Chiral Stationary Phase HPLC Research Articles

Chromatographic evaluation of poly ( trans-1,2-cyclohexanediyl-bis acrylamide) as a chiral stationary phase for HPLC

Chiral stationary phases (CSPs) based on polymeric ( R, R)- or ( S, S)-1,2-diaminocyclohexane (DACH) derivatives are synthesized. When bonded to 5 μm porous spherical silica gel, the poly ( trans-1,2-cyclohexanediyl-bis acrylamide) based poly-cyclic amine polymer (P-CAP) stationary phases is proved to be effective chiral stationary phases that could be used in the normal-phase mode, polar organic mode and with halogenated solvents mobile phases, if desired. Since these are entirely synthetic CSPs, the elution order of all enantiomers can be reversed between the ( R, R) P-CAP and ( S, S) P-CAP columns. Because of the high loading of chiral selectors, the columns exhibit very high sample capacities. Thus, P-CAP columns are useful for preparative and semi-preparative enantiomeric separations. The application of these CSPs and optimization of their separations are discussed.

New hybrid polymeric liquid chromatography chiral stationary phase prepared by surface-initiated polymerization

A new hybrid organic/inorganic HPLC chiral stationary phase (CSP1) has been synthesized by the grafting from (g-from) radical polymerization of an enantiopure diacryloyl derivative of trans-1,2-diaminocyclohexane in the presence of mesoporous, azo-activated silica particles. The new chiral stationary phase has been fully characterized by elemental analysis, differential scanning calorimetry, diffuse reflectance infrared spectroscopy, scanning electron microscopy, inverse size exclusion chromatography and Van Deemter analysis. CSP1 shows improved chromatographic performances compared to its analog CSP2 synthesized by the alternative grafting to (g-to) approach in which the azo initiator is kept in solution. CSP1 can successfully resolve several chemically diverse chiral compounds, using both organic and water-based eluents (normal phase, polar organic, etc.).

Enhanced biosynthesis of dihydrodaidzein and dihydrogenistein by a newly isolated bovine rumen anaerobic bacterium

A rod-shaped and Gram-positive anaerobic bacterium, named Niu-O16, which was isolated from bovine rumen contents, was found to be capable of anaerobically converting isoflavones daidzein and genistein to dihydrodaidzein (DHD) and dihydrogenistein (DHG), respectively. The metabolites DHD and DHG were identified using EI-MS and NMR spectrometric analyses. Stereoisomeric metabolites, which were separated on chiral stationary phase HPLC, were formed in equal amounts by the strain Niu-O16. Tautomerization reaction occurred on the B-ring of DHD and DHG seems to be attributed to the equal production of stereoisomeric metabolites. For the synthesis of DHD, the strain Niu-O16 showed an optimal pH range from 6.0 to 7.0 and completely reduced up to 800 μM of daidzein to DHD with the initial OD 600 nm = 1.0 and pH 7.0 for 3 days incubation. The strain Niu-O16, showed relatively faster reduction activity toward daidzein to produce DHD than the previously isolated human intestinal bacterium Clostridium sp. HGH6.

Cyclopolymerization of Dianhydro Sugar Leading to Novel Carbohydrate Polymers as Macromolecular Ionophores

AbstractFor Abstract see ChemInform Abstract in Full Text.

Convenient synthesis of mono(6A-N-allylamino-6A-deoxy)permethylated $beta;-cyclodextrin: a promising chiral selector for an HPLC chiral stationary phase

Convenient synthesis of mono(6A-N-allylamino-6A-deoxy)permethylated $beta;-cyclodextrin: a promising chiral selector for an HPLC chiral stationary phase

Convenient synthesis of mono(6 A- N-allylamino-6 A-deoxy)permethylated β-cyclodextrin: a promising chiral selector for an HPLC chiral stationary phase

Mono(6-( p-toluenesulfonyl))permethylated β-cyclodextrin, a versatile precursor for a wide variety of mono-functionalized permethyl β-cyclodextrins, has been generated successfully by the direct methylation of monotosylated cyclodextrin. This afforded a convenient synthesis of mono(6 A- N-allylamino-6 A-deoxy)permethylated β-cyclodextrin. Hydrosilylation of the chiral selector with (EtO) 3SiH and reaction of the resultant reactive siloxane with pristine silica gel afforded a facile entry into a structurally well-defined chiral HPLC stationary phase.

Theoretical reassessment of Whelk-O1 as an enantioselective receptor for 1-(4-halogeno-phenyl)-1-ethylamine derivatives.

A combination of molecular mechanics and first principles calculations was used to explore the enantioselectivity of receptors, taking into account experimental data from the CHIRBASE database. Interactions between the Whelk-O1 HPLC chiral stationary phase with the complete series of 1-(4-halogeno-phenyl)-1-ethylamine derivative racemates were studied. The objective was to extract information from the interactions between the chiral Whelk-O1 stationary phase and the enantiomers, hence probing the origin of the enantioselective behavior. Calculations correctly reproduce the elution orders and reasonably describe the experimental enantioselectivities and retention factors. Different binding modes were observed for the first eluted enantiomer complexes, whereas the second eluted show only one prevalent diastereomeric binding fashion. Natural bond orbital (NBO) analysis was used on the global minima bound-complexes to quantify donor-acceptor interactions among chiral stationary phase and ligand moieties. Intermolecular hydrogen bonding was found to be the essential energetic interaction for all systems studied. CH-pi, aromatic stacking and various charge transfer interactions were found to be smaller in magnitude but still important for the global enantioselective behavior. The three-point interaction model is discussed, pointing out the difficulty of its application for the qualitative prediction of elution orders (absolute configurations).

Ruffling-induced chirality: synthesis, metalation, and optical resolution of highly nonplanar, cyclic, benzimidazole-based ligands.

Expedient five-step syntheses of a cyclic bis(benzimidazole)-based amide 5 and two sterically more hindered analogues 23-24 have been developed. These amides are chiral due to the inherent ruffling of the macrocyclic plane. Racemization of the optical antipodes of these compounds has been studied using dynamic chiral stationary phase HPLC. These studies reveal that, while the parent amide 5 racemizes rapidly, for the sterically more hindered amides 23-24, the rate of racemization is significantly reduced. Bis(benzimidazole)-based amides 5 and 23-24 form stable Ni(II) complexes 25-27, respectively. Like their parent ligands, complexes 25-27 are chiral due to their highly ruffled geometry. Studies of these complexes by chiral stationary phase HPLC reveal that metalation leads to a much lower rate of racemization. Incorporation of a strap can slow racemization even further. A series of strapped cyclic amides 54-57, along with their corresponding dimers 58-61, have been prepared. The rate of racemization for amides 54-57 is strongly dependent on the length of the strap. X-ray single-crystal structure analysis of the Ni(II) complex of strapped amide 54 reveals that the bis(benzimidazole) core retains its highly ruffled shape, with the two phenyl rings of the macrocycle located anti to the strap. Chiral separation of strapped ligands 54-57 and their corresponding Ni(II) complexes is shown to be facile by chiral stationary phase HPLC.

Enantioseparation of dansyl amino acids on terguride‐based chiral selectors. Part I: Capillary electrophoretic separation

AbstractA method of separation of dansyl amino acid (Dns‐AA) enantiomers by capillary electrophoresis (CE) using terguride‐based chiral selectors (CSs) was developed using linear polyacrylamide coated capillaries. Due to the high UV absorption of all studied CSs, only the capillary was filled with CS, while the inlet and outlet vials contained buffer solution only. Conditions were found for baseline separation of all Dns‐AA under study. L‐Dns‐AAs were eluted prior to D‐Dns‐AAs on all studied CSs. Use of the optical antipode of allyl‐terguride resulted in reversal of elution order of all Dns‐AAs (the D‐form was eluted prior to the L‐form). The best enantioselectivity was achieved for hydroxyl group‐containing AAs, i. e. serine and threonine. This finding is in good agreement with the proposed mechanism of chiral recognition of these AAs determined by X‐ray studies. Seven CSs with different N(1)‐side chain units, five of them designed for the preparation of silica based HPLC chiral stationary phases, were tested for their ability to resolve enantiomers of Dns‐AAs. Different polarity and hydrophobicity of N(1)‐side chains of the CSs resulted in different CS mobilities, thus influencing the effective separation time in the capillary. 1‐(5‐Aminopentyl)‐terguride showed the best enantioseparation and resolution for the Dns‐AAs studied.

Mixed cellulose-derived benzoates bonded on allylsilica gel as HPLC chiral stationary phases: influence of the introduction of an aromatic moiety in the fixation substituent

Several mixed alkenoxybenzoyl/benzoates and 10-undecenoyl/benzoates of cellulose were fixed onto allylsilica gel by the radical coupling of double bonds. The introduction of an aromatic group in the fixation substituent modifies the chiral recognition ability of the resulting chiral stationary phases (CSPs) in comparison with the 10-undecenoate/benzoate cellulose derivatives. Better enantioselectivity values are achieved when the electronic and geometric characteristics of both substituents, fixating and derivatizing, are similar.

Mono(6 A- N-allylamino-6 A-deoxy)perphenylcarbamoylated β-cyclodextrin: synthesis and application as a chiral stationary phase for HPLC

A novel cyclodextrin derivative: mono(6 A- N-allylamino-6 A-deoxy)perphenylcarbamoylated β-cyclodextrin was synthesized. Hydrosilylation with (EtO) 3SiH and then reaction of the reactive siloxane with pristine silica gel afforded a facile entry into a durable, structurally well-defined chiral stationary phase capable of enantioseparation of a variety of racemic drugs.

Uniformly Sized Molecularly Imprinted Polymer for (S)-Nilvadipine. Comparison of Chiral Recognition Ability with HPLC Chiral Stationary Phases Based on a Protein

Uniformly sized molecularly imprinted polymers (MIPs) for (S)-nilvadipine have been prepared by a multistep swelling and polymerization method using methacrylic acid, 2-(trifluoromethyl)acrylic acid, 2-vinylpyridine, or 4-vinylpyridine (4-VPY) as a functional monomer and ethylene glycol dimethacrylate (EDMA) as a cross-linker. The chiral recognition abilities of the MIPs for nilvadipine and other dihydropyridine calcium antagonists were evaluated using a mixture of sodium phosphate buffer (or water) and acetonitrile or only acetonitrile as the mobile phase. The (S)-nilvadipine-imprinted 4-VPY-co-EDMA polymers gave the highest resolution for nilvadipine among the MIPs prepared. In addition, the enantioseparation of nilvadipine was attained using the (S)-nilvadipine-imprinted EDMA polymers, without use of a functional monomer. 1H NMR and molecular modeling studies suggested a one-to-one hydrogen-bonding-based complex formation of (S)-nilvadipine with 4-VPY in chloroform. These results reveal that the (S)-nilvadipine-imprinted EDMA polymers could recognize the template molecule by its molecular shape, and that in addition to this recognition, hydrophobic and hydrogen-bonding interactions seems to play important roles in the retention and chiral recognition of nilvadipine on the 4-VPY-co-EDMA polymers in hydroorganic mobile phases. By optimizing chromatographic conditions such as column temperature and flow rate, the baseline separation of nilvadipine enantiomers was attained with a short analysis time and with a column efficiency comparable to commercially available chiral stationary phases based on a protein, such as ovomucoid or alpha1-acid glycoprotein.

Resolution of Enantiomers Using Sugar-Carrying Polyisocyanides as Chiral Stationary Phases for HPLC

Abstract 3,5-Dimethylphenylcarbamate derivatives of α-/β-glucose and α-/β-galactose-carrying helical poly(phenyl isocyanide)s were used as chiral stationary phases (CSPs) for HPLC to estimate their chiral recognition abilities. CD spectroscopy suggested that the helix sense in these rigid helical polymers may be regulated by the chirality of the α- or β-anomeric center of the sugar moieties. Some 10 different types of racemates with functional groups were completely or partially resolved depending on the stereostructure of the pendant sugars. The enhanced chiral recognition ability is attributable to the three-dimensionally regulated sugar arrays along the helical backbone; that is, the CSP of the α-glucose-carrying helical poly(phenyl isocyanide) exhibited more effective enantioseparation than that of the corresponding flexible poly(N-phenylacrylamide). The CSPs of the galactose-carrying poly(phenyl isocyanide)s showed the resolving ability for broader racemates than those of the glucose-type poly(phenyl isocyanide)s. Especially, the CSP of the α-galactose-carrying poly(phenyl isocyanide) separated the largest number of racemates.

Preparation of chiral stationary phase for HPLC based on immobilization of cellulose 3,5-dimethylphenylcarbamate derivatives on silica gel.

The immobilization of cellulose 3,5-dimethylphenylcarbamate derivatives having a polymerizable vinyl group, i.e., 4-vinylphenylcarbamate or 2-methacyloyloxyethylcarbamate, on silica gel was examined under various conditions. The immobilization was basically conducted through the radical copolymerization of the derivatives with a vinyl monomer. Several factors, such as the vinyl monomer content and the type and amount of the vinyl group of cellulose derivatives, were varied. The introduction of a vinyl group onto the silica surface resulted in a more efficient immobilization of the cellulose phenylcarbamate derivatives on the silica gel. As the content of the vinyl group on the cellulose derivatives was reduced, the immobilization became more difficult, although the obtained phase exhibited higher chiral recognition abilities. These immobilized CSPs could be stably used with the eluent containing 10% chloroform, which cannot be used for the phase prepared by coating the derivatives on silica gel. Some racemates were better resolved on the immobilized CSP by using chloroform as a component of the eluent.

A comparison of silica and porous graphitic carbon as support materials for a chiral selector in HPLC

The tartaramide based chiral selector 2 was synthesized, coated onto porous graphitic carbon (PGC), and evaluated as a chiral stationary phase for HPLC. Its performance was compared to the silica based sorbent 1, containing the same chiral moiety. The retention and separation characteristics of the two columns were found to be correlated, but separation factors and column efficiencies were constantly lower on the PGC column. Coating of PGC as a novel and simple means to evaluate chiral selectors was evaluated.

Liquid Chromatographic Reaolution of N-Protected α -Amino Acids as Their Anilide and 3,5-Dimethylanilide Derivatives on Chiral Syationary Phases Derived fron (S)-Leucine

Various racemic N-protected α-amino acids such as N-t-BOC-(tert-butoxycarbonyl), N-CBZ-(benzyloxycarbonyl) and N-FMOC-(9-fluorenylmethyloxycarbonyl) α-amino acids were resolved as their anilide and 3,5-dimethylanilde derivatives on an HPLC chiral stationary phase (CSP) developed by modifying a commercial (S)-leucine CSP. The chromatographic resolution results were compared to those on the commercial (S)-leucine CSP. The resolutions were greater on the modified CSP than those on the commercial CSP with only one exception, the resolution of N-t-BOC-phenylglycine anilide. In addition, the chromatographic resolution behaviors were quite consistent except for the resolution of N-protected phenylglycine derivatives, the (S)-enantiomers being retained longer. Based on the chromatographic resolution behaviors and with the aid of CPK molecular model studies, we proposed a chiral recognition mechanism for the resolution of N-protected α-amino acid derivatives. However, for the resolution of N-protected phenylglycine derivatives, a second chiral recognition mechanism, which competes in the opposite sense with the first chiral recognition mechanism, was proposed. The two competing chiral recognition mechanisms were successfully used in the rationalization of the chromatographic behaviors for the resolution of N-protected phenylglycine derivatives.

Amylose Tris(3,5-dimethylphenylcarbamate)-Coated Zirconia as a Chiral Stationary Phase for Micro HPLC

Amylose Tris(3,5-dimethylphenylcarbamate)-Coated Zirconia as a Chiral Stationary Phase for Micro HPLC

Conformational studies by dynamic NMR. 86. Structure, stereodynamics, and cryogenic enantioseparation of the stereolabile isomers of o-dinaphthylphenyl derivatives.

Static and dynamic stereochemistry of the hydrocarbon comprising a phenyl ring bearing two alpha-naphthyl substituents in the ortho positions, i.e., 1,2-di-(4-methyl-naphth-1-yl)-benzene 1, has been studied by a combination of variable temperature NMR, cryogenic HPLC, and MM calculations. Whereas in solution both syn (meso) and anti (chiral) forms were observed and the corresponding interconversion barrier was determined (Delta G(++) = 19.5 kcal mol(-1)), only the diastereoisomer anti was found to be present in the crystalline state (X-ray diffraction). When the molecule is rendered asymmetric by introduction of a nitro group in the phenyl ring as in 1,2-di-(4-methyl-naphth-1-yl)-4-nitrobenzene 2, the chiral syn and anti diastereoisomers are simultaneously present both in solution and in the solid state, albeit in different proportions. Cryogenic chromatography on a HPLC chiral stationary phase at 20 degrees C allowed the stereolabile diastereoisomers and the corresponding enantiomers to be separated.

Chiral and Chirality Discrimination on Helical Polyacetylenes

Synthesis and function of helical polyphenylacetylenes are briefly reviewed. Optically active helical polyphenylacetylenes were synthesized by polymerization of the corresponding chiral phenylacetylene monomers and their chiral recognition abilities were evaluated as a chiral stationary phase for HPLC. The polyphenylacetylenes showed high chiral recognition and could resolve some racemic compounds. Conceptually new helical polyphenylacetylenes were found to be induced on optically inactive polyphenylacetylenes bearing functional groups upon complexation with optically active small molecules capable of interacting with the polymers. The complexes exhibited a characteristic induced circular dichroism in the UV-visible region in organic solvents as well as in water, which could be used to determine the absolute configuration of chiral molecules. A unique helical polyacetylene bearing a β-cyclodextrin as the side group that exhibits the inversion of macromolecular helicity accompanied with a color change is also described.

NMR studies of chiral discrimination relevant to the enantioseparation of N‐acylarylalkylamines by an (R)‐phenylglycinol‐derived chiral selector

Recently, it was reported that the chiral recognition ability of (R)-N-3,5-dinitrobenzoyl phenylglycinol derivative was examined as a new HPLC chiral stationary phase (CSP 1) for the resolution of racemic N-acylnaphthylalkylamines. However, the mechanism of chiral discrimination on the CSP remained elusive until now. In this study, a spectroscopic investigation of the chiral discrimination mechanism of CSP 1 was undertaken using mixtures of (R)-N-3,5-dinitrobenzoyl phenylglycinol-derived chiral selector (2) and each of the enantiomers of N-acylnaphthylalkylamines (3) by NMR study. First, the differences in free energy changes (DeltaDeltaG) upon diastereomeric complexation in solution between the complex of each isomer with chiral selector 2 by NMR titration were calculated. The values were then compared with those estimated by chiral HPLC. The chemical shift changes of each proton on the chiral selector and analytes were also checked and it was found that the chemical shift changes decreased continuously as the acyl group on analytes increased in length. This observation was consistent with the HPLC data. From these experimental results, the interaction mechanism of chiral discrimination between the chiral selector and the analytes is more precisely explained.