Separation or Resolution of Enantiomers

Because the physical properties of enantiomers are identical, they seldom can be separated by simple physical methods, such as fractional crystallization or distillation. It is only under the influence of another chiral substance that enantiomers behave differently, and almost all methods of resolution of enantiomers are based upon this fact. We include here a discussion of the primary methods of resolution. 

19-3A Chiral Amines as Resolving Agents. Resolution of Racemic Acids 

The most commonly used procedure for separating enantiomers is to convert them to a mixture of diastereomers that will have different physical properties  

Resolution of chiral acids through the formation of diastereomeric salts requires adequate supplies of suitable chiral bases. Brucine, strychnine, and quinine frequently are used for this purpose because they are readily available, naturally occurring chiral bases. Simpler amines of synthetic origin, such as 2-amino-1-butanol, amphetamine, and 1-phenylethanamine, also can be used, but first they must be resolved themselves. 

Chiral acids, such as (+)-tartaric acid, (—)-malic acid, (—)-mandelic acid, and (+)-camphor-10-sulfonic acid, are used for the resolution of a racemic base. 

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Crystallization methods are widely used for the separation, or resolution, of enantiomer pairs. Enantiomer mixtures may essentially crystallize in two different ways. In around 8 per cent of cases, each enantiomer crystallizes separately, giving rise to a mechanical mixture of crystals of the two forms, known as a conglomerate. Conglomerates may usually be separated by physical methods... 

In contrast to the case with diastereomers, separation or resolution of enantiomers is generally difficult because of the identity of their chemical properties their differing effect on plane-polarized light is not helpful for separation. Nonetheless, various experimental techniques have been developed to accomplish resolutions, and one of them is presented in Section 7.6. 

Most methods for the resolution of enantiomers contained in a reaction mixture consist in the conversion of the compounds into stable or transient diastereoisomers and separation of the latter on the basis of their different physico-chemical properties. 

Figure 2b shows the other extreme, whereby the rate of epimerization is fast relative to the rate of substitution. In this case, Curtin-Hammett kinetics apply, and the product ratio is determined by AAG. In the specific case of organolithium enantiomers that are rendered diastereomeric by virtue of an external chiral ligand, such a process may be termed a dynamic kinetic resolution. Both of these processes are also known by the more general term asymmetric transformation One should be careful to restrict the term resolution to a separation (either physical or dynamic) of enantiomers. An asymmetric transformation may also afford dynamic separation of equilibrating diastereomers, but such a process is not a resolution. "... 

Chromatography. Under certain conditions, even homochiral and het-erochiral self-assemblies can be separated by achiral methods. Thus, chromatography of partially resolved enantiomers can cause depletion or enrichment of enantiomers on achiral stationary phases with an achiral mobile phase. 14C-Labeled nicotine was first resolved into its enantiomers by high-performance liquid chromatography (HPLC) on an achiral stationary phase (Partisil-ODS or -SCX) through coinjection with optically active nicotine (59). This observation was followed by resolution of a number of chiral compounds by chromatography (<50-62) (Scheme 34). When a chiral diamide in 74% ee was separated on a Kieselgel 60... 

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. 

Capillary approaches have been shown to be useful for many chiral separations as well as achiral separations. For chiral separations, separation buffer additives containing chirogenic centers (tecoplainin, erythromycin, vancomycin, or cyclodextrans) have facilitated the resolution of enantiomers [26,30,31]. Chiral capillary separations could readily be combined with mass spectrometry because the volume of effluent moving from the separation capillary to the ion source is small and makeup solvent is commonly added by means of an union to stabilize the ion beam. Chiral capillary separations provide an attractive alternative to analytical-scale normal-phase separations when using atmospheric pressure ion-ization mass spectrometry. 

We became interested in synthesizing both the enantiomers of differolide to clarify whether one or both of them are bioactive. Our synthesis is summarized in Figure 5.14 and 5.15.23 Because both the enantiomers 135 and 135 were necessary for bioassay, we adopted enantiomer separation (optical resolution) of an intermediate as our key step (Figure 5.14). A crystalline acetal (-)-B was obtained from ( )-A and (—)-menthol, and analysed by X-ray to reveal its structure as (—)-B, basing on the known absolute configuration of (-)-menthol. When (+)-menthol was used for acetal formation, crystalline (+)-B was obtained in a similar manner. We thus secured both (-)-B and (-t-)-B as pure crystals. 

MIP membrane adsorbers for the specific sample enrichment from large volumes by membrane SPE, and for the specific decontamination of large process streams will be among first examples for applications (cf Section V.D). Other promising continous separations are the resolution of enantiomers or the product removal from bioreactors, both feasible by electrodialysis or dialysis (cf. Section V.B). 

If the starting ester is chiral and present as a mixture of its enantiomers, the lipase enzyme reacts selectively with one enantiomer to release the corresponding chiral carboxylic acid and an alcohol, while the other ester enantiomer remains unchanged or reacts much more slowly. The result is a mixture that consists predominantly of one stereoisomer of the reactant and one stereoisomer of the product, which can usually be separated easily on the basis of their different physical properties. Such a process is called a kinetic resolution, where the rate of a reaction with one enantiomer is different than with the other, leading to a preponderance of one product stereoisomer. We shall say more about the resolution of enantiomers in Section 5.16. The following hydrolysis is an example of a kinetic resolution using lipase ... 

Terpene-derived bis-N-oxide 21.22 represent the most recent addition to the successful catalyst series. The catalyst was shown to be particularly efficient in the allylation and crotylation of aromatic and a,p-unsaturated aldehydes (<99% enantiomeric excess at —60°C), however, with aliphatic aldehydes the selectivity dropped to 50% ee. It is noteworthy that 21.22 was synthesised in four easy steps from inexpensive (I )-myrtenal and the protocol is amendable to scaling up. In contrast, synthesis of enantiopure catalysts 21.19-21.21 requires either resolution of enantiomers or separation of diastereoisomeric mixtures, which hampers their larger-scale application. In this group of polydentate IV-oxides it is also worth mentioning terpyridine N,IV IV"-trioxide, the related bis-imidazole Af,M -dioxides and chiral dinitrones,but their efficiency was inferior to the best pyridine-type dioxides, such as 21.20-21.22. 

The most popular thin layer chromatography (TLC) techniques for separation of enantiomers are described here 1) use of non-chiral phases for indirect resolution of optical isomers after derivatization to obtain the corresponding diastereoisomers and 2) direct resolution of enantiomers using chiral stationary phases or chiral mobile phases. Advantages and limits of all reported techniques are discussed. 

In the area of cyclodextrin ethers the -compound has been converted into a set of five tris-Tbdms ethers, all substituted at their various 6-positions, which were separated by hplc and characterized by n.m.r. spectroscopy. Related work applied to y-cyclodextrin gave the various 6,6 -disubstituted ethers. 5-Bromo-l-pentene was used to produce the 2-0-mono-4-pentenyl ether of P-cyclodextrin which was then permethylated and the product was chemically bonded to silica gel to form an efficient hplc stationary phrase for the separation of enantiomers. Peroctyl a-cyclodextrin has been studied as a chiral receptor for the ephedrinium ion. Various octyl ethers of a-, P- and y-cyclodextrin ranging in their substitution from the diethers to completely alkylated products were characterized by electrospray mass spectrometry and n.m.r. methods applied to methylated derivatives. The 2,6-didodecyl derivative of p-cyclodextrin has been used as a potentiometric sensor. In the field of aromatic ethers, naphthyl carboxylate substituents have been bonded at the 6-positions and the products were able to transfer excitation energy to complexed merocyanine held in the cavities of those molecules. These phototransfer processes were extremely efficient.P-Substituted cyclodextrin derivatives with p-allyloxybenzoyl or various benzyl substituents at 0-2 or 0-3 were incorporated by hydrosilylation to give hydromethylpolysiloxane polymers used as chiral phases for chromatographic resolution of enantiomers. Cyclodextrins with complex benzyl-like eth are illustrated in 22 and 23. The latter were prepared as artificial redox enzymes. 

Chromatography and Capillary Electrophoresis Chromatographic resolution is oriented toward the exclusive separation of chiral substances using columns with a chiral stationary phase from which the enantiomers are separated through diastereomeric interactions. Resolution of enantiomers contained in the mobile phase (gas or liquid)... 

Macrocyclic antibiotics, for example, erythromycin and vancomycin, have also been used as impregnating agents for TLC resolution of enantiomers of dansyl amino acids by Bhushan and Parshad [6] and Bhushan and Thiong o [7]. Separation in this case is due to chirality of macrocyclic antibiotic molecule, which is ionizable and contains hydrophobic and hydrophilic moieties as well providing enantioseparation via n-n complexation, H-bonding, inclusion in a hydrophobic pocket, dipole stacking, steric interactions, or combination thereof. In view of the scope of the chapter and nonavailability of reports related to NSAIDs, these are not being discussed here. 

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