re-crystallization. 

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