Macrocyclic selectors

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

LI Separations in the reversed-phase mode — chiral recognition mechanisms and structural features of selectands. The primary mechanism of interaction between the macrocyclic selectors and the selectands in the reversed-pha.se mode (employing aqueous buffered mobile phases) is (partial) inclusion of hydrophobic molecules or parts of the molecules, such as (substituted) aromatic rings, into the apolar cavity of the CD. It is clear that the dimensions of the CD cavity play a dominant role to facilitate this... 

The macrocyclic glycopeptide chiral selectors are now a very important class of CSPs that must be part of the column set of any laboratory involved in enantiomeric separations. The variety of functionalities found in these relatively small molecules allow for many different interactions leading to successful enantioseparations [29]. The similarities between members of this class of chiral selectors produced the complementary separation property [14, 30, 31]. If a partial separation of an enantiomeric pair is observed on a macrocyclic selector, say vancomycin, a baseline separation may very likely be observed on a different selector, say teicoplanin. This interesting property in method development illustrates the large number of selector-selectand possible interactions. Such complementarities are due to the... 

The chemistry of the macrocyclic linkage to the silica surface was recently improved and the Chirobiotic V2 and T2 were proposed along with the classical V and T products. The active macrocyclic selector being the same, the newly proposed CSPs essentially separate the same families of enantiomers, the enantios-electivity factors can be higher. The loading capability of the second-generation CSPs is also higher [29, 31]. 

Chiral stationary phases in tic have been primarily limited to phases based on normal or microcrystalline cellulose (44,45), triacetylceUulose sorbents or siHca-based sorbents that have been chemically modified (46) or physically coated to incorporate chiral selectors such as amino acids (47,48) or macrocyclic antibiotics (49) into the stationary phase. 

Macrocyclic glycopeptides. The first of these CSPs - based on the cavity of the antibiotic vancomycin bound to silica - was introduced by Armstrong [25]. Two more polycyclic antibiotics teicoplanin and ristocetin A, were also demonstrated later. These selectors are quite rugged and operate adequately in both normal-phase and reversed-phase chromatographic modes. However, only a limited number of such selectors is available, and their cost is rather high. 

This relatively new class of CSPs incorporates glycopeptides attached covalently to silica gel. These CSPs can be used in the normal phase, reversed phase, and polar organic modes in LC [62]. Various functional groups on the macrocyclic antibiotic molecule provide opportunities for tt-tt complexation, hydrogen bonding, and steric interactions between the analyte and the chiral selector. Association of the analyte... 

Mixing the additive in the eluent used as a mobile phase can also modify the chromatographic system (dynamic modification), but the use of modified adsorbents has led to an improvement of resolution. Example works include that by Armstrong and Zhou [11], who used a macrocyclic antibiotic as the chiral selector for enantiomeric separations of acids, racemic drugs, and dansyl amino acid on biphenyl-bonded silica. 

Many chiral compounds can be used as selectors, for example, chiral metal complexes, native and modified cyclodextrins, crown ethers, macrocyclic antibiotics, noncyclic oligosaccharides, and polysaccharides all have been shown to be useful for efficient separation of different types of compounds. 

Vancomycin was the first macrocyclic antibiotic evaluated as selector for the synthesis of HPLC chiral stationary phases (CSPs) [7], along with rifamycin B (among ansamycins) and thiostrepton (among polypeptides). 

Avoparcin was the fourth macrocyclic antibiotic evaluated as selector for the synthesis of HPLC CSPs [37]. 

This strategy consists in the initial modification of the silica surface with organosilanes having suitable anchoring groups, which are either reactive themselves or can be additionally activated for the final attachment of the chiral selector. The choice of the proper silane will depend on the presence of suitable functional groups on the chiral entity to be fixed to the matrix. As macrocyclic antibiotics contain hydroxyl, amine, and carboxylic acid functionalities, they can be linked to the silica surface in a variety of different ways [7, 55]. The obvious drawback of the stepwise assemblage of chiral selectors on the silica surface is the eventual formation of additional polar or ionizable sites on the matrix, which may cause unselective retention of chiral analytes. 

In this synthetic strategy, the macrocyclic antibiotic is covalently bonded to the silica matrix in two steps (1) chemical modification of the selector via reaction between suitable groups of the antibiotic and proper groups of the spacer, reacting also as a di- or trialkoxysilane (2) immobilization of the functionalized selector on unmodified silica particles. 

In a typical reaction, the macrocycle is treated with a 2-3 M excess of (3-isocyanatopropyl)triethoxysilane in dry DMF. The derivative is then added to a dry DMF slurry of silica gel ( 2 g of functionalized selector to 4 g of silica gel). The solution is stirred, allowed to react for 20 h at 107°C, and then cooled, filtered, and washed with methanol, 50% aqueous methanol, and methanol again, and finally dried. Surface coverage data are reported only in the case of the commercially available vancomycin CSP (see Table 2.2), immobilized by joining on average three linkers per vancomycin molecule, probably via a 3-isocyanatopropyl-silane [48]. 

Cyclic amines (including local anesthetic drugs) and amides were among the first classes of chiral compounds investigated in the early stages of the application of macrocyclic antibiotics as chiral selectors therefore, they were screened on vancomycin [7], teicoplanin [30], and ristocetin A [33] CSPs, under RPmode systems. Cyclic imides (including barbiturates, piperidine-2,6-diones, and mephenytoin) have been separated on a vancomycin CSP [157], under NP and RP mobile phase conditions. 

Aboul-Enein, H.Y. and Ali, I., Macrocyclic antibiotics as effective chiral selectors for enantiomeric resolntion by liquid chromatography and capillary electrophoresis, Chromatographia, 52, 679, 2000. 

Ward, T.J., Dann III, C., and Blaylock, A., Enantiomeric resolution using the macrocyclic antibiotics rifamycin B and rifamycin SV as chiral selectors for capillary electrophoresis, J. Chromatogr. A, 715, 337, 1995. 

Armstrong, D.W. et al., Macrocyclic antibiotics as a new class of chiral selectors for liquid chromatography. Anal. Chem., 66, 1473, 1994. 

Strege, M.A., Huff, B.E., and Risley, D.S., Evaluation of macrocyclic antibiotic A82846B as a chiral selector for capillary electrophoresis separations, LC-GC, 14, 144, 1996. 

Ekborg-Ott, K.H., et ah. Evaluation of the macrocyclic antibiotic avoparcin as a new chiral selector for HPLC, Chirality, 10, 627, 1998. 

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