Resolution—The Separation of Enantiomers

Since all living cells and organisms involve reactions of enantiomerically pure materials such as carbohydrates, proteins, and DNA, most naturally occurring chiral compounds exist in enantiomerically pure form. Chemical reactions, however, often produce racemic mixtures. This is always the case if only racemic and/or achiral reactants, reagents, catalysts, and solvents are used. The products of chemical reactions can be enantiomerically enriched or enantiopure only if chiral starting materials, reagents, catalysts or solvents are used. (See Section 2.5 for a discussion of enantiose-lective reactions.) Racemic mixtures can be separated into the two enantiomeric forms. The process of separating a racemic mixture into its enantiomers is called resolution, and it can be accomplished in several different ways. 

Carry out Incomplete reaction with enantiomerically pure reagent 

If rate for f -enantiomer S-enantiomer Unreacted material is enriched in S-enantiomer product enriched in derivative of f -enantiomer 

In a resolution with the initial concentrations being equal, [5] = [/ ] the enantiomeric selectivity ratio E is the relative rate given by 

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We discussed resolution, the separation of enantiomers by the formation of dfaster ofeomers with en optically active resolving agent, in Chapter 16. 

Another means of resolution depends on the difference in rates of reaction of two enantiomers with a chiral reagent. The transition-state energies for reaction of each enantiomer with one enantiomer of a chiral reagent will be different. This is because the transition states and intermediates (f -substrate... f -reactant) and (5-substrate... R-reactant) are diastereomeric. Kinetic resolution is the term used to describe the separation of enantiomers based on different reaction rates with an enantiomerically pure reagent. 

Recently, two examples of the separation of enantiomers using CCC have been published (Fig. 1-2). The complete enantiomeric separation of commercial d,l-kynurenine (2) with bovine serum albumin (BSA) as a chiral selector in an aqueous-aqueous polymer phase system was achieved within 3.5 h [128]. Moreover, the chiral resolution of 100 mg of an estrogen receptor partial agonist (7-DMO, 3) was performed using a sulfated (3-cyclodextrin [129, 130], while previous attempts with unsubstituted cyclodextrin were not successful [124]. The same authors described the partial resolution of a glucose-6-phosphatase inhibitor (4) with a Whelk-0 derivative as chiral selector (5) [129]. 

A lot of published data on the separation of enantiomers of flavors and fragrances by GC is reviewed by Chirbase/Flavor database. Table 1. summarizes the enantiomer separation of oxygenated monoterpenes on chiral stationary phases of cyclodextrin derivatives by high resolution gas chromatography. 

Optical. - This adjective exclusively refers to measurements of optical rotation. It must not be used other than in the combinations optical rotation and optical purity. In particular, the terms optical resolution (better separation of enantiomers) and optical yield must be avoided. 

Several types of CSPs are commercially available. However, many of these columns provide only adequate separation of enantiomers at best and often do not provide baseline resolution. In addition, many CSPs are expensive, costing as much as 18,000 per column. CSPs using mesoporous silica may be able to enhance the separation of enantiomers and are also produced more cheaply than commercial materials... 

In view of the importance of chiral resolution and the efficiency of liquid chromatographic methods, attempts are made to explain the art of chiral resolution by means of liquid chromatography. This book consists of an introduction followed by Chapters 2 to 8, which discuss resolution chiral stationary phases based on polysaccharides, cyclodextrins, macrocyclic glyco-peptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ethers. The applications of other miscellaneous types of CSP are covered in Chapter 9. However, the use of chiral mobile phase additives in the separation of enantiomers is discussed in Chapter 10. 

The properties of partially acetylated cellulose for the separation of enantiomers were recognized in 1966 by Luttringhaus and Peters [14]. But the full potential of cellulose acetate was developed by Hesse and Hagel in 1973 [15]. Since then, this stationary phase has been frequently used for the resolution of various racemic compounds. Native cellulose acetylated heterogeneously yields a crystallographic form of cellulose triacetate (CTA) known as CTA-I, which corresponds to its source, an indication that the original supramolecular structure of the starting... 

LIPASE-CATALYZED KINETIC RESOLUTION OF RACEMATES A VERSATILE METHOD FOR THE SEPARATION OF ENANTIOMERS... 

The lipase-catalysed access to enantiomerically pure compounds remains a versatile method for the separation of enantiomers. The selected examples shown in this survey demonstrate the broad applicability of lipases in terms of substrate structures and enantioselectivity. More recently, modem molecular biology methods such as rational protein design and especially directed evolution103 will further boost the development of tailor-made lipases for future applications in the synthesis of optically pure compounds. It has been already shown that a virtually non-enantioselective lipase (E=l.l in the resolution of 2-methyldecanoate) could be evolved to become an effective biocatalyst (E>50). Furthermore, variants were identified which showed opposite enantiopreference. 

If we need one pure enantiomer of butan-2-ol, we must find a way of separating it from the other enantiomer. The separation of enantiomers is called resolution, and it is a different process from the usual physical separations. A chiral probe is necessary for the resolution of enantiomers such a chiral compound or apparatus is called a resolving agent. 

The oldest example of molecular chiral recognition described by Pasteur [1] is the separation of enantiomers based on diastereoisomeric salt formation and subsequent fractionated crystallisation. The principle of the enantiomeric differentation is that one of the salts formed with a chiral reagent is less soluble than the other, and thus precipitates from the solution. This enrichment of one of the enantiomers leads to the optical resolution... 

On this basis the choice of a specific catalytic step is usually determined by an economic analysis of the catalytic route vi. alternative routes (such as the separation of enantiomers by resolution, organic synthesis starting from chiral natural products used as building blocks (the chiral pool concept), or the use of enzymatic and microbial transformations) and time to market considerations (see also Chapter 1). 

While Pasteur made the historical discovery, subsequent advances in the resolution of enantiomers by crystallization were based on empirical results. Several attempts to separate enantiomers using paper chromatography were met with unsystematic results. In 1952 Dalgliesh postulated that three points of simultaneous interaction between the enantiomeric analyte and the stationary phase are required for the separation of enantiomers [2]. 

The advent of modern column LC in the 1970s rapidly led to the use of this chromatographic technique in the separation of enantiomers as dia-stereomeiic derivatives. Today, most of the reported new developments in indirect enantiomer resolutions use LC, and LC is particularly important in the resolution of chiral pharmaceuticals. 

If an ordinary chemical synthesis yields a racemic modification, and if this cannot be separated by our usual methods of distillation, crystallization, etc., how do we know that the product obtained is a racemic modification It is optically inactive how do we know that it is actually made up of a mixture of two optically active substances The separation of enantiomers (called resolution) can be accomplished by special methods these involve the use of optically active reagents, and will be discussed later (Sec. 7.9). 

Resolution is the separation of enantiomers from a racemic mixture and is usually achieved by one of three methods ... 

Resolution is the separation of enantiomers. We have seen resolution before in Chapter 22. A kinetic resolution is a resolution which works simply because, under the right conditions, one enantiomer of a racemic mixture reacts faster than the other.1... 

A commercial CE system and a micropacked capillary was used to separate N—, O—, and S-containing heterocyclic compounds. Migration time reproducibility, linearity, and detector response was found to be comparable to HPLC. A study of the heterocyclic compound s elution order followed that predicted by the octanol-water partition coefficients (354). While chiral CEC provides improved resolution and higher efficiencies, additional work is needed since chiral CEC capillaries are not available commercially. The separation principles and chiral recognition mechanism for the separation of enantiomers have been reviewed (355). Furthermore, a comprehensive collection of drug applications and other compounds of interest has been reported (356). Direct enantiomeric separations by CEC were studied using a capillary packed with alpha-1-acid glycoprotein chiral stationary phase (357). Chiral resolution was achieved for enantiomers of benzoin, hexobarbital, pentobarbital, fosfamide, disopyramide, methoprolol, oxprenolol, and propanolol. The effects of pH, electrolyte concentration, and con-... 

From amongst the numerous applications of adsorption column chromatography, an interesting example is provided by the separation of a mixture of two enantiomers into its constituents whose separation cannot be carried out by the other usual physical methods like fractional crystallization or fractional distillation. A well-known illustration of the separation of enantiomers without their having to be converted into diastereoisomers by chemical reaction with optically active acids is the resolution of Troger s base, effected by V. Prelog and P. Wieland in 1944,... 

Resolution The separation of a racemic mixture into (at least one of) its component enantiomers. See also kinetic resolution. 

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