Efficient access to all four stereoisomers of phenylalanine cyclopropane analogues by chiral HPLC

Bonded polysaccharide‐derived chiral stationary phases were found to be useful for the preparation of the four stereoisomers of the cyclopropane analogue of phenylalanine (c₃Phe) as well as for the direct determination of the enantiomeric purity of c₃Phe derivatives by HPLC. Three chiral stationary phases, consisting of cellulose and amylose derivatives chemically bonded on allylsilica gel, were tested. The mixed 10‐undecenoate/3,5‐dimethylphenylcarbamate of cellulose, 10‐undecenoate/3,5‐dimethylphenylcarbamate of amylose and 10‐undecenoate/p‐methylbenzoate of cellulose were the starting polysaccharide derivatives for CSP‐1, CSP‐2, and CSP‐3, respectively. Using mixtures of n‐hexane/chloroform/2‐propanol as mobile phase on a semi‐preparative column (150 mm × 20 mm ID) containing CSP‐2, we separated about 1.7 g of racemic cis‐methyl 1‐tert‐butoxycarbonylamino‐2‐phenylcyclopropanecarboxylate (cis‐ 6 ) and 1.2 g of racemic trans‐methyl‐1‐tert‐butoxycarbonylamino‐2‐phenylcycloprop‐anecarboxylate (trans‐ 6 ) by successive injections. Chirality 11:583–590, 1999. © 1999 Wiley‐Liss, Inc.     

Chiral stationary phases in HPLC for the stereoselective determination of methadone

An extensive study of the behavior of three chiral stationary phases (CSP) used in liquid chromatography (LC) is presented for the stereoselective determination of methadone. The following chromatographic columns were selected: a cellulose, Chiralcel OJ; a modified cyclodextrin, Cyclobond I 2000 RSP, and a protein, Chiral‐AGP. Retention factors, enantioselectivity, efficiency, and resolution were tested by modifying the composition of the mobile phase as well as the temperature. The mechanism for the chiral recognition of methadone on each support was discussed. Optimal chromatographic parameters were obtained for the three supports tested, and methadone enantiomers were separated in less than 20 minutes. The cellulose‐based column gave the best resolution, but this CSP was not adapted to clinical analyses of methadone. Under optimized conditions, the cyclodextrin‐ and protein‐based columns allowed an excellent separation of methadone enantiomers, but no interference with the primary metabolite was found only with Chiral‐AGP. Chirality 11:319–325, 1999. © 1999 Wiley‐Liss, Inc.     

Enantioselective potential of polysaccharide‐based chiral stationary phases in supercritical fluid chromatography

The enantioselective potential of two polysaccharide‐based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5 μm silica particles were tris‐(3,5‐dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose‐based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose‐based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose‐based chiral stationary phase were achieved particularly with propane‐2‐ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO₂, respectively. Methanol and basic additive isopropylamine were preferred on amylose‐based chiral stationary phase. The complementary enantioselectivity of the cellulose‐ and amylose‐based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.     

Study on the HPLC‐based separation of some ezetimibe stereoisomers and the underlying stereorecognition process

The enantioseparation of ezetimibe stereoisomers by high‐performance liquid chromatography on different chiral stationary phases, ie, 3 polysaccharide‐based chiral columns, was studied. It was observed that cellulose‐based Chiralpak IC column exhibited the best resolving ability. After the optimization of mobile phase compositions in both normal and reversed phase modes, satisfactory separation could be obtained on Chiralpak IC column, especially in normal phase mode. The use of prohibited solvents as nonstandard mobile phase gave rise to better resolution than that of standard mobile phases (n‐hexane/alcohol system). In addition, the presence of ethanol in nonstandard mobile phase has played an important role in enhancing chromatographic efficiency and resolution between ezetimibe stereoisomers. Various attempts were made to comprehensively compare the chiral recognition capabilities of immobilized versus coated polysaccharide‐based chiral columns, amylose‐based versus cellulose‐based chiral stationary phases, reversed versus normal phase modes, and standard versus nonstandard mobile phases. Moreover, possible solute‐mobile phase‐stationary phase interactions were derived to explain how stationary and mobile phases affected the separation. Then the method validation with respect to selectivity, linearity, precision, accuracy, and robustness was carried out, which was demonstrated to be suitable and accurate for the quantitative determination of (RRS)‐ezetimibe impurity in ezetimibe bulk drug.     

Synthesis and HPLC chiral recognition of regioselectively carbamoylated cellulose derivatives

Four regioselective-carbamoylated cellulose derivatives having two different substituents at 2-, 3-, and 6-position were prepared and evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography. Investigations showed that the nature and arrangement of the substituents significantly influenced the chiral recognition abilities of the heterosubstituted cellulose derivatives and each derivative exhibited characteristic enantioseparation. Some racemates were better resolved on these derivatives than the corresponding homogeneously substituted cellulose derivatives including a commercial CSP, Chiralcel OD. Racemic compounds shown in this study were most effectively discriminated on cellulose 2,3-(3-chloro-4-methylphenylcarbamate)-6-(3,5-dimethylphenylcarbamate) and 2,3-(3,5-dimethylphenylcarbamate)-6-(3-chloro-4-methylphenylcarbamate).     

Direct resolution of propranolol and bupranolol by thin-layer chromatography using cellulose derivatives as stationary phase

Cellulose triphenylcarbamate derivatives have been used as stationary phases for resolution of the enantiomers of the β-blockers propranolol and bupranolol by TLC. The derivatives examined were: cellulose trisphenylacarbamate (1), cellulose tris(2,3-dichlorophenyl carbamate) (2), cellulose tris(2,4-dichlorophenyl carbamate) (3), cellulose tris(2,6-dichlorophenyl carbamate) (4), cellulose tris (2,3-dimethylphenyl carbamate) (5), cellulose tris(3,4-dichlorophenyl carbamate) (6), cellulose tris(3,5-dichlorophenyl carbamate) (7), and cellulose tris(3,5-dimethylphenyl carbamate) (8). A variety of mobile phases were used to achieve useful separations and the effects of solvent polarity are also discussed. The best resolution of rac-propranolol was obtained on CSP 8 (R_fR = 0.26, R_fS = 0.06, α = 4.33) in mobile phase hexane:propan-2-ol (80:20 v/v). The best resolution of rac-bupranolol was obtained on CSP 5 (R_fR = 0.29, R_fS = 0.09, α = 3.22) in mobile phase hexane:propan-2-ol (80:20 v/v). These results demonstrated the potential of cellulose triphenylcarbamates as chiral stationary phases in TLC and indicate that this is potentially a useful method for the direct, simple, and rapid (within 30 min) resolution of racemates in the analytical control of enantiomeric purity. Physical aspects such as problems in cracking of the CSP, adhesion to plate, and interference of spot detection due to triphenylcarbamate chromphores are also discussed, along with the method employed to overcome them. Chirality 9:139–144, 1997. © 1997 Wiley-Liss, Inc.     

First direct resolution of gossypol enantiomers on a chiral high‐performance liquid chromatography phase

The first direct resolution of gossypol enantiomers has been achieved by HPLC on a chiral stationary phase consisting of cellulose tris‐(3,5‐dimethylphenyl carbamate) coated onto microporous aminopropyl‐silica eluted in the reverse phase mode. Chirality 11:46–49, 1999. © 1999 Wiley‐Liss, Inc.     

Reversed-phase liquid chromatographic enantioseparation by cycloalkylcarboxylates of cellulose and amylose

Three novel cycloalkylcarboxylates, cyclopentyl, cyclohexyl, and 1-adamantylcarboxylates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated using a methanol-water mobile phase. Among these esters, cellulose tris(cyclohexylcarboxylate) showed a relatively high chiral recognition ability. The 1-adamantylcarboxylates of cellulose and amylose showed dissimilar chiral recognition abilities from the other two, probably due to the low degree of substitution and the high hydrophobicity of this group.     

New chiral stationary phases based on xanthone derivatives for liquid chromatography

Six chiral derivatives of xanthones (CDXs) were covalently bonded to silica, yielding the corresponding xanthonic chiral stationary phases (XCSPs). The new XCSPs were packed into stainless‐steel columns with 150 x 4.6 mm i.d. Moreover, the greening of the chromatographic analysis by reducing the internal diameter (150 x 2.1 mm i.d.) of the liquid chromatography (LC) columns was also investigated. The enantioselective capability of these phases was evaluated by LC using different chemical classes of chiral compounds, including several types of drugs. A library of CDXs was evaluated in order to explore the principle of reciprocity as well as the chiral self‐recognition phenomenon. The separation of enantiomeric mixtures of CDXs was investigated under multimodal elution conditions. The XCSPs provided high specificity for the enantiomeric mixtures of CDXs evaluated mainly under normal‐phase elution conditions. Furthermore, two XCSPs were prepared with both enantiomers of the same xanthonic selector in order to confirm the inversion order elution.     

Optical resolution of glycerin sulfides and glycerin selenides on the chiral stationary phase of cellulose‐tris(3,5‐dimethylphenylcarbamate)

Cellulose‐tris(3,5‐dimethylphenylcarbamate) was prepared after a reported method and was coated onto an aminopropylated mesopore spherical silica gel. The final product was used as a chiral stationary phase of high performance liquid chromatography for the enantioseparation of a series of glycerin sulfides and glycerin selenides. Mixtures of hexane and 2‐propanol were used as mobile phases. The effects of 2‐propanol concentration in the mobile phase on the retention and resolution were investigated. Some enantiomers of the glycerin monosulfides and monoselenides could be separated satisfactorily, but none of the disulfides could be separated. The structural features of the solutes that influence chiral separation were discussed. Chirality 11:598–601, 1999. © 1999 Wiley‐Liss, Inc.     

Use of cellulose-based stationary phases for chiral separation in open tubular column chromatography

Application of cellulose-based chiral stationary phases was extended to open tubular columns. These chiral materials were mixed with achiral matrix stationary phases. Compromises were found among the polarity and the ratio of achiral matrix polymers against the content of the chiral cellulose derivative in order to optimize the resolution of the investigated racemates. In GC, the high efficiency feature of open tubular columns allows fast analysis, however, compounds which express strong H-bond interaction with cellulose derivatives elute with a bad peak shape. The application of these stationary phases for open tubular SFC was more successful, because the solvation power of the mobile phase can compensate the strong interaction between the solute and the cellulose derivative. Immobilization of the stationary phases were achieved for SFC purposes. © 1992 Wiley-Liss, Inc.     

Enantioselective separation of 1,4-disubstituted piperazine derivatives on two cellulose chiralcel OD and OJ and one amylose chiralpak AD chiral selectors

A series of 1,4-disubstituted piperazine derivatives with hypnotic activity were examined on three polysaccharide-based chiral stationary phases, namely, Chiracel OD, Chiracel OJ and Chiralpak AD. It was possible to resolve all the compounds on all the phases examined (1.13 相似文献   

Chromatographic enantioseparation by cycloalkylcarbamate derivatives of cellulose and amylose

Cyclopentyl and (+/-)-exo-2-norbornylcarbamates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography (HPLC) were evaluated. Among these carbamates, cellulose tris(cyclopentylcarbamate) and amylose tris((+/-)-exo-2-norbornylcarbamate) showed particularly high chiral recognition, which is comparable to that of several well-known phenylcarbamate derivatives. The chiral recognition mechanism of cellulose tris(cyclohexylcarbamate), which was previously found to be an effective chiral stationary phase for HPLC, was investigated using NMR spectroscopy. The derivative dissolved in chloroform exhibited the chiral discrimination of several enantiomers in NMR as well as in HPLC. For example, the 1,1'-bi-2-naphthol enantiomers were distinctly discriminated in the (1)H, (13)C, and 2D-NOESY spectra.     

Vancomycin‐based chiral stationary phases for micro‐column liquid chromatography

Vancomycin selectively immobilized to silica via either one of its two amino groups has been investigated and compared with columns made from native vancomycin. The chemical modification of vancomycin prior to immobilization involved protection of one amino group as a 9‐fluorenylmethyl carbamate. The immobilization and the subsequent cleavage of the protecting group was performed on‐column. The types of compounds that can be separated with the vancomycin chiral stationary phases resemble those separated previously by capillary electrophoresis and thin‐layer chromatography. The protected chiral stationary phases were also investigated and in some cases very high enantioselectivity were obtained. One example of this is a separation of thalidomide with an α‐value as high as 5.4. The soft immobilization procedure preserves the structure of native vancomycin, in contrast to other approaches. Good repeatability and stability of the columns have also been obtained. Chirality 11:121–128, 1999. © 1999 Wiley‐Liss, Inc.     

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.     

Comparison of separation performances of amylose‐ and cellulose‐based stationary phases in the high‐performance liquid chromatographic enantioseparation of stereoisomers of β‐lactams

High‐performance liquid chromatographic methods were developed for the separation of the enantiomers of 19 β‐lactams. The direct separations were performed on chiral stationary phases containing either amylose‐tris‐3,5‐dimethylphenyl carbamate, (Kromasil® AmyCoat? column) or cellulose‐tris‐3,5‐dimethylphenyl carbamate, (Kromasil® CelluCoat? column) as chiral selector. The different methods were compared in systematic chromatographic examinations. The separations were carried out with good selectivity and resolution. The AmyCoat? and CelluCoat? columns appear to be highly complementary. The best separations of bi‐ and tricyclic β‐lactam stereoisomers were obtained with the AmyCoat? column, whereas the 4‐aryl‐substituted β‐lactams were better separated on the CelluCoat? column. The elution sequence was determined in all cases; no general rule could be established. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

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.     

Enantioresolution of Chiral Derivatives of Xanthones on (S,S)‐Whelk‐O1 and l‐Phenylglycine Stationary Phases and Chiral Recognition Mechanism by Docking Approach for (S,S)‐Whelk‐O1

The resolution of seven enantiomeric pairs of chiral derivatives of xanthones (CDXs) on (S,S)‐Whelk‐O1 and l ‐phenylglycine chiral stationary phases (CSPs) was systematically investigated using multimodal elution conditions (normal‐phase, polar‐organic, and reversed‐phase). The (S,S)‐Whelk‐O1 CSP, under polar‐organic conditions, demonstrated a very good power of resolution for the CDXs possessing an aromatic moiety linked to the stereogenic center with separation factor and resolution factor ranging from 1.91 to 7.55 and from 6.71 to 24.16, respectively. The chiral recognition mechanisms were also investigated for (S,S)‐Whelk‐O1 CSP by molecular docking technique. Data regarding the CSP–CDX molecular conformations and interactions were retrieved. These results were in accordance with the experimental chromatographic parameters regarding enantioselectivity and enantiomer elution order. The results of the present study fulfilled the initial objectives of enantioselective studies of CDXs and elucidation of intermolecular CSP–CDX interactions. Chirality 25:89–100, 2013. © 2012 Wiley Periodicals, Inc.     

Enantiomeric resolution of kielcorin derivatives by HPLC on polysaccharide stationary phases using multimodal elution

Analytical HPLC methods using carbamate chiral stationary phases of polysaccharide derivatives were developed for the enantiomeric resolution of five racemic mixtures of xanthonolignoids: rac-trans-kielcorin C, rac-cis-kielcorin C, rac-trans-kielcorin D, rac-trans-isokielcorin D, and rac-trans-kielcorin E. The separations were evaluated with the stationary phases cellulose tris-3,5-dimethylphenylcarbamate, amylose tris-3,5-dimethylphenylcarbamate, amylose tris-(S)-1-phenylethylcarbamate, and amylose tris-3,5-dimethoxyphenylcarbamate under normal, reversed-phase, and polar organic elution conditions. Chiral recognition of those chiral stationary phases, the influence of mobile phases on the enantiomers separation, and the effects of structural features of the solutes on the chiral discrimination observed are discussed. The best performance was achieved on an amylose tris-3,5-dimethylphenylcarbamate phase. Polar organic conditions gave shorter retention factors and better resolutions and were a valuable alternative to the alcohol-hexane or reversed-phase conditions.     

Comparison between cellulose and amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phases for enantiomeric separation of 17 amidotetralins

Direct enantiomeric separations of 17 chiral amidotetralins by means of high performance liquid chromatography were performed on stationary phases composed of tris(3,5-dimethylphenylcarbamate) derivatives of cellulose and amylose, coated on silica gel. The enantiomers of 15 out of 17 amidotetralins were resolved with a resolution of more than 1.5 by at least one of the chiral stationary phases. The stationary phases showed complementary results with regard to the separation of the amidotetralins, that is, pairs that did not separate on the cellulose-type column were well separated on the amylose-type column, and vice versa. There was no significant correlation between the chromatographic properties of the chiral stationary phases. © 1993 Wiley-Liss, Inc.