Chiral Packing Materials Research Articles

Virtual chiral recognition of eugenol derivatives on amylose tris(3-chloro-5-methylphenylcarbamate) chiral stationary phase in unusual normal-phase mode

Chromatographic enantioseparation on polysaccharide-based chiral stationary phases has undergone explosive development over the last three decades as a method for separating the enantiomers of chiral compounds on an analytical and preparative scale. In this context, understanding the nature of the intermolecular interactions involved in retention and recognition processes is an interesting scientific challenge. In the present study, three eugenol derivatives were used as chiral references to elucidate some unexplored aspects of the enantioselective and retention properties of the Chiralpak IG-U chiral stationary phase based on amylose-tris(3-chloro-5-methylphenylcarbamate).The performance of the ultra-high performance liquid chromatography chiral packing material Chiralpak IG-U was evaluated using a two-step approach. First, binary mixtures containing variable proportions of alcohol (ethanol or 2-propanol) in n-hexane were used as mobile phases and the retention factors were recorded at three different temperatures. A rational analysis of this set of chromatographic data shows the leading role played by hydrogen bond between the OH group linked to the stereogenic centre of the analytes and the active sites of the chiral chromatographic material in obtaining a high degree of enantioseparation.The retention factors were then plotted against the percentage of alcohol modifiers to obtain retention maps with a non-linear performance trend with correlation factors >0.9990.The proposed retention map model was used to extrapolate and describe virtual chiral recognition of chiral analytes on the Chiralpak IG-U chiral stationary phase under extreme elution conditions with expected run times of hundreds or thousands of years. The presented virtual chiral recognition approach is based on a generic concept and therefore opens new possibilities for understanding the performance of other polysaccharide-based chiral stationary phases.

Cellulose phenylcarbamate-derived hybrid bead-type chiral packing materials for efficient chiral recognition

For efficient enantioseparation, eight organic–inorganic hybrid bead-type chiral packing materials (CPMs) with high organic contents have been developed using cellulose tris(3,5-dimethylphenylcarbamates) bearing small amount of 3-(triethoxysilyl)propyl groups and tetraethyl orthosilicate by a modified Stober process in the basic condition. The hybrid beads could be formed more efficiently in the basic condition without using surfactant, compared to that in the acidic condition. The preparation conditions, including type and amount of catalyst, amount of water and temperature, significantly affected the organic contents and morphology of the hybrid CPMs. Their chiral recognition properties were then examined by high-performance liquid chromatography. The obtained hybrid CPMs with a higher organic content possessed better enantioseparation ability than the traditional CPMs and previous analogue derived from the acidic condition. For some racemates, the hybrid CPMs exhibited even higher enantioselectivity than the commercial Chiralpak IB N, which is one of the most powerful immobilized-type CPMs and prepared from cellulose tris(3,5-dimethylphenylcarbamate).

Structural dependence on the property of chiral stationary phases derived from chitosan bis(arylcarbamate)-(amide)s

The goal of present study was to investigate the structural dependence of chitosan derivatives on enantioseparation and mobile phase tolerance of the corresponding chiral packing materials for liquid chromatography. Hence, a series of chitosan bis(arylcarbamate)-(n-pentyl amide)s and the related chiral stationary phases (CSPs) were prepared from chitosans with different molecular weights. Because of the H-bond formed via CH3-π interaction, the CSP bearing methyl substituent exhibited high tolerance than the ones bearing dichloro substituents. The CSP derived from the chitosan bis(3,5-dichlorophenylcarbamate)-(n-pentyl amide) with a higher molecular weight possessed high tolerance to mobile phases, whereas the enantioseparation capability of this CSP was not as good as that of the one prepared from the chitosan derivative with a lower molecular weight. Therefore, enantioseparation capability and mobile phase tolerance have to be counterbalanced in designing chiral selectors for the CSPs derived from chitosan bis(arylcarbamate)-(amide)s.

Eluent Tolerance and Enantioseparation Recovery of Chiral Packing Materials Based on Chitosan Bis(Phenylcarbamate)-(n-Octyl Urea)s for High Performance Liquid Chromatography.

The goal of the present work was to study the influence of the swelling of chitosan derivatives on the enantioseparation and the separation performance recovery of chiral stationary phases (CSPs) based on these derivatives. Therefore, six chitosan bis(phenylcarbamate)-(n-octyl urea)s were synthesized, which were coated on macroporous 3-aminopropyl silica gel affording new CSPs. Most of the CSPs demonstrated strong enantioseparation capability for the tested chiral compounds. The swelling capacity of the chitosan bis(phenylcarbamate)-(n-octyl urea)s in ethyl acetate, acetone and tetrahydrofuran (THF) was evaluated. Among the chitosan derivatives, the chitosan bis(3,5-dichlorophenylcarbamate)-(n-octyl urea) polymer showed the highest swelling capacity in ethyl acetate and THF. The polymer-based CSPs could be utilized with pure ethyl acetate and a normal phase containing 70% THF, but was damaged by pure THF. On the other hand, the separation performance of the damaged CSP could be recovered after it was allowed to stand for a period of time. The observations are important for the development and application of polysaccharide derivative-based CSPs.

Nanocellulose Derivative/Silica Hybrid Core-Shell Chiral Stationary Phase: Preparation and Enantioseparation Performance.

Core-shell silica microspheres with a nanocellulose derivative in the hybrid shell were successfully prepared as a chiral stationary phase by a layer-by-layer self-assembly method. The hybrid shell assembled on the silica core was formed using a surfactant as template by the copolymerization reaction of tetraethyl orthosilicate and the nanocellulose derivative bearing triethoxysilyl and 3,5-dimethylphenyl groups. The resulting nanocellulose hybrid core-shell chiral packing materials (CPMs) were characterized and packed into columns, and their enantioseparation performance was evaluated by high performance liquid chromatography. The results showed that CPMs exhibited uniform surface morphology and core-shell structures. Various types of chiral compounds were efficiently separated under normal and reversed phase mode. Moreover, chloroform and tetrahydrofuran as mobile phase additives could obviously improve the resolution during the chiral separation processes. CPMs still have good chiral separation property when eluted with solvent systems with a high content of tetrahydrofuran and chloroform, which proved the high solvent resistance of this new material.

Synthesis and Chiral Recognition Ability of a Novel Fullerene-functionalized Cellulose Derivative

Abstract The fullerene-functionalized cellulose derivative 1 was synthesized and coated onto silica gel as chiral stationary phase (CSP) for high-performance liquid chromatography (HPLC). The chiral recognition ability of the obtained chiral packing material (CPM) was evaluated by HPLC with ten types of racemates 2–11. This CPM could be used in the eluents including chloroform and terahydrofuran (THF), which are usually unsuitable solvents for conventional coating-type CPMs. Hence the scope of application of this CPM is wider than conventional coating CPM.

New approach for chiral separation: from polysaccharide-based materials to chirality-responsive polymers

Chiral separation that is closely related to daily life is a meaningful research. Polysaccharide-(e.g., cellulose, amylose derivatives) based chiral packing materials afford powerful chiral stationary phases (CSPs) toward a broad range of racemic compounds. However, considering the explosive growth of specific chiral drugs, the separation efficiencies of these CSPs need further improvement, which calls for new approaches and strategies. Smart polymers can change their physical or chemical properties dynamically and reversibly according to the external stimuli (e.g., thermo-, pH, solvent, ion, light, critical parameters for chromatographic separation) exerted on them, subsequently resulting in tunable changes in the macroscopic properties of materials. In addition to their excellent controllability, the introduction of chiral characteristics into the backbones or side-chains of smart polymers provides a promising route to realize reversibly conformational transition in response to the chiral analytes. This dramatic transition may significantly improve the performance of materials in chiral separation through modulating the enantioselective interactions between materials and analytes. With the help of chirality-responsive polymers, intelligent and switchable CSPs could be developed and applied in column-liquid chromatography. In these systems, the elution order or enantioselectivity of chiral drugs can be precisely modulated, which will help to solve many challenging problems that involve complicated enantiomers. In this paper we introduce some typical examples of smart polymers that serve as the basis for a discussion of emerging developments of CPSs, and then briefly outline the recent CSPs based on natural and certain synthetic polymers.

Enantiomeric separation of proton pump inhibitors on new generation chiral columns using LC and supercritical fluid chromatography

The enantioselectivity of proton pump inhibitors, namely, omeprazole, lansoprazole, rabeprazole, pantoprazole, tenatoprazole, and ilaprazole were studied using new generation chiral packing materials: CHIRALPAK IA, CHIRALPAK IB, and CHIRALPAK IC. Two versatile techniques, HPLC and supercritical fluid chromatography (SFC) were used in this study. CHIRALPAK IC has shown superior selectivity under both LC and SFC conditions, whereas CHIRALPAK IA has shown good selectivity in SFC when compared to LC under primary screening conditions. The chiral recognition ability in LC and SFC modes were found to be in the order CHIRALPAK IC > CHIRALPAK IA > CHIRALPAK IB. In addition to diode array detection, chiral detection was carried out using a laser polarimeter and the elution orders were found to be the same in both LC and SFC elution modes. Mobile phase modifiers and column temperature effects were also studied. In SFC, modifiers (cosolvent) elution strength was found to be in the order ethanol > methanol > 2-propanol > acetonitrile. In both LC and SFC, a decrease in retention and increase in resolution with an increase in temperature was noticed for all the proton pump inhibitors.

Enantioseparation on Helical Poly(phenylacetylene)s Bearing Cinchona Alkaloid Pendants as Chiral Stationary Phases for HPLC

Abstract A series of optically active helical poly(phenylacetylene)s bearing cinchona alkaloid pendant groups was coated on macroporous silica gel to obtain novel chiral packing materials for high-performance liquid chromatography, which could resolve diverse racemic compounds into enantiomers whose chiral recognition abilities were significantly influenced by the macromolecular helicity induced by the alkaloid pendants.

Controlled synthesis and chiral recognition of immobilized cellulose and amylose tris(cyclohexylcarbamate)s/3-(triethoxysilyl)propylcarbamates as chiral packing materials for high-performance liquid chromatography

The cyclohexylcarbamates of cellulose and amylose bearing a controlled amount of 3-(triethoxysilyl)propyl residue were synthesized by a one-pot process and efficiently immobilized onto silica gel through the intermolecular polycondensation of triethoxysilyl group. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high-performance liquid chromatography (HPLC). The immobilized CPMs exhibited comparable or higher recognition abilities than the conventional coated-type CPMs. The universal solvent compatibility of the immobilized CPMs clearly contributes to the improvement of chiral recognition for most racemates used in the present study. Interestingly, a significantly improved resolution for racemic trans-stilbene oxide (α=2.23) could be attained on the immobilized CPM using the eluent containing 30vol.% chloroform in hexane, which cannot be used for the conventional coated-type CPMs. On the CPMs, almost no resolution of trans-stilbene oxide was attained by a typical eluent, hexane-2-propanol mixture (90/10, v/v). The novel immobilized CPM can also be used in thin-layer chromatography (TLC) due to the absence of an aromatic group.

Enantioseparation on poly(phenyl isocyanide)s with macromolecular helicity memory as chiral stationary phases for HPLC

A series of novel optically active poly(phenyl isocyanide)s bearing achiral benzanilide pendant groups was prepared based on the noncovalent “helicity induction and memory strategy”. The polymers that exhibited an optical activity due to the macromolecular helicity memory were immobilized to 3-aminopropyl-silanized silica gel through chemical bonding or coated on the silica gel to obtain chiral packing materials for high-performance liquid chromatography (HPLC). The chiral recognition abilities toward a variety of racemic compounds with different functionalities were investigated by HPLC. The obtained CSPs separated various racemates into enantiomers, in particular, a right-handed helical poly(phenyl isocyanide) carrying achiral anilide pendants showed an excellent chiral recognition ability and completely or partially resolved nine racemates, including cyclic ether, amine, ketones and metal acetylacetonate complexes, among twelve racemates.

Immobilization and chromatographic evaluation of novel regioselectively substituted amylose-based chiral packing materials for HPLC

The regioselectively substituted amylose derivatives bearing a 4-tert-butylbenzoate or 4-chlorobenzoate group at 2-position, and 3,5-dichlorophenylcarbamate and a small amount of 3-(triethoxysilyl)propylcarbamate groups at 3- and 6-positions were synthesized by a two-step process based on the esterification of 2-position of a glucose unit. The obtained derivatives were effectively immobilized onto macroporous silica gel by intermolecular polycondensation of triethoxysilyl groups. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high-performance liquid chromatography. These CPMs showed high chiral recognition as well as the conventional coated-type CPM, and can be used with the eluents-containing chloroform and tetrahydrofuran. With the extended use of these eluents, improvement of chiral recognition and reversed elution orders were realized. For some racemates, the immobilized CPM exhibited ability comparable or better to the commercial immobilized amylose- or cellulose-based columns, Chiralpak IA, IB, and IC.

Controlled Immobilization of Polysaccharide Derivatives for Efficient Chiral Separation

AbstractPolysaccharide‐based chiral packing materials (CPMs) for high‐performance liquid chromatography (HPLC) have been widely used to determine the enantiomeric excess of a wide range of chiral compounds and also to resolve them on an industrial scale. These CPMs have conventionally been prepared by physically coating a chromatographic support with the polysaccharide derivatives. However, solvents that swell or dissolve the polysaccharide derivatives cannot be used as an eluent component because the column performance will be drastically decreased. This restriction on eluent selection is sometimes a serious issue for efficient analytical and preparative enantioseparations. In order to enhance the diversity of the eluent selection, immobilized CPMs, in which the polysaccharide derivatives are immobilized onto chromatographic supports, have been developed since 1987. This review summarizes the various immobilization methods of the polysaccharide derivatives mainly onto silica gel as a chromatographic support.

Synthesis and Chiral Recognition Ability of a Poly(phenylenevinylene)-Encapsulated Amylose Derivative

Abstract Poly(p-phenylenevinylene) (PPV) was found to be encapsulated in amylose during the polymerization of the precursor monomer in an aqueous solution. The resulting amylose–PPV composite can be further chemically modified by introducing various substituents into the hydroxy groups of the amylose using isocyanates and acetic anhydride. Here we report on the chemical modification of the amylose–PPV composite with 3,5-dimethylphenyl isocyanate in order to obtain a novel chiral packing material for resolving enantiomers by high-performance liquid chromatography (HPLC). The obtained 3,5-dimethylphenylcarbamated amylose–PPV was soluble in pyridine and exhibited a lyotropic liquid crystalline phase in a concentrated pyridine solution. The chiral recognition ability of the 3,5-dimethylphenylcarbamated amylose–PPV was different from that of amylose tris(3,5-dimethylphenylcarbamate) having no PPV rods in the helical cavity of the amylose and can resolve many racemic compounds into enantiomers. Among racemic compounds, cyclic dibenzamide and dibenzanilide derivatives were resolved on the 3,5-dimethylphenylcarbamated amylose–PPV better than amylose tris(3,5-dimethylphenylcarbamate). The difference in the enantioseparation abilities of these amylose-based chiral stationary phases for HPLC is discussed based on the difference in their helical structures.

Preparation and chiral recognition in HPLC of cellulose 3,5-dichlorophenylcarbamates immobilized onto silica gel

The 3,5-dichlorophenylcarbamates (2) of cellulose bearing a small amount of 3-(triethoxysilyl)propyl residues were synthesized by a one-pot process and immobilized onto a silica gel through intermolecular polycondensation of the triethoxysilyl groups. The obtained cellulose derivatives were characterized by (1) H NMR and elemental analysis (EA), and their recognition abilities were evaluated by high-performance liquid chromatography (HPLC). The cellulose derivatives containing about 1-5% of the 3-(triethoxysilyl)propyl residue were efficiently immobilized with a high chiral recognition ability. The immobilized chiral packing materials (CPMs) could be used with the eluents containing chloroform and tetrahydrofuran (THF), which cannot be used with the conventional coated-type chiral packing materials. By using these eluents, the chiral recognition for many racemates was improved.

Immobilization of 3,5‐dimethylphenylcarbamates of cellulose and amylose onto silica gel using (3‐glycidoxypropyl)triethoxysilane as linker

The 3,5-dimethylphenylcarbamates of cellulose and amylose were effectively immobilized onto plain silica gels as chiral packing materials (CPMs) for HPLC by means of intermolecular polycondensation of triethoxysilyl groups introduced with (3-glycidoxypropyl)triethoxysilane. The immobilization and chiral recognition abilities of the obtained CPMs prepared with different amounts of (3-glycidoxypropyl)triethoxysilane were investigated. In addition, the solvent compatibilities of the immobilized-type CPMs were examined with eluents containing chloroform and THF. When these eluents were used, for most of the tested racemates, the chiral resolving abilities of the obtained CPMs were improved.

Synthesis and Immobilization of Amylose Derivatives Bearing a 4-tert-Butylbenzoate Group at the 2-Position and 3,5-Dichlorophenylcarbamate/3-(Triethoxysilyl)propylcarbamate Groups at 3- and 6-Positions as Chiral Packing Material for HPLC

Abstract Two novel amylose derivatives 2a and 2b bearing 4-tert-butylbenzoate at the 2-position and 3,5-dichlorophenylcarbamate/3-(triethoxysilyl)propylcarbamate residues at 3- and 6-positions were successfully synthesized and immobilized onto silica gel, and their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high-performance liquid chromatography. These immobilized CPMs exhibited recognition ability similar to conventional coated CPM, and for some racemates, comparable or better resolutions were observed even compared to commercial immobilized amylose- or cellulose-based columns.

Preparation and HPLC application of chiral stationary phase from 4‐tert‐butylphenylcarbamates of cellulose and amylose immobilized onto silica gel

The 4-tert-butylphenylcarbamates of cellulose and amylose bearing a small amount of 3-(triethoxysilyl)propyl residues were synthesized by a one-pot process and efficiently immobilized onto a silica gel through intermolecular polycondensation of the triethoxysilyl groups. The obtained chiral packing materials (CPMs) were evaluated by HPLC. The polysaccharide derivatives containing about 1-2% of the 3-(triethoxysilyl)propyl residue were efficiently immobilized with a high chiral recognition ability. The immobilized CPMs could be used with the eluents containing chloroform and tetrahydrofuran (THF), which cannot be used with the conventional coated-type CPMs. By using these eluents, the chiral recognition for many racemates was improved.

Immobilization and chiral recognition of 3,5‐dimethylphenylcarbamates of cellulose and amylose bearing 4‐(trimethoxysilyl)phenylcarbamate groups

A small amount of 4-(trimethoxysilyl)phenyl groups was randomly introduced onto the 3,5-dimethylphenylcarbamates of cellulose and amylose by a one-pot method. The obtained derivatives were then effectively immobilized onto silica gel as chiral packing materials (CPMs) for high-performance liquid chromatography through intermolecular polycondensation of the trimethoxysilyl groups. The effects of the amount of 4-(trimethoxysilyl)phenyl groups on immobilization and enantioseparation were investigated. Also, the solvent durability of the immobilized-type CPMs was examined with the eluents containing chloroform and tetrahydrofuran. When these eluents were used, the chiral recognition abilities of the CPMs for most of the tested racemates were improved to some extent depending on the compounds.

Chiral polymers for resolution of enantiomers

AbstractIn 1979, the formation of one‐handed helical poly(triphenylmethyl methacrylate) (PTrMA) was found through the helix‐sense‐selective polymerization of methacrylate using chiral anionic initiators, and the existence of a stable helical polymer without chiral side chains was proved. The chiral polymer exhibited unexpected high chiral recognition of various racemic compounds when used as the chiral packing material (CPM) for HPLC, which was commercialized in 1982 as the first chiral column based on an optically active polymer. This success encouraged us to develop further useful commercial chiral packing materials (CPMs) based on polysaccharides, cellulose, and amylose. By using these polysaccharide‐based CPMs, particularly phenylcarbamate derivatives, nearly 90% of chiral compounds can be resolved not only analytically but also preparatively, and several chiral drugs have been produced using the CPMs. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1731–1739, 2009