Exploring the enantioseparation of amino-naphthol analogues by supercritical fluid chromatography.

The direct separation of the enantiomers of 1-(α-aminoarylmethyl)-2-naphthol, 1-(α-aminoalkyl)-2-naphthol, 2-(α-aminoarylmethyl)-1-naphthol analogues and 2-(1-amino-2-methylpropyl)-1-naphthol) was investigated in supercritical fluid chromatography. Five commercially available chiral stationary phases based on immobilized polysaccharide chiral selectors (Chiralpak IA, IB, IC, ID and IE) were evaluated. Chiralpak IB was by far the most efficient to achieve the separation of these racemates and was further selected for optimization of elution conditions. The effects of column temperature (varying between 5 and 45 °C) and co-solvent added to carbon dioxide (methanol, ethanol, isopropanol and dichloromethane) were investigated. A particular attention was paid to mobile-phase additives. Several of them, acids, bases or salts (namely water, formic acid, acetic acid, trifluoroacetic acid, diethylamine, diethanolamine, triethylamine, triethanolamine, dimethylethanolamine, ammonia and ammonium acetate), were tested in order to improve peak shapes while maintaining selectivity. With the help of other achiral naphthol derivatives, the additive effects were examined.

Cyclodextrin-mediated enantioseparation of phenylalanine amide derivatives and amino alcohols by capillary electrophoresis-role of complexation constants and complex mobilities.

The separation of the enantiomers of phenylalanine amide and N-methyl derivatives as well as some amino alcohols was studied by CE in acidic BGEs using CDs as chiral selectors. The native CDs displayed only low chiral recognition ability at a concentration of 15 mg/mL in 20 mM sodium phosphate buffer, pH 2.5. In contrast, the analyte enantiomers were separated in the presence of randomly sulfated CDs under reversed polarity of the applied voltage. Sulfated ß-CD proved to be the most universal selector resulting in the enantioseparation of all analytes. Opposite enantiomer migration order depending on the size of the CD cavity was observed for phenylalanine amide and 2-amino-2-phenylethanol. The R-enantiomers migrated first in the presence of sulfated α-CD and γ-CD while the S-enantiomers were detected first in the presence of sulfated ß-CD. The enantioseparations could be rationalized based on analyte complexation by the respective CDs except for 2-amino-2-phenylethanol and sulfated ß-CD where essentially equal complexation constants were derived for the enantiomers. In this case, the migration behavior could be attributed to the mobilities of the enantiomer-CD complexes adding another example to the CE-specific phenomenon of enantioseparations based primarily on complex mobilities.

Enantioseparation of ß(2)-amino acids on cinchona alkaloid-based zwitterionic chiral stationary phases. Structural and temperature effects.

The enantiomers of sixteen unusual ß(2)-amino acids were directly separated on chiral stationary phases containing quinine- or quinidine-based zwitterionic selectors. The effects of the mobile phase composition, the structure of the analyte and temperature on the separations were investigated. Experiments were performed at constant mobile phase compositions in the temperature range -5 to 55°C in order to study the effects of temperature, and thermodynamic parameters were estimated from plots of lnk or lnα vs. 1/T. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes. It was found that the enantiomeric separations were in most cases enthalpically driven, but entropically driven separation was also observed. The sequence of elution of the enantiomers was determined in some cases.

Chiral derivatizations applied for the separation of unusual amino acid enantiomers by liquid chromatography and related techniques.

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the proteinogenic α-amino acids contain at least one stereogenic center (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past 25 years and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers of unusual amino acids and related compounds, with emphasis on the literature published from 1980s. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea and thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Enantioseparations by high-performance liquid chromatography using macrocyclic glycopeptide-based chiral stationary phases: an overview.

Since their introduction by Armstrong in 1994, macrocyclic antibiotic-based chiral stationary phases have proven their applicability for the chiral resolution of various types of racemates. The unique structure of macrocyclic glycopeptides and their large variety of interactive sites (e.g., hydrophobic pockets, hydroxyl, amino and carboxyl groups, halogen atoms, aromatic moieties, etc.) are the reason for their wide-ranging selectivity. The commercially available Chirobiotic™ phases, which display complementary characteristics, are capable of separating a broad variety of enantiomeric compounds with good efficiency, good column loadability, high reproducibility, and long-term stability. These are the major reasons for the use of macrocyclic antibiotic-based stationary phases in HPLC enantioseparations. This overview chapter provides a brief summary of general aspects of macrocyclic antibiotic-based chiral stationary phases including their preparation and their application to direct enantioseparations of various racemates focusing on the literature published since 2004.

Development of the high-performance liquid chromatographic method for the enantioseparation of unusual glycine ester analogs on polysaccharide-based chiral stationary phases.

The stereoisomers of ten unusual amino acid analogs, 1- and 2-naphthol-substituted glycine and its ester derivatives, were separated on chiral stationary phases containing the chiral selectors cellulose tris-(3,5-dimethylphenylcarbamate) (Cellulose-1), cellulose tris-(3-chloro-4-methylphenylcarbamate) (Cellulose-2), cellulose tris-(4-methylbenzoate) (Cellulose-3), cellulose tris-(4-chloro-3-methylphenylcarbamate) (Cellulose-4) and amylose tris-(5-chloro-2-methylphenylcarbamate) (Amylose-2). Experiments were performed in normal-phase mode with n-heptane/alcohol/diethylamine mobile phases in a wide temperature range: 5-50°C. Thermodynamic parameters were calculated from plots of lnk or lnα vs. 1/T. Δ(ΔH°) ranged from -10.1 to 6.2kJmol(-1), Δ(ΔS°) from -31.5 to 22.5Jmol(-1)K(-1) and -Δ(ΔG°) from 0.4 to 1.4kJmol(-1), and both enthalpy and entropy-controlled enantioseparations were observed. The latter was advantageous with regard to the shorter retention and greater selectivity at high temperature. The sequence of elution of the stereoisomers was determined in some cases.

High-performance liquid chromatographic enantioseparation of unusual isoxazoline-fused 2-aminocyclopentanecarboxylic acids on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid-based chiral stationary phases.

The enantiomers of four unusual isoxazoline-fused 2-aminocyclopentanecarboxylic acids were directly separated on chiral stationary phases containing (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as chiral selector. The nature of the alcoholic modifier (MeOH, EtOH, IPA) exerted a great effect on the retention, whereas the selectivity and resolution did not change substantially. Two types of dependence of retention on alcohol content were detected: k(1) increased continuously with increasing alcohol content or a U-shaped retention curve was observed. A comparison of the chromatographic data obtained with HCOOH, AcOH, TFA, HClO(4), H(2)SO(4), or H(3)PO(4) as acidic modifier at a constant concentration demonstrated that in most cases, larger k values were obtained on the application of AcOH or HCOOH, and an increase of the acid content resulted in a decrease of retention. Some mechanistic aspects of the chiral recognition process are discussed with respect to the structures of the analytes and selector. The sequence of elution of the enantiomers was determined in all cases.

Enantiomeric separation of nonproteinogenic amino acids by high-performance liquid chromatography.

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the amino acids contain a chiral carbon atom, the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The development of methods for the separation of enantiomers has attracted great interest, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This special review surveys indirect and direct high-performance liquid chromatographic (HPLC) methods of biologically and pharmaceutically important enantiomers of nonproteinogenic amino acids and related compounds, with emphasis on the literature published from the beginning.

Recent advances in the direct and indirect liquid chromatographic enantioseparation of amino acids and related compounds: a review.

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise to complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain protein as well as non-protein aminoacids in many instances. Since most of the proteinogenic α-amino acids contain a chiral carbon atom (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past decade and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic methods. This review surveys indirect and direct HPLC separations of biologically and pharmaceutically important enantiomers of amino acids and related compounds, with emphasis on the literature published from 2005.

High-performance liquid chromatographic enantioseparation of amino compounds on newly developed cyclofructan-based chiral stationary phases.

Direct separation of the enantiomers of amino acid amines, amino alcohols, and diamines was performed on recently developed chiral stationary phases containing isopropyl carbamate-cyclofructan 6 (IP-CF6), (R)-naphthylethylcarbamate cyclofructan 6 (RN-CF6), or dimethylphenylcarbamate cyclofructan 7 (DMP-CF7) as chiral selectors, using n-hexane/alcohol/TFA as mobile phase. The effects of the mobile phase composition, the nature and concentrations of the alcoholic and acidic modifiers, and the structures of the analytes on the retention and resolution were investigated. In some cases, separations were carried out at constant mobile phase composition in the temperature range 5-40 °C. Thermodynamic parameters and T(iso) values were calculated from plots of lnk versus 1/T. It was found that the enantioseparations were enthalpy driven. The sequences of elution of the stereoisomers were determined but no general rule could be established.

High-performance liquid chromatographic enantioseparation of Betti base analogs on a newly developed isopropyl carbamate-cyclofructan6-based chiral stationary phase.

The direct separation of the enantiomers of 1-(α-aminoarylmethyl)-2-naphthol, 1-(α-aminoalkyl)-2-naphthol, 2-(α-aminoarylmethyl)-1-naphthol analogs, and 2-(1-amino-2-methylpropyl)-1-naphthol) was performed on a newly developed chiral stationary phase containing isopropyl carbamate-cyclofructan6 as chiral selector, with n-heptane/alcohol/trifluoroacetic acid as mobile phase. The effects of the mobile-phase composition, the nature and concentration of the alcoholic and acidic modifiers, and the structures of the analytes on the retention and resolution were investigated. In some cases, separations were carried out at constant mobile-phase compositions in the temperature range 5-40°C. Thermodynamic parameters and T(iso) values were calculated from plots of ln k' or ln α versus 1/T. -Δ(ΔH°) ranged from 2.8 to 3.2 kJ mol(-1) , -Δ(ΔS°) from 7.7 to 10.1 J mol(-1) K(-1) , and -Δ(ΔG°) from 0.2 to 0.5 kJ mol(-1) . It was found that the enantioseparations were enthalpy driven. The sequence of elution of the stereoisomers determined in some cases was (R) < (S).

High-performance liquid chromatographic enantioseparation of 1-(phenylethylamino)- or 1-(naphthylethylamino)methyl-2-naphthol analogs and a temperature-induced inversion of the elution sequence on polysaccharide-based chiral stationary phases.

The stereoisomers of five 1-(phenylethylamino)methyl-2-naphthol analogs or 1-(naphthylethylamino)methyl-2-naphthol analogs containing two chiral centers were directly separated on chiral stationary phases containing the chiral selectors cellulose tris-(3,5-dimethylphenyl) carbamate (Lux Cellulose-1), cellulose tris-(3-chloro-4-methylphenyl) carbamate (Lux Cellulose-2) and amylose tris-(5-chloro-2-methylphenyl) carbamate (Lux Amylose-2). Experiments were performed in normal-phase mode in a wide temperature range -5 to 70°C. Thermodynamic parameters and T(iso) values were calculated from plots of lnk or lnα vs. 1/T. -Δ(ΔH°) ranged from 1.0 to 4.7 kJ mol(-1), -Δ(ΔS°) from 1.6 to 11.0 J mol(-1) K(-1) and -Δ(ΔG°) from 0.1 to 1.5 kJ mol(-1). The sequence of elution of the stereoisomers was determined in all cases and in one case a temperature-induced inversion of the elution sequence was observed.