Cinchonan carbamate CSPs

If the hydroorganic elution (RP) mode does fail to separate neutral and basic compounds, respectively, the NP mode may be the method of choice. In this mode, the cinchonan carbamate CSPs appear to function according to principles well known... 

The favorable effect of the introduction of a carbamate moiety into the cinchonan selectors was already proven by the prototype cinchonan carbamate CSPs (type I and type II) (Figure 1.9) [30], which showed enhanced enantioselectivities and a widened application range as compared to the CSPs with native cinchona alkaloid selectors and those reported earlier in the literature. 

FIGURE 1.9 Selection of cinchonan carbamate CSPs that have been prepared in the course of selector optimization studies (type I prototype type II, O-9-linked thiol-silica supported prototype type III, C-ll-linked thiol-silica supported CSPs type IV, dimeric selectors). (Adapted from M. Lammerhofer and W. Lindner, J. Chromatogr. A, 741 33 (1996) W. Lindner et al., PCT/EP97/02888, US 6,313,247 B1 (1997) P. Franco et ah, J. Chromatogr. A, 869 111 (2000) C. Czerwenka et ah. Anal. Chem., 74 5658 (2002).)... 

Separation of Various Chiral Acids on Complementary Cinchonan Carbamate CSPs ... 

The chiral distinction capability of cinchonan carbamate CSPs for underivatized amino acids has not been fully elucidated yet, in contrast to the large embodiment of A-acylated and A-arylated amino acid derivatives vide infra). However, it seems that chiral amino acids can be successfully resolved into enantiomers if the amino acid side chain R residue) contains a functionality that represents a strongly interactive binding site with the selector such as an extended aromatic ring system like in thyroxin (T4). 

A number of nonnatural amino acids were resolved into individual enantiomers on 0-9-(2,6-diisopropylphenylcarbamoyl)quinine-based CSPby Peter and coworkers [48,90,113,114] after derivatization with Sanger s reagent, chloroformates (DNZ-Cl, FMOC-Cl, Z-Cl), Boc-anhydride, or acyl chlorides (DNB-Cl, Ac-Cl, Bz-Cl). For example, the four stereoisomers of P-methylphenylalanine, P-methyltyrosine, P-methyltryptophan, and P-methyl-l,2,3,4-tetrahydroisoquinoline-3-carboxylic acid could be conveniently resolved as various A-derivatives [113]. The applicability spectrum of cinchonan carbamate CSPs comprises also P-amino carboxylic acid derivatives, which were, for example, investigated by Peter et al. [114]. A common trend in terms of elution order of DNP-derivatized P-amino acids was obeyed in the latter study On the utilized quinine carbamate-based CSP, the elution order was S before R for 2-aminobutyric acid, while it was R before S for the 3-amino acids having branched R substituents such as wo-butyl, iec-butyl, tert-butyl, cyclohexyl, or phenyl residues. 

The applicability of cinchonan carbamate CSPs for bioanalytical investigations using HPLC-ESI-MS/MS has been demonstrated by Fakt et al. [120]. The goal was the stereoselective bioanalysis of (R)-3-amino-2-fluoropropylphosphinic acid, a y-aminobutyric acid (GABA) receptor agonist, in blood plasma in order to determine whether this active enantiomer is in vivo converted to the 5-enantiomer. In this enantioselective HPLC-MS/MS bioassay, sample preparation consisted of... 

The broad and nearly universal applicability of the cinchonan carbamate CSPs for chiral acid separations is further corroborated by successful enantiomer separations of acidic solutes having axial and planar chirality, respectively. For example, Tobler et al. [124] could separate the enantiomers of atropisomeric axially chiral 2 -dodecyloxy-6-nitrobiphenyl-2-carboxylic acid on an C-9-(tert-butylcarbamoyl)quinine-based CSP in the PO mode with a-value of 1.8 and Rs of 9.1. This compound is stereolabile and hence at elevated temperatures the two enantiomers were interconverted during the separation process on-column revealing characteristic plateau regions between the separated enantiomer peaks. A stopped-flow method was utilized to determine the kinetic rate constants and apparent rotational energy barriers for the interconversion process in the presence of the CSP. Apparent activation energies (i.e., energy barriers for interconversion) were found to be 93.0 and 94.6 kJ mol for the (-)- and (-l-)-enantiomers, respectively. 

Other examples of successful enantiomer separations of miscellaneous chiral acids on cinchonan carbamate CSPs are collected in Table 1.11. 

The PO mode is a specific elution condition in HPLC enantiomer separation, which has received remarkable popularity especially for macrocyclic antibiotics CSPs and cyclodextrin-based CSPs. It is also applicable and often preferred over RP and NP modes for the separation of chiral acids on the cinchonan carbamate-type CSPs. The beneficial characteristics of the PO mode may arise from (i) the offset of nonspecific hydrophobic interactions, (ii) the faster elution speed, (iii) sometimes enhanced enan-tioselectivities, (iv) favorable peak shapes due to improved diffusive mass transfer in the intraparticulate pores, and last but not least, (v) less stress to the column, which may extend the column lifetime. Hence, it is rational to start separation attempts with such elution conditions. Typical eluents are composed of methanol, acetonitrile (ACN), or methanol-acetonitrile mixtures and to account for the ion-exchange retention mechanism the addition of a competitor acid that acts also as counterion (e.g., 0.5-2% glacial acetic acid or 0.1% formic acid) is required. A good choice for initial tests turned out to be a mobile phase being composed of methanol-glacial acetic acid-ammonium acetate (98 2 0.5 v/v/w). 

Although the cinchonan carbamate-based CSPs are of primary interest for the separation of chiral acids, it needs to be stressed that the scope of application is, however, not restricted to chiral acids. A few reports in the literature deal with the separation of the enantiomers of neutral and weakly basic chiral compounds, respectively, on quinine carbamate-type CSPs [50-54]. Both RP and NP modes may be applicable. 

Various papers have been published that investigated the thermodynamics of host-guest associations of cinchonan carbamate derivatives either in solution with the chiral selectors (i.e., the soluble precursors of CSPs) on the one hand [87,88] or... 

The enantiomer separation capability of the cinchonan carbamate selectors and CSPs, respectively, is extremely broad what acidic compounds is concerned (success rate close to 100% if the acidic functional group is close to the chiral center). Hence... 

FIGURE 1.12 CSPs on the basis carbamate and urea-linked cinchonan-calixarene hybrid selectors as examples for CSPs with complementary enantioselectivity profiles. (Reproduced from K.H. Krawinkler et al., J. Chromatogr. A, 1053 119 (2004). With permission.)... 

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