Chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether for enantiomer separation of amino compounds using a normal mobile phase

In order to apply the excellent chiral recognition ability of chiral pseudo-18-crown-6 ethers that we developed to chiral separation, we prepared a chiral stationary phase (CSP) by immobilizing a chiral pseudo-18-crown-6-type host on 3-aminopropyl silica gel. A chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. A liquid chromatography system using this CSP combined with the detection by mass spectrometry was used for enantiomer separation of amino compounds. A normal mobile phase can be used on this CSP as opposed to conventional dynamic coating-type CSPs. Enantiomers of 18 common natural amino acids were efficiently separated. The chiral separation observed for amino acid methyl esters, amino alcohols, and lipophilic amines was fair using this HPLC system. In view of the correlation between the enantiomer selectivity observed in chromatography and the complexion in solution, the chiral recognition in host-guest interactions might contribute to this enantiomer separation.     

Dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins as chiral stationary phases for high-performance liquid chromatography

New dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins (CDs) were prepared and their enantiomeric recognition abilities were evaluated as chiral stationary phases (CSPs) in normal phase high-performance liquid chromatography (HPLC). The effects of the type of cyclodextrins, the nature and position of the substituents on the phenyl ring, binding mode and spacer on the chiral recognition were studied in detail. No marked change of chiral recognition abilities was established by reversing the binding side of CDs (i.e., by the narrower [primary] opening of the cone-shaped CD to silica gel with the wider [secondary] opening sides). This result indirectly proves the previously drawn conclusion about the minor role of inclusion phenomena in chiral recognition in this case. Nevertheless, chiral recognition of these CSPs toward some compounds critically depends on the type of CDs used. All CD derivatives described in this study show rather low enantiomeric resolving abilities compared with corresponding polysaccharide (cellulose and amylose) derivatives, although very high enantioselectivity of separation was observed for a few compounds, such as racemic flavanone and cyclopropanedicarboxilic acid dianilide. © 1996 Wiley-Liss, Inc.     

Comparative HPLC enantioseparation of ferrocenylalcohols on two cellulose-based chiral stationary phases

The direct HPLC enantiomeric separation of several ferrocenylalcohols on the commercially available Chiralcel OD and Chiralcel OJ columns has been evaluated in normal-phase mode. Almost all the compounds were resolved on one or both chiral stationary phases (CSPs) with separation factor (alpha) ranging from 1.06 to 2.88 while the resolution (R(s)) varied from 0.63 to 12.70 In the separation of the alpha-ferrocenylalcohols 1a-e and the phenyl analogues 2a-e, which were all resolved except 1c, a similar trend in the retention behavior for the two series of alcohols was evidenced and the selectivity was roughly complementary on the two investigated CSP. For three ferrocenylacohols, chosen as model compounds, the influence of the mobile phase composition and temperature on the enantioseparation were investigated and additional information on the chiral recognition mechanism were deduced from the chromatographic behavior of their acetylderivatives.     

Significance of mobile phase composition in enantioseparation of chiral drugs by HPLC on a cellulose-based chiral stationary phase

Eight randomly selected pharmaceuticals, which included ibuprofen, ketoprofen, albuterol, acebutolol, propafenone, betaxolol, methylphenidate, and homatropine, were directly separated on a cellulose tris(4-methylbenzoate) chiral stationary phase (CSP) without derivatization via normal phase mode HPLC. Enantioresolution was achieved by the optimization of the type and the ratio of mobile phase modifiers and additives. The modifiers included alcohols; the mobile phase additives were trifluoroacetic acid (TFA) and triethylamine (TEA). It was found that methanol and ethanol were superior to isopropanol as mobile phase modifiers for enhancing chiral separation of some of the chiral drugs. The results also demonstrated that TFA has a dominant effect on chiral separations for both acidic and basic chiral drugs, although for some basic drug such as homatropine, TEA was more beneficial at improving enantioseparation. The separation of acebutolol enantiomers was achieved for the first time by adding both TFA and TEA to the mobile phase. The purpose of this paper is to demonstrate that the applicability of cellulose based CSPs can be expanded by controlling the mobile phase compositions through the addition of trace amounts of achiral additives and the selection of the appropriate alcoholic modifier. © 1996 Wiley-Liss, Inc.     

Enantioseparation on 4-halogen-substituted phenylcarbamates of amylose as chiral stationary phases for high-performance liquid chromatography

Four 4-halogen-substituted phenylcarbamate derivatives of amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated and compared with those of the corresponding cellulose derivatives. The amylose derivatives with fluoro, chloro, bromo, or iodo group at the four-position on the phenyl group were found to show higher chiral resolving ability than the corresponding cellulose derivatives. Among four amylose derivatives 4-fluoro- and 4-chlorophenylcarbamates showed an excellent chiral recognition ability. Especially, amylose tris(4-chlorophenylcarbamate) resolved (±)-1,2,2,2-tetraphenylethanol with a very high α value (α = 8.29). In order to obtain useful information concerning the chiral recognition mechanism of this resolution, we also performed enantioseparation of a variety of analogous racemic alcohols, and found that both the hydroxy and bulky triphenylmethyl groups of the racemate are essential for the effective chiral recognition. Chirality 9:63–68, 1997. © 1997 Wiley-Liss, Inc.     

Liquid chromatographic resolution of 3-amino-1,4-benzodiazepin-2-one derivatives on various Pirkle-type chiral stationary phases

The two enantiomers of N-acyl amide and N-ureide derivatives of 3-amino-5-phenyl-1,4-benzodiazepin-2-ones, which have been known to show anti-respiratory syncytial virus (RSV) activity, were resolved on seven different Pirkle-type chiral stationary phases (CSPs) with the use of 10% isopropyl alcohol in hexane as a mobile phase. Among the seven Pirkle-type CSPs, the one based on (S)-leucine derivative named as N-Phe-L-Leu was found to be most successful, the separation factors (α) and the resolutions (R(S) ) for seven analytes being in the range of 1.78-4.21 and 5.94-15.08, respectively. By resolving N-benzyloxycarbonyl derivatives of 3-amino-5-phenyl(or 5-methyl)-1,4-benzodiazepin-2-ones on Pirkle-type CSPs, the phenyl ring at the 5-position and the N?H hydrogen at the 1-position of analytes were found to play an important role in the chiral recognition.     

Chromatographic optical resolution on trans-1,2-diaminocyclohexane derivatives: Theory and applications

An overall view on some new chiral stationary phases based on (trans)-1,2-diaminocyclohexane is illustrated. The selected chiral moiety, derivatized with different aroyl groups, has been linked to a silica matrix in order to give chiral stationary phases (CSPs) enabling them to be used efficiently in the normal and reverse phase, both for analytical and preparative purposes. In addition new polymeric CSPs have been prepared by using the same selector, suitably modified, as monomer. The new chiral stationary phases have been characterised by physicochemical methods and used for the resolution of various racemic compounds classes such as α-aryloxyacetic acids, alcohols, sulfoxides, selenoxides, phosphinates, tertiaryphosphine oxides, benzodiazepines etc. without prederivatization or as amines, amino acids, amino alcohols, nonsteroidal antiinflammatory agents in a derivatized form. The separated solutes structural variety suggests that multiple interaction sites are involved in the recognition process: some thermodynamic data relative to the CSPs—selectands interactions are also illustrated. © 1992 Wiley-Liss, Inc.     

HPLC enantioseparation on cellulose tris(3,5-dimethylphenylcarbamate) as a chiral stationary phase: Influences of pore size of silica gel,coating amount,coating solvent,and column temperature on chiral discrimination

Cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) was coated on large-pore silica gels and used as a chiral stationary phase (CSP) for high-performance liquid chromatographic separation of enantiomers. The influences of pore size of silica gel, coating amount of CDMPC, coating solvent, and column temperature on chiral discrimination were investigated. CSPs prepared with a large-pore silica gel having a small surface area showed higher chiral recognition. The amount of CDMPC adsorbed on the silica gel influenced the chiral recognition of some racemates. Loading capacity of racemates increased with an increase of the amount of CDMPC supported on the silica gel, and a CSP coated with 45% CDMPC by weight can be used for both analytical and semi-preparative scale separations. The CDMPC, coated using acetone as the coating solvent, exhibited, in many cases, higher enantioselectivity than that obtained with tetrahydrofuran F as the coating solvent. © 1996 Wiley-Liss, Inc.     

A hierarchical screening approach to enantiomeric separation

The screening of a number of chiral stationary phases (CSPs) with different modifiers in supercritical fluid chromatography to find a chromatographic method for separation of enantiomers can be time‐consuming. Computational methods for data analysis were utilized to establish a hierarchical screening strategy, using a dataset of 110 drug‐like chiral compounds with diverse structures tested on 15 CSPs with two different modifiers. This dataset was analyzed using a combinatorial algorithm, principal component analysis (PCA), and a correlation matrix. The primary goal was to find a set of eight columns resolving a large number of compounds, but also having complementary enantioselective properties. In addition to the hereby defined hierarchical experimental strategy, quantitative structure enantioselective models (QSERs) were evaluated. The diverse chemical space and relatively limited size of the training set reduced the accuracy of the QSERs. However, including separation factors from other CSPs increased the accuracies of the QSERs substantially. Hence, such combined models can support the experimental strategy in prioritizing the CSPs of the second screening phase, when a compound is not separated by the primary set of columns.     

Synthesis and Enantioseparation Ability of Xylan Bisphenylcarbamate Derivatives as Chiral Stationary Phases in HPLC

Ten novel xylan bisphenylcarbamate derivatives bearing meta‐ and para‐substituents on their phenyl groups were synthesized and their chiral recognition abilities were evaluated as the chiral stationary phases (CSPs) for high‐performance liquid chromatography (HPLC) after coating them on macroporous silica. The chiral recognition abilities of these CSPs depended on the nature, position, and number of the substituents on the phenyl moieties. The introduction of an electron‐donating group was more attractive than an electron‐withdrawing group to improve the chiral recognition ability of the xylan phenylcarbamate derivatives. Among the CSPs discussed in this study, xylan bis(3,5‐dimethylphenylcarbamate)‐based CSP seems to possess the highest resolving power for many racemates, and the meta‐substituted CSPs showed relatively better chiral recognition than the para‐substituted ones. For some racemates, the xylan bis(3,5‐dimethylphenylcarbamate) derivative exhibited higher enantioselectivity than the CSP based on cellulose tris(3,5‐dimethylphenylcarbamate). Chirality 27:518–522, 2015 © 2015 Wiley Periodicals, Inc.     

A convenient and validated enantiomer separation of chiral aliphatic amines as nitrobenzoxadiazole derivatives on polysaccharide‐derived chiral stationary phases under simultaneous ultraviolet and fluorescence detection

A convenient method using a fluorogenic agent, 4‐chloro‐7‐nitro‐1,2,3‐benzoxadiazole (NBD‐Cl), was developed for enantiomer separation of chiral aliphatic amines including amino alcohols by normal high‐performance liquid chromatography. The enantiomer separation of chiral aliphatic amines as NBD derivatives was performed on six covalently bonded and four coated‐type polysaccharide‐derived chiral stationary phases (CSPs) under simultaneous ultraviolet (UV) and fluorescence detection (FLD). Among the covalently bonded CSPs, Chiralpak IE showed the best enantiomer separation for most analytes. The other CSPs also showed good enantioselectivity except for Chiralpak IB. On the other hand, Chiralpak AD‐H and Amylose‐1 generally exhibited better enantiomer separation of NBD derivatized chiral amines among the coated CSPs. The developed analytical technique was also applied to determine the optical purity of commercially available (R)‐ and (S)‐leucinol; the impurity was found to be 0.06%. The developed method was validated and proved to be an accurate, precise, sensitive, and selective method suitable for separation of chiral aliphatic amines as NBD derivatives under simultaneous UV and FLD.     

Enantioseparation on Riboflavin Derivatives Chemically Bonded to Silica Gel as Chiral Stationary Phases for HPLC

Acetylated and/or 3,5‐dimethylphenylcarbamated riboflavins were prepared and the resulting riboflavin derivatives as well as natural riboflavin were regioselectively immobilized on silica gel through chemical bonding at the 5’‐O‐ or 3‐N‐position of the riboflavin to develop novel chiral stationary phases (CSPs) for enantioseparation by high‐performance liquid chromatography (HPLC). The chiral recognition abilities of the obtained CSPs were significantly dependent on the structures of the riboflavin derivatives, the position of the chemical bonding on the silica gel, and the structures of the racemic compounds. The CSPs bonded at the 5’‐O‐position on the silica gel tended to well separate helicene derivatives, while the CSPs bonded at the 3‐N‐position composed of acetylated and 3,5‐dimethylphenylcarbamated riboflavins showed a better resolving ability toward helicene derivatives and bulky aromatic racemic alcohols, respectively, and some of them were completely separated into the enantiomers. The observed difference in the chiral recognition abilities of these riboflavin‐based CSPs is discussed based on the difference in their structures, including the substituents of riboflavin and the positions immobilized on the silica gel. Chirality 27:507–517, 2015. © 2015 Wiley Periodicals, Inc.     

Enantioseparation of napropamide by supercritical fluid chromatography: Effects of the chromatographic conditions and separation mechanism

Supercritical fluid chromatography (SFC) is already used for enantioseparation in the pharmaceutical industry, but it is rarely used for the separation of chiral pesticides. Comparing with high performence liquid chromatography, SFC uses much more environmnetal friendly and economic mobile phase, supercritical CO₂. In our work, the enantioseparation of an amide herbicide, napropamide, using three different polysaccharide‐type chiral stationary phases (CSPs) in SFC was investigated. By studying the effect of different CSPs, organic modifiers, temperature, back‐pressure regulator pressures, and flow rates for the enantioseparation of napropamide, we established a rapid and green method for enantioseparation that takes less than 2 minutes: The column was CEL2, the mobile phase was CO₂ with 20% 2‐propanol, and the flow rate was 2.0 mL/min. We found that CEL2 demonstrated the strongest resolution capability. Acetonitrile was favored over alcoholic solvents when the CSP was amylose and 2‐propanol was the best choice when using cellulose. When the concentration of the modifiers or the flow rate was decreased, resolutions and analysis times increased concurrently. The temperature and back‐pressure regulator pressure exhibited only minor influences on the resolution and analysis time of the napropamide enantioseparations with these chiral columns. The molecular docking analysis provided a deeper insight into the interactions between the enantiomers and the CSPs at the atomic level and partly explained the reason for the different elution orders using the different chiral columns.     

Chromatographic properties of composite chiral stationary phases based on cellulose derivatives

Chiral stationary phases (CSPs) prepared by mixing together two different cellulose derivatives, before or after being coated on macroporous silica gel, were developed in order to determine the mutual influence of two different polymers on global chiral recognition capacity. The chromatographic properties of these CSPs were evaluated using a wide range of racemic test solutes. The mixing method does not significantly affect the enantioselectivities. The composite CSPs obtained by cocoating of two different cellulose derivatives on silica generally exhibit chiral recognition capacities intermediate between those of the two individual phases, and thus broadening the application range of a single column. These results indicate that the simultaneous coating of two different cellulose derivatives does not significantly alter the optical resolution power of each chiral material and are discussed in relationship with the supramolecular structure of the polymeric stationary phases. © 1995 Wiley-Liss, Inc.     

Evaluation of "click" binaphthyl chiral stationary phases by liquid chromatography

Two "click" binaphthyl chiral stationary phases were synthesized and evaluated by liquid chromatography. Their structures incorporate S-(-)-1,1'-binaphthyl moiety as the chiral selector and 1,2,3-triazole ring as the spacer. These chiral stationary phases (CSPs) allowed the efficient resolution for a wide range of racemic BINOL derivatives, particularly for nonpolar diether derivatives and 3-phenyl indolin-2-one analogs. The chromatographic data showed that the π-π interaction was crucial for enantiorecognition of these CSPs. Loss of enantioselectivity observed on CSP3, which are lacking the triazole ring linkage, indicated that the triazole ring linkage took part in the enantioseparation process, although it was remote from the chiral selector of the CSP. The substitution of the phenyl group at 6 and 6' positions can significantly improve the separation ability of the CSP. The chiral recognition mechanism was also investigated by tracking the elution orders and studying the thermodynamic parameters.     

Liquid Chromatographic Resolution of Amino Acid Esters of Acyclovir Including Racemic Valacyclovir on Crown Ether‐Based Chiral Stationary Phases

Valacyclovir, a potential prodrug for the treatment of patients with herpes simplex and herpes zoster, and its analogs were resolved on two chiral stationary phases (CSPs) based on (3,3’‐diphenyl‐1,1’‐binaphthyl)‐20‐crown‐6 covalently bonded to silica gel. In order to find out an appropriate mobile phase condition, various mobile phases consisting of various organic modifiers in water containing various acidic modifiers were applied to the resolution of valacyclovir and its analogs. When 30% acetonitrile in water containing any of 0.05 M, 0.10 M, or 0.15 M perchloric acid was used as a mobile phase, valacyclovir and its analogs were resolved quite well on the two CSPs with the separation factors (α) in the range of 2.49 ~ 6.35 and resolutions (R_S) in the range of 2.95 ~ 12.21. Between the two CSPs, the CSP containing residual silanol protecting n‐octyl groups on the silica surface was found to be better than the CSP containing residual silanol groups. Chirality 27:268–273, 2015. © 2015 Wiley Periodicals, Inc.     

New chiral stationary phases for liquid chromatography based on small molecules: Development,enantioresolution evaluation and chiral recognition mechanisms

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150 × 2.1 mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.     

Liquid chromatographic direct resolution of beta-amino acids on a doubly tethered chiral stationary phase containing N--H amide linkage based on (+)-(18-crown-6)- 2,3,11,12-tetracarboxylic acid

A doubly tethered chiral stationary phase (CSP) prepared by bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid to doubly tethered primary aminoalkyl silica gel was used for the resolution of various beta-amino acids. All the beta-amino acids tested were resolved quite well, the separation (alpha) and the resolution factors (RS) being in the ranges 1.34-2.09 and 2.52-7.45, respectively, with a mobile phase of methanol-water (50:50, v/v) containing 10 mM acetic acid. The chiral recognition efficiency of the doubly-tethered CSP was found to be generally superior to that of the corresponding singly-tethered CSP in the resolution of beta-amino acids. The chiral recognition behaviors for the resolution of beta-amino acids on the doubly tethered CSP were examined by varying the type and content of organic and acidic modifiers in the aqueous mobile phase and the column temperature.     

Enantiomeric discrimination of pyrethroic acid esters on polysaccharide derived chiral stationary phases

Enantiomeric separation of pyrethroic acid methyl and ethyl esters was examined on cellulose-based chiral stationary phases (CSPs): chiralcel OD (cellulose tris(3,5-dimethylphenyl carbamate)) and chiralcel OF (cellulose tris(4-chlorophenyl carbamate)). The good resolution of pyrethroic acid esters was achieved on chiralcel OD and OF. Separation factors ranged from 1.19-5.12 for Chiralcel OD and 1.00-1.59 for chiralcel OF. Hexane/2-propanol (100:0.15, v/v %) was used as the eluent. The resolution capability of CSPs was greater chiralcel OD than chiralcel OF in the case of the pyrethroic acid esters. The flow rate was 0.8 ml/min and detection was set at 230 nm. The results of the chromatographic data and molecular mechanics suggest that steric effect was a major factor in the enantioseparation. Furthermore, the hydrogen bond between analytes and CSP played an important role in the chiral recognition.     

Liquid Chromatographic Resolution of Mexiletine and Its Analogs on Crown Ether‐Based Chiral Stationary Phases

Mexiletine, an effective class IB antiarrhythmic agent, and its analogs were resolved on three different crown ether‐based chiral stationary phases (CSPs), one (CSP 1 ) of which is based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid and the other two (CSP 2 and CSP 3 ) are based on (3,3’‐diphenyl‐1,1’‐binaphthyl)‐20‐crown‐6. Mexiletine was resolved with a resolution (R_S) of greater than 1.00 on CSP 1 and CSP 3 containing residual silanol group‐protecting n‐octyl groups on the silica surface, but with a resolution (R_S) of less than 1.00 on CSP 2 . The chromatographic behaviors for the resolution of mexiletine analogs containing a substituted phenyl group at the chiral center on the three CSPs were quite dependent on the phenoxy group of analytes. Namely, mexiletine analogs containing 2,6‐dimethylphenoxy, 3,4‐dimethylphenoxy, 3‐methylphenoxy, 4‐methylphenoxy, and a simple phenoxy group were resolved very well on the three CSPs even though the chiral recognition efficiencies vary with the CSPs. However, mexiletine analogs containing 2‐methylphenoxy group were not resolved at all or only slightly resolved. Among the three CSPs, CSP 3 was found to show the highest chiral recognition efficiencies for the resolution of mexiletine and its analogs, especially in terms of resolution (R_S). Chirality 26:272–278, 2014. © 2014 Wiley Periodicals, Inc.