Functionalities tuned enantioselectivity of phenylcarbamate cyclodextrin clicked chiral stationary phases in HPLC

The mixed chloro‐ and methyl‐ functionalities can greatly modulate the enantioselectivities of phenylcarbamate cyclodextrin (CD) clicked chiral stationary phases (CSPs). A comparison study is herein reported for per(4‐chloro‐3‐methyl)phenylcarbamate and per(2‐chloro‐5‐methyl)phenylcarbamate β‐CD clicked CSPs (i.e., CCC4M3‐CSP and CCC2M5‐CSP). The enantioselectivity dependence on column temperature was studied in both normal‐phase and reversed‐phase mode high performance liquid chromatography (HPLC). The thermodynamic study revealed that the stronger intermolecular interactions can be formed between CCC4M3‐CSP and chiral solutes to drive the chiral separation. The higher enantioselectivities of CCC4M3‐CSP were further demonstrated with the enantioseparation of 17 model racemates in HPLC.     

Gas chromatographic separation of PCB atropisomers on cyclodextrin stationary phases

Gas chromatographic study on chiral separation of PCBs was performed in a series of capillary columns coated with 0.1-μm film of modified cyclodextrin (CD) stationary phases. The preparation of columns included the investigation into the effect of the content of cyclodextrin derivative in polysiloxane, the type of polysiloxane and temperature of analysis on the quality of separation and retention of atropisomers of 15 selected PCB congeners. The separation properties towards PCBs of stationary phase heptakis(2,3-di-O-methyl-6-O-tert-butyl-dimethylsilyl)-β-CD dissolved in SE-30, SE-54, and OV-1701, were compared with those of 6-monokis-octamethylene-permethyl-β-CD anchored to polydimethylsiloxane polymer (ChirasilDex column, Chrompack, Middelburg, The Netherlands) and octakis(2,6-di-O-methyl-3-O-pentyl)-γ-CD in OV-1701 (MEGA, Legnano (MI), Italy). The correctness of quantitative enantiomer ratio determination was assesed by splitless analysis of PCBs reference solutions in concentration of 1.25–125 ng/ml (PCBs 45 and 91) and 2.5–250 ng/ml (PCB 95) (the PCB congeners are numbered according to IUPAC). Chirality 10:540–547, 1998. © 1998 Wiley-Liss, Inc.     

A new class of network-polymeric chiral stationary phases

A strategy based on the use of homo bi- and multifunctional building blocks for the synthesis of a new class of network-polymeric chiral stationary phases has been evaluated. The key steps comprise acylation of N,N′-diallyl-L-tartardiamide (DATD) and reaction with a multifunctional hydrosilane, yielding a network polymer incorporating the bifunctional C₂-symmetric chiral selector. Covalent bonding to a functionalized silica takes place during the latter process. Many of these chiral sorbents show interesting enantioselective properties toward a wide variety of racemic solutes under normal-phase (hexane-based) conditions. The retention is mainly caused by the hydrogen-bonding ability of the analyte, which is regulated by mobile phase additives like alcohol or ether cosolvents. The most interesting chiral stationary phases, in terms of broad enantioselectivity, were obtained from O,O′-diaryol-DATD-derivatives, particularly those containing the 3,5-dimethylbenzoyl and the 4-(tert-butyl)benzoyl moieties. Since high column efficiencies can be obtained with these chiral sorbents, an α-value of ca. 1.2 is usually sufficient to produce baseline separation. A large number of neutral as well as acidic or basic drug racemates are resolved without derivatization. © 1995 Wiley-Liss, Inc.     

Enantioseparation of racecadotril using polysaccharide‐type chiral stationary phases in polar organic mode

Enantioseparation of the antidiarrheal drug, racecadotril, was investigated by liquid chromatography using polysaccharide‐type chiral stationary phases in polar organic mode. The enantiodiscrimininating properties of 4 different chiral columns (Chiralpak AD, Chiralcel OD, Chiralpak AS, Chiralcel OJ) with 5 different solvents (methanol, ethanol, 1‐propanol, 2‐propanol, and acetonitrile) at 5 different temperatures (5–40 °C) were investigated. Apart from Chiralpak AS column the other 3 columns showed significant enantioseparation capabilities. Among the tested mobile phases, alcohol type solvents were superior over acetonitrile, and significant differences in enantioselective performance of the selector were observed depending on the type of alcohol employed. Van't Hoff analysis was used for calculation of thermodynamic parameters which revealed that enantioseparation is mainly enthalpy controlled; however, enthropic control was also observed. Enantiopure standard was used to determine the enantiomer elution order, revealing chiral selector—and mobile‐phase dependent reversal of enantiomer elution order. Using the optimized method (Chiralcel OJ stationary phase, thermostated at 10 °C, 100% methanol, flow rate: 0.6 mL/min) baseline separation of racecadotril enantiomers (resolution = 3.00 ± 0.02) was achieved, with the R‐enantiomer eluting first. The method was validated according to the ICH guidelines, and its application was tested on capsule and granules containing the racemic mixture of the drug.     

Solution-phase synthesis and evaluation of tetraproline chiral stationary phases

A protocol was developed for the solution-phase synthesis of multigram amounts of two 9-fluorenylmethoxycarbonyl (Fmoc)-protected tetraproline peptides. These tetraproline peptides were then attached to amino derivatized silica gel. The replacement of the Fmoc group with the trimethylacetyl group lead to two tetraproline chiral stationary phases (CSPs). A comparison of the chromatographic behavior of these two solution-phase-synthesized tetraproline CSPs with that prepared by stepwise solid-phase synthesis revealed that all three had similar chromatographic performance for resolving 53 model analytes. This suggests that the solution-phase synthesis of oligoprolines, which allows for the specific benefits of good batch reproducibility, selector homogeneity, and possibly low cost, is a feasible alternative to the solid-phase synthesis of oligoproline CSPs.     

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.     

Comparison of the factors that contribute to retention on immobilized polysaccharide-based chiral stationary phases and macrocyclic glycopeptide chiral stationary phases with the Abraham model

The Abraham model of linear solvation energy relationship (LSER) was utilized to characterize three recently commercialized chiral stationary phases (CSPs), the Chiralpak IA, IB and IC. Normal phase system constants were determined by HPLC for these three CSPs and compared to literature values for the Chirobiotic T and V CSPs. The results indicate that the Chirobiotic T and V CSPs participate in more polar interactions, such as hydrogen bonding and dipolar interactions, than the three immobilized derivatized polysaccharide CSPs. Additionally, differences were noted for the e and b terms of the Abraham model (polarizable interactions and hydrogen bond acidity) for the IA and IB CSPs, which are nominally similar CSPs in their chemical makeup.     

Enantioselective separation of cyclic chiral ketones and their corresponding diastereomeric alcohols by HPLC on chiral and chiral/chiral coupled stationary phases

Enantioselective HPLC methods have been developed for the resolution of (RS)-2-phenylcyclohexanone (compound 1) and (RS)-2-phenyltetrahydropyran-4-one (compound 4) and the diastereoselective and enantioselective separations of their respective cis- and trans-alcohols; reduction of compound 1 yields trans- and cis-2-phenyl-1-cyclohexanol (compounds 2 and 3, respectively) and reduction of compound 4 yields trans- and cis-2-phenyl-tetrahydropyran-4-ol (compounds 5 and 6, respectively). Compounds 1, 2, and 3 were stereochemically resolved using a chiral stationary phase (CSP) based upon amylose tris(3,5-dimethylphenyl carbamate) coated on 10 μm silica-gel (Chiralpak AD-CSP). Compounds 4, 5, and 6 were stereochemically resolved on a coupled column system where a column containing a CSP based upon cellulose tris(3,5-dimethylphenyl carbamate) coated on 5 μm silica (Chiralcel OD-H-CSP) was coupled in series to the AD-CSP. The strategy employed in the identification of the peaks in the respective chromatograms is also discussed in this presentation. Chirality 8:551–555, 1996. © 1997 Wiley-Liss, Inc.     

Chiral separations on polysaccharide stationary phases using polar organic mobile phases

About 30% of a chemically diverse set of compounds were found to separate on four polysaccharide chiral stationary phases using polar organic mobile phases. No structural features appeared to correlate to successful separations. Titrations between normal and polar organic mobile phases suggested that separation mechanisms do not differ between these mobile phases. Attempts made to control retention met with varying degrees of success. Addition of hexane to alcohols had minor effects on retention although this was occasionally beneficial. Addition of water to alcohols increased retention. Addition of water to acetonitrile decreased retention. Addition of alcohol to acetonitrile also proved beneficial to the separation of some compounds. Loading studies performed to mimic preparative separations indicated that the benefits of polar organic mobile phases are largely due to increased solubility.     

Synthesis of polymer-type chiral stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two new chiral polymers of different molecular weights were synthesized by the copolymerization of (1R,2R)-(+)-1,2-diphenylethylenediamine, phenyl diisocyanate and terephthaloyl chloride. The polymers were immobilized on aminated silica gel to afford two chiral stationary phases. The polymers and the corresponding chiral stationary phases were characterized by Fourier transform-IR, elemental analysis, 1H and 13C NMR. The surface coverages of chiral structural units on the chiral stationary phases were estimated as 0.27 and 0.39 mmol/g, respectively. The enantioseparation ability of these chiral stationary phases was evaluated with a variety of chiral compounds by high-performance liquid chromatography. The effects of the organic additives, the composition of mobile phases, and the injection amount of sample on enantioseparation were investigated. A comparison of enantioseparation ability between these two chiral stationary phases was made. It was believed that the chain length of polymeric chiral selector significantly affected the enantioseparation ability of corresponding chiral stationary phase.     

Normal phase chiral HPLC of methylphenidate: comparison of different polysaccharide-based chiral stationary phases.

A comparison of the enantiomeric resolution of (+/-)-threo-methylphenidate (MPH) (Ritalin) was achieved on different polysaccharide based chiral stationary phases. The mobile phase used was hexane-ethanol-methanol-trifluoroacetic acid (480:9.75:9.75:0.5, v/v/v/v). Benzoic acid and phenol were used as the mobile phase additives for the enantiomeric resolution of MPH on Chiralcel OB column only. The alpha values for the resolved enantiomers were 1.34, 1.29, 1.30, and 1.24 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase) columns, respectively. The R(s) values were 1.82, 1.53, 1.19, and 1.10 on Chiralpak AD, Chiralcel OD, Chiralcel OB (containing 0.2 mM benzoic acid in mobile phase), and Chiralcel OB (containing 0.2 mM phenol in mobile phase), respectively. The role of benzoic acid and phenol as mobile phase additives is discussed.     

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.     

Enantiomeric LC separation of calcium antagonists on protein-based chiral stationary phases

Three chiral calcium antagonist drugs, bepridil and two dihydropyridine derivatives (nicardipine and REC 15/2375), have been successfully separated within short retention times using either the α₁-acid glycoprotein chiral stationary phase (Chiral AGP) or the ovomucoid column (Ultron ES-OVM). Aqueous buffer at defined pH is modified by the addition of an organic component (propan-2-ol, acetonitrile, ethanol) in order to modulate the retention properties of each system. The influence of pH and percentage of organic modifier on retention, selectivity, resolution, and column performance are discussed for bepridil analyzed on Chiral AGP and for the two dihydropyridines (nicardipine and REC 15/2375) analyzed on Ultron ES-OVM stationary phases. © 1993 Wiley-Liss, Inc.     

High-performance liquid chromatography of imine stereoisomers on cellulose-based chiral stationary phases

A high-performance liquid chromatographic method has been developed for the analysis of the intermediate imines and end products in an asymmetric isomerization route toward optically active amines. Separation of the imine enantiomers was performed on commercially available Chiralcel OD-H, Chiralcel OJ, and Chiralpak AD chiral stationary phases. All substituted imine enantiomers could be readily resolved with selectivities (α) higher than 1.10 using the Chiralpak AD column. By derivatization with ring-substituted benzaldehydes, aromatic amines were converted into Schiff base derivatives and the enantiopurity of these amines was determined. Chirality 9:727–731, 1997. © 1997 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.     

Lipases in catalytic reactions of organic chemistry

Aspects of enzymatic catalysis in lipase-catalyzed reactions of organic synthesis are discussed in the review. The data on modern methods of protein engineering and enzyme modification allowing a broader range of used substrates are briefly summarized. The application of lipase in the preparation of pharmaceuticals and agrochemicals containing no inactive enantiomers and in the synthesis of secondary alcohol enantiomers and optically active amides is demonstrated. The subject of lipase involvement in the C-C bond formation in the Michael reaction is discussed. Data on the enzymatic synthesis of construction materials—polyesters, siloxanes, etc.—are presented. Examples demonstrating the application of lipase enzymatic catalysis in industry are given.     

Direct HPLC enantioseparation of chiral aptazepine derivatives on coated and immobilized polysaccharide-based chiral stationary phases

The direct HPLC enantioseparation of Mianserin and a series of aptazepine derivatives is accomplished on polysaccharide-based chiral stationary phases (CSPs). The resolutions are performed on the coated-type Chiralcel OD and Chiralpak AD CSPs and on the first commercially available immobilized-type Chiralpak IA CSP, in normal-phase and polar-organic modes. The complete separation of enantiomers of all racemates investigated was successfully achieved under at least one of CSP/eluent combinations employed. Pure alcohols such ethanol or 2-propanol, with a fixed percentage of DEA added, serve as valuable alternatives to the more common n-hexane-based normal-phase eluents in resolution of Mianserin on the AD CSP. In order to study the chiroptical properties of aptazepine derivatives, chromatographic resolutions are carried out at semipreparative scale using Chiralpak AD and Chiralpak IA as CSPs. Nonconventional dichloromethane-based eluents have permitted to expand the chiral resolving ability of the immobilized Chiralpak IA CSP and to perform mg-scale enantioseparations with an analytical-size column. Assignment of the absolute configuration of the separated enantiomers is empirically established by comparing their chiroptical data with those of structurally related Mianserin.     

Enantiomer self-disproportionation and chiral stationary phase based selective chiral separation of organic compounds

In modern chromatography, chiral stationary phase (CSP) and enantiomer self-disproportionation (ESD) are new inventions of packing material offer a guarantee for a successful enantiomeric separation. All CSPs were synthesized by chemical bonding of the relevant organic moieties onto a porous parent silica material for the separation of various racemic mixtures whereas achiral silica matrix was used for separation of non-racemic mixtures in ESD. Our present study provides to establish an understanding on the entire enantio-selective profile of amino alcohol based CSP as well as ESD and their precise utilization for high success rates for selective enantiomer separation with its appropriateness.     

Chiral discrimination with regioselectively substituted cellulose esters as chiral stationary phases

Four kinds of cellulose derivatives, including two regioselectively substituted cellulose esters (6-O-acetyl-2,3-di-O-benzoyl cellulose and 2,3-di-O-acetyl-6-O-benzoyl cellulose), were synthesized so that the effects of their functional group distribution on their chiral discrimination ability could be examined. The degree of substitution by functional groups appeared to have a critical effect on the separation in most cases, but the type of the functional group at the C-6 position also significantly influenced chiral discrimination when a series of neutral arylalcohol derivatives were used as racemates. Copyright 2000 Wiley-Liss, Inc.     

Benzoylcellulose derivatives as chiral stationary phases for open tubular column chromatography

The application of cellulose-based stationary phases for chiral separations has been extended to open tubular column chromatography. Efficient columns were obtained by coating the capillaries with mixtures of chiral cellulose materials and conventional achiral stationary phases for gas chromatography. In this study, various siloxane and polyethylene glycol polymers were used as achiral components and mixed with different substituted benzoylcellulose derivatives as chiral components. Systematic investigations were carried out to determine the optimal ratio for the components of the stationary phase. Depending on the chromatographic mode—gas chromatography (GC) or supercritical fluid chromatography (SFC)—the stationary phases were found to behave differently. The applicability of the technique was demonstrated by the resolution of various racemic compounds. © 1993 Wiley-Liss, Inc.