A colorimetric chiral sensor based on chiral crown ether for the recognition of the two enantiomers of primary amino alcohols and amines

A new colorimetric chiral sensor material consisting of three different functional sites such as chromophore (2,4-dinitrophenylazophenol dye), binding site (crown ether), and chiral barrier (3,3'-diphenyl-1,1'-binaphthyl group) was prepared and applied to the recognition of the two enantiomers of primary amino alcohols and amines. Among five primary amino alcohols and two primary amines tested, the two enantiomers of phenylalaninol show the highest difference in the absorption maximum wavelength (Δλ(max)=43.5 nm) and in the association constants (K(S)/K(R)=2.51) upon complexation with the colorimetric chiral sensor material and, consequently, the two enantiomers of phenylalaninol were clearly distinguished from each other by the color difference.     

Development of a chiral stationary phase based on cinchonidine. Comparison with a quinine-based chiral column

A chiral anion-exchanger stationary phase based on cinchonidine (CD) was developed. Two columns were packed with and without endcapping (EC) treatment (CD-chiral stationary phase[CD-CSP(EC)] and [CD-CSP], respectively) and studied for their ability to separate N-2,4-dinitrophenyl α-amino acids (DNP-amino acids) enantiomers over a temperature range of 10-40 °C with a hydro-organic buffer mobile phase. The more hydrophobic, endcapped stationary phase showed significantly larger retentive capacity than the non-endcapped one. The apparent thermodynamic transfer parameters of the enantiomers from the mobile to both CSPs were estimated from van't Hoff plots within the cited temperature range. Similar studies with two natural quinine-based columns (QN-CSP and QN-CSP(EC)) were previously reported. In this work, a critical comparison in the chiral recognition ability to DNP-amino acids of these cinchonidine and QN-based chiral columns was drawn. It has been found that QN-based CSPs show greater chiral recognition capability towards these derivatives than CD-CSPs. The influence of the QN methoxy group on the equilibrium constants of the enantioselective interaction between these DNP-amino acids with these two cinchona CSPs could be assessed.     

Application of L-proline derivatives as chiral shift reagents for enantiomeric recognition of carboxylic acids

Four proline-derived chiral receptors 5-8 were readily synthesized starting from L-proline. The enantiomeric recognition ability of chiral receptors was examined with a series of carboxylic acids by (1) H NMR spectroscopy. The molar ratio and the association constants of the chiral compounds with each of the enantiomers of guest molecules were determined by using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate that the hosts form 1:1 instantaneous complexes with all guests. The receptors exhibited different chiral recognition abilities toward the enantiomers of racemic guests. Among the chiral receptors used in this study, prolinamide 6 was found to be the best chiral shift reagent and is effective for the determination of the enantiomeric excess of chiral carboxylic acids.     

Determination of enantiomeric composition by negative-ion electrospray ionization-mass spectrometry using deprotonated N-(3,5-dinitrobenzoyl)amino acids as chiral selectors

The ability to use mixtures of deprotonated N-(3,5-dinitrobenzoyl)amino acids as chiral selectors for the determination of enantiomeric composition by electrospray ionization-mass spectrometry is demonstrated. For each experiment, two N-(3,5-dinitrobenzoyl)amino acids were chosen such that each would have opposite selectivity for the enantiomers of the analyte. Electrospray ionization-mass spectrometry, monitored in the negative ion mode, of solutions containing the two N-(3,5-dinitrobenzoyl)amino acids, sodium hydroxide, and the analyte, in a one-to-one mixture of methanol and water, afford peaks in the mass spectrum that correspond to the deprotonated 1:1 analyte-selector complexes. The ratio of the intensities of the complexes in the mass spectrum can be related to the enantiomeric composition of the analyte. Additionally, the sense and extent of chiral recognition is consistent with chromatographic observations, using chiral stationary phases derived from N-(3,5-dinitrobenzoyl)amino acids. Each analysis of enantiomeric composition requires less than 10 s to complete, indicating that this method has great potential for the development of fast-/high-throughput chiral analyses.     

Biphasic recognition chiral extraction: A novel method for separation of mandelic acid enantiomers

This article reports a new chiral separation method-biphasic recognition chiral extraction for the separation of mandelic acid enantiomers. Distribution behavior of mandelic acid enantiomers was studied in the extraction system with O,O'-di-benzoyl-(2S,3S)-4-toluoyl-tartaric acid (D-(+)-DTTA) in organic phase and beta-CD derivatives in aqueous phase, and the influence of the types and concentrations of extractants and pH on extraction efficiency was investigated. Hydroxypropyl-beta-cyclodextrin (HP-beta-CD), hydroxyethyl-beta-cyclodextrin (HE-beta-CD), and methyl-beta-cyclodextrin (Me-beta-CD) have stronger recognition abilities for S-mandelic acid than those for R-mandelic acid, among which HP-beta-CD has the strongest ability. D-(+)-DTTA preferentially recognizes R-mandelic acid. pH and the concentrations of extractants have great effects on chiral separation ability. A high enantioseparation efficiency with a maximum enantioselectivity of 1.527 is obtained at pH of 2.7 and the ratio of 2:1 of [D-(+)-DTTA] to [HP-beta-CD]. The obtained results indicate that the biphasic recognition chiral extraction is of stronger chiral separation ability than the monophasic recognition chiral extraction. It may be very helpful to optimize the extraction systems and realize the large-scale production of pure enantiomers.     

Evaluation of dalbavancin as chiral selector for HPLC and comparison with teicoplanin‐based chiral stationary phases

Dalbavancin is a new compound of the macrocyclic glycopeptide family. It was covalently linked to 5 μm silica particles using two different binding chemistries. Approximately 250 racemates including (a) heterocyclic compounds, (b) chiral acids, (c) chiral amines, (d) chiral alcohols, (e) chiral sulfoxides and sulfilimines, (f) amino acids and amino acid derivatives, and (g) other chiral compounds were tested on the two new chiral stationary phases (CSPs) using three different mobile phases. As dalbavancin is structurally related to teicoplanin, the same set of chiral compounds was screened on two commercially available teicoplanin CSPs for comparison. The dalbavancin CSPs were able to separate some enantiomers that were not separated by the teicoplanin CSPs and also showed improved separations for many racemates. However, there were other compounds only separated or better separated on teicoplanin CSPs. Therefore, the dalbavancin CSPs are complementary to the teicoplanin CSPs. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Enantiomeric separation of imidazolinone herbicides using chiral high-performance liquid chromatography

Chiral high-performance liquid chromatography (HPLC) is one of the most powerful tools to prepare enantiopure standards of chiral compounds. In this study, the enantiomeric separation of imidazolinone herbicides, i.e., imazethapyr, imazapyr, and imazaquin, was investigated using chiral HPLC. The enantioselectivity of Chiralpak AS, Chiralpak AD, Chiralcel OD, and Chiralcel OJ columns for the three analytes was compared under similar chromatographic conditions. Chiralcel OJ column showed the best chiral resolving capacity among the test columns. The resolved enantiomers were distinguished by their signs of circular dichroism detected at 275 nm and their structures confirmed with LC-mass spectrometric analysis. Factors affecting the chiral separation of imidazolinones on Chiralcel OJ column were characterized. Ethanol acted as a better polar modifier than the other alcohols including 2-propanol, 1-butanol, and 1-pentanol. Although the acidic modifier in the mobile phase did not influence chiral recognition, it was necessary for reducing the retention time of enantiomers and suppressing their peak tailing. Thermodynamic evaluation suggests that enantiomeric separation of imidazolinones on Chiralcel OJ column is an enthalpy-driven process from 10 to 40 degrees C. This study also shows that small amounts of pure enantiomers of imidazolinones may be obtained by using the analytical chiral HPLC approach.     

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.     

The development of biphasic chiral recognition in chiral separation

In chiral separation, enantioseparation factor is an important parameter which influences the resolution of enantiomers. In this current overview, a biphasic chiral recognition method is introduced to the readers. This method can significantly improve the enantioseparation factor in two‐phase solvent through adding lipophilic and hydrophilic chiral selectors which have opposite chiral recognition ability to organic and aqueous phases, respectively. This overview presents the development and applications of biphasic chiral recognition in liquid‐liquid extraction and counter current chromatography. It mainly focuses on the topics of mechanism, advantages and limitations, applications, and key factors of biphasic chiral recognition. In addition, the future outlook on development of biphasic chiral recognition also has been discussed in this overview.     

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.     

One-step hydrothermal preparation of chiral carbon quantum dots and enantioselective sensing of glutamine enantiomeric isomers

A novel method for chiral identification of glutamine enantiomers based on chiral carbon quantum dots (cCQDs) fluorescent probes. cCQDs were prepared using a one-step hydrothermal method with L-tryptophan as the carbon source and chiral source, producing spherical nanoparticles exhibiting a blue colour luminescence. The fluorescence intensity (F) of cCQDs was enhanced or quenched following the addition of chiral enantiomeric glutamine (L/D-Gln), and therefore cCQDs, as a fluorescence probe, could be used for enantioselective sensing of the L/D-Gln. The fluorescence enhancement value (∆F_E) exhibited good linearity with L-Gln concentration in the range 0.23–10.00 mM, and the limit of detection was 0.14 mM. The fluorescence quenching value (∆F_Q) showed a good linear relationship with D-Gln concentration in the range 0.29–10.00 mM, and the detection limit was 0.18 mM. The mechanism of fluorescence enhancement/quenching was explored by molecular modelling and the type of quenching. The method was applied to the determination of L-Gln content in real samples, and the recovery rate was satisfactory. This study provided a novel approach for the synthesis of cCQDs and the recognition of amino acid enantiomers.     

Electrochromatographic enantioseparation of amino acids using polybutylmethacrylate-based chiral monolithic column by capillary electrochromatography

This article describes the development of a polybutylmethacrylate‐based monolithic capillary column as a chiral stationary phase. The chiral monolithic column was prepared by polymerization of butyl methacrylate (BMA), ethylene dimethacrylate (EDMA), and N‐methacryloyl‐l ‐glutamic acid (MAGA) in the presence of porogens. The porogen mixture included N,N‐dimethyl formamide and phosphate buffer. MAGA was used as a chiral selector. The effect of MAGA content was investigated on electrochromatographic enantioseparation of d,l ‐histidine, d,l ‐tyrosine, d,l ‐phenyl alanine, and d,l ‐glutamic acid. The effect of acetonitrile (ACN) content in mobile phase on electro‐osmotic flow was also investigated. It was demonstrated that the poly(BMA‐EDMA‐MAGA) monolithic chiral column can be used for the electrochromatographic enantioseparation of amino acids by capillary electrochromatography (CEC). The mobile phase was ACN/10 mM phosphate buffer (45:55%) adjusted to pH 2.7. It was observed that l ‐enantiomers of the amino acids migrated before d ‐enantiomers. The separation mechanism of electrochromatographic enantioseparation of amino acids in CEC is discussed. Chirality 24:606–609, 2012. © 2012 Wiley Periodicals, Inc.     

Protein domain of chicken alpha(1)-acid glycoprotein is responsible for chiral recognition

Ovoglycoprotein from chicken egg whites (OGCHI) has been used as a chiral selector to separate drug enantiomers. However, neither the amino acid sequence of OGCHI nor the responsible part for the chiral recognition (protein domain or sugar moiety) has yet to be determined. First, we isolated a cDNA clone encoding OGCHI, and clarified the amino acid sequence of OGCHI, which consists of 203 amino acids including a predictable signal peptide of 20 amino acids. The mature OGCHI shows 31-32% identities to rabbit and human alpha(1)-acid glycoproteins (alpha(1)-AGPs). Thus, OGCHI should be the chicken alpha(1)-AGP. Second, the recombinant chicken alpha(1)-AGP was prepared by the Escherichia coli expression system, and its chiral recognition ability was confirmed by capillary electrophoresis. Since proteins expressed in E. coli are not modified by any sugar moieties, this result shows that the protein domain of the chicken alpha(1)-AGP is responsible for the chiral recognition.     

Enantioseparation of Racemic Flurbiprofen by Aqueous Two‐Phase Extraction With Binary Chiral Selectors of L‐dioctyl Tartrate and L‐tryptophan

A novel method for chiral separation of flurbiprofen enantiomers was developed using aqueous two‐phase extraction (ATPE) coupled with biphasic recognition chiral extraction (BRCE). An aqueous two‐phase system (ATPS) was used as an extracting solvent which was composed of ethanol (35.0% w/w) and ammonium sulfate (18.0% w/w). The chiral selectors in ATPS for BRCE consideration were L‐dioctyl tartrate and L‐tryptophan, which were screened from amino acids, β‐cyclodextrin derivatives, and L‐tartrate esters. Factors such as the amounts of L‐dioctyl tartrate and L‐tryptophan, pH, flurbiprofen concentration, and the operation temperature were investigated in terms of chiral separation of flurbiprofen enantiomers. The optimum conditions were as follows: L‐dioctyl tartrate, 80 mg; L‐tryptophan, 40 mg; pH, 4.0; flurbiprofen concentration, 0.10 mmol/L; and temperature, 25 °C. The maximum separation factor α for flurbiprofen enantiomers could reach 2.34. The mechanism of chiral separation of flurbiprofen enantiomers is discussed and studied. The results showed that synergistic extraction has been established by L‐dioctyl tartrate and L‐tryptophan, which enantioselectively recognized R‐ and S‐enantiomers in top and bottom phases, respectively. Compared to conventional liquid–liquid extraction, ATPE coupled with BRCE possessed higher separation efficiency and enantioselectivity without the use of any other organic solvents. The proposed method is a potential and powerful alternative to conventional extraction for separation of various enantiomers. Chirality 27:650–657, 2015. © 2015 Wiley Periodicals, Inc.     

Liquid chromatographic enantiomer resolution of N-hydrazide derivatives of 2-aryloxypropionic acids on a crown ether derived chiral stationary phase

The liquid chromatographic separation of the enantiomers of several N-hydrazide derivatives of 2-aryloxypropionic acids was performed on a crown ether type chiral stationary phase derived from (18-crown-6)-2,3,11,12-tetracarboxylic acid. The behavior of chromatographic parameters by the change of mobile phases and additives for the resolution of these analytes was investigated. The enantiomers of all analytes were base-line resolved with a mobile phase of 100% methanol containing 20 mM H2SO4. These results are the first reported for enantiomer resolution of chiral acids of 2-aryloxypropionic acids as their N-hydrazide derivatives.     

Optimum operational conditions for chiral separation of tryptophan enatiomers using ligand exchange liquid chromatography

A simple and precise method for chiral separation of tryptophan enantiomers using high performance liquid chromatography with aligand exchange mobile phase was developed. Chiral separation was performed on a conventional C₁₈ column, using a mobile phase that consisted of a water-methanol solution (88∶12, v/v) containing 10 mmol/Ll-leucine and 5 mmol/L copper sulfate as a chiral ligand additive at a flow rate of 1.0 mL/min. This method allowed baseline separation of two enantiomers with a resolution of 1.84 in less than 30 min. The effect of various conditions, including concentration, type of ligand, organic modifier, pH, flow rate, and temperature, on enantioseparation were evaluated and chiral recognition mechanisms were investigated. Thermodynamic data (ΔΔH and ΔΔS) obtained by van't Hoff plots revealed that enantioseparation is an enthalpy-controlled process.     

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.     

The direct chiral separations of fungicide enantiomers on amylopectin based chiral stationary phase by HPLC

Amylopectin-tris(phenylcarbamate) was synthesized and coated to aminopropylsilica to prepare chiral stationary phase. The chiral separations of fungicide enantiomers were performed by the CSP using high-performance liquid chromatography. Mobile phase was n-hexane and isopropanol, and flow rate was 1.0 ml/min. Detection wavelength was 230 nm. The influence of the percentage of isopropanol in the mobile phase on the separations was studied. Twelve chiral fungicides were tested and seven of them were found to show stereoselectivity on the CSP. The enantiomers of metalaxyl and benalaxyl got near baseline separations and myclobutanil, hexconazole, tebuconazole, uniconazole, and paclobutrazol enantiomers were completely separated. The decreasing percentage of isopropanol in the mobile phase resulted in better separation and longer analysis time. The enantiomers were identified by a circular dichroism (CD) detector and the CD spectra of the individual enantiomers were also studied by online scanning.     

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.     

Role of the weak interactions in enantiorecognition of racemic dihydropyrimidinones by novel brush-type chiral stationary phases

The chiral discrimination ability of two recently prepared chiral stationary phases (CSP 1 and CSP 2), based on a leucine derived chiral selector, was tested for the enantiomers of dihydropyrimidone (DHPM) derivatives and compared with the commercially available Hyun-leucine CSP 3 and classical Pirkle-leucine CSP 4. By combining all of these CSPs, the enantiomers of all DHPM derivatives used in this study can be properly resolved. Particularly good enantioresolutions were achieved for thioureide derivatives, such as Monastrol. The results presented show that sulfur-aromatic interactions are meritorious for these very good separations.