Achiral stationary phase

An alternative model has been proposed in which the chiral mobile-phase additive is thought to modify the conventional, achiral stationary phase in situ thus, dynamically generating a chiral stationary phase. In this case, the enantioseparation is governed by the differences in the association between the enantiomers and the chiral selector in the stationary phase. 

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

This strategy is the one most commonly used for the analytical determination of ena-tiopurity. A given racemate is reacted with a unichiral derivatizing agent, and the resulting pair of diastereomers is separated on an achiral stationary phase, in most of the cases on a reversed-phase type (Fig. 7-2). 

In general, retention decreases as the modifier concentration increases because the modifier competes with the analytes for sites on the stationary phase. The effect on retention of changes in modifier concentration seems to be more pronounced for CSPs than for achiral stationary phases in SFC, and peak shapes are apt to degrade rapidly at low modifier concentrations [12]. Efficiency tends to decrease as the modifier concentration increases because analyte diffusion is slowed by the increased viscosity of the eluent [39]. 

Walhagen, A., Edholm, L.E. (1991). Chiral separation of achiral stationary phases with different functionalities using P-cyclodextrin in the mobile phase and applications to bioanalysis and coupled columns. Chromatographia 32, 215-223. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

By adding a chiral molecule to the mobile phase, the direct separation of enantiomers can be obtained on an achiral stationary phase. This... 

The separation can be based on one or more of three possible mechanisms as follows (1) The two enantiomers of a solute have a tendency to form complexes with the selector in the mobile phase to different extents. The diastereomeric complexes formed and the free enantiomers have a different distribution to the achiral stationary phase. (2) The diastereomeric complexes formed have a different distribution to the achiral stationary phase. (3) The chiral selector adsorbs to the achiral stationary phase to form a chiral pseudostationary phase [49]. 

Three approaches can be employed to separate peptide stereoisomers and amino acid enantiomers separations on chiral columns, separations on achiral stationary phases with mobile phases containing chiral selectors, and precolumn derivatization with chiral agents [111]. Cyclodextrins are most often used for the preparation of chiral columns and as chiral selectors in mobile phases. Macrocyclic antibiotics have also been used as chiral selectors [126]. Very recently, Ilsz et al. [127] reviewed HPLC separation of small peptides and amino acids on macrocyclic antibiotic-based chiral stationary phases. 

The presence of these 12 stereoisomers was experimentally confirmed by the partial resolution of 12 peaks in HPLC chromatography using an achiral stationary phase (octadecyl polysiloxane, ODS) and different mobile phase compositions (CH3CN/THF 55 45-58 42) and temperatures (288-258 K). Figure 7 shows one of these chromatograms in which 10 partially resolved peaks are clearly distinguished. ... 

Figure 7. Representative HPLC separations of the stereoisomers of (a) dendrimer 5 (G = 1) at 298 K and (b) dendrimer 6 (G = 1) at 258 K using a HPLC achiral stationary phase of ODS and ACN/THF (55 45) as mobile phase.

Chromatography the enantiomers are converted into a pair of diastereomers by chemical reaction with an auxiliary (enantiomerically pure, chiral compound) and the ratio of the peak areas is determined by chromatography on an achiral stationary phase. 

The possibility of enantiomeric enrichment of nonracemic mixtures by chromatography on achiral stationary phases has clearly been established23-24 and is the result of an EE-effect 25. Indeed, it has been possible to separate fractions of high enantiomeric purity from the residual racemic mixture by chromatography on achiral stationary phases26. 

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

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... 

This method can be used when the enantiomers of interest are not coeluting with other compounds in the sample and when accurate quantitative information is not the highest priority of the analysis. The sample will have been prepared by an extraction method selected from those in unitgi.i and should have a concentration of 50 to 100 ppm. The identity of the components of the sample should be known from gas chromatography-mass spectrometry (GC-MS) together with their retention indices on the achiral stationary phase. Additional sample cleanup procedures may be needed to ensure the optimum results that are evaluated below ... 

Using a different achiral stationary phase will alter the order of elution. This may eliminate some coelution problems, but new ones may occur. It may alter the order of elution of the R and S isomers. 

Both techniques can be applied in two ways. In the first method the enantiomeric mixture (or racemate) is converted into a diastereoisomeric mixture with a suitable optically pure reagent, and this mixture chromatographed on a column having an achiral stationary phase. Separation then depends on the differential molecular interactions of the diastereoisomers with the stationary phase. In the second method, the stationary phase on the support material (usually chemically bonded) contains a chiral, optically pure residue. In this case the mixture of enantiomers which is loaded directly on to the column is separated by virtue of differential diastereoisomeric molecular interactions between each enantiomer and the optically pure stationary phase. 

No reports are yet available on the separation of enantiomerically enriched fractions arising from incomplete asymmetric reactions. It should be noted that enantiomerically enriched mixtures may be fractionated in racemate and excess enantiomer also on achiral stationary phases (Gil-Av and Schurig, 1994 Nicoud et al., 1996) by adopting the principle of re-crystallization of mixtures enriched in one enantiomer leading to the pure enantiomer. 

A. G. Adams and J. T. Stewart, A HPLC method for the determination of albuterol enantiomers on human serum usage SPE and a sumichiral-achiral stationary phase, J. Liquid Chromatogr., 16 3863 (1993). 

Finally, a multireaction system will also be considered. The example is related to the separation of binaphthol enantiomers, and was reported by Morbidelli, Mazzotti and co-workers in some detail [1]. Separation of the enantiomers is even possible with an achiral stationary phase due to dimerization reactions taking place in the fluid phase. 

Maleic anhydride reacts with cyclopenta-1,3-diene in a Diels-Alder reaction. Since there is a plane of symmetry, the reaction can lead to two achiral compounds, which are diastereomers of each other, containing an endo- or exo-oriented dicarboxylic anhydride group. These differ in absolute and relative configuration at the bond shared by both rings. Under normal conditions the Diels-Alder reaction proceeds stereospecifically to yield preferentially the endo product. Note that in the tricyclic product no trans fusion in the ring system is possible as a consequence of the reaction mechanism. Subsequent reduction of the products therefore affords two diols, which are also diastereomers of each other. These may be separated by chromatography on an achiral stationary phase. 

In recent years several sensitive and specific methods for the simultaneous determination of ephedrine alkaloids in plant material have been published. These include thin-layer chromatography (292,426), gas chromatography (251), straight-phase and reversed-phase high-performance liquid chromatography (253, 255, 302, 355, 427), isotachophoresis (303, 356), and 13C-NMR (304). Resolution of enantiomeric alkaloids by HPLC has been achieved on chiral stationary phases (417, 418) or after derivatization with a chiral agent on an achiral stationary phase (419). Chromatographic separation and analytical detection of... 

Karlsson, A. et al. Enantioselective Ion-Pair Chromatography of Phenolic 2-Dipropylam-inotetrahn Derivatives on Achiral Stationary Phases an Experimental and Theoretical Study of Chiral Discrimination. Acta Chem. Scand. 1993,47,469-481. 

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