Enantioresolution of five β-blockers by reversed-phase high-performance liquid chromatography using fifteen chiral derivatizing reagents having amino acids or their amides as chiral auxiliaries on a cyanuric chloride platform

Enantioseparation of five β-blockers, namely, (R,S)-atenolol, (R,S)-propranolol, (R,S)-bisoprolol, (R,S)-metoprolol and (R,S)-carvedilol, was achieved as their diastereomers prepared with chiral derivatizing reagents (CDRs) synthesized on a cyanuric chloride platform. Fifteen CDRs were synthesized by nucleophilic substitution of the Cl atom in cyanuric chloride or its 6-methoxy derivative with amino acids (namely, L-Leu, L-Val, D-Phg, L-Met and L-Ala) or their amides as chiral auxiliaries. The diastereomers were synthesized under microwave irradiation for 70 or 100 s at 85% power. Separation of diastereomers was carried out on a C(18) column and gradient eluting mixtures of methanol with aqueous trifluoroacetic acid with UV detection at 230 nm. Separation efficiencies of the reagents were compared on the basis of effect of chiral auxiliaries (i.e. amino acids or amino acid amides) and achiral substituents (i.e. chlorine or methoxy group) in the CDRs. The method was validated for detection limit, linearity, accuracy and precision.     

Enantioseparation of phenylsuccinic acid by high speed counter-current chromatography using hydroxypropyl-β-cyclodextrin as chiral selector

High speed counter-current chromatography (HSCCC) was successfully applied to resolution of phenylsuccinic acid (PSA) with hydroxypropyl-β-cyclodextrin (HP-β-CD) as chiral selector (CS). The two-phase solvent system composed of n-hexane–methyl tert-butyl ether–0.1 mol L⁻¹ phosphate buffer solution with pH = 2.51 (0.5:1.5:2, v/v/v) was selected. Influence factors involved in the chiral separation were investigated, including the concentration of chiral selector, pH value of the aqueous phase, the separation temperature, and the thermodynamic parameters of inclusion complex were calculated. The complex formation constants were determined using analytical instrument. Two HSCCC elution modes were studied and peak resolution equation was discussed. Under optimum separation conditions, 712 mg of PSA racemate was separated using preparative apparatus. The purities of both of the fractions including (+)-PSA and (−)-PSA from the preparative CCC separation were over 98.5% determined by HPLC and enantiomeric excess of (+)-PSA and (−)-PSA reached 97.6% and 98.6%, respectively. Recovery for the target compounds from the CCC fractions reached 80–82% yielding 285 mg of (+)-PSA and 292 mg of (−)-PSA.     

Analysis of multicomponent mixture and simultaneous enantioresolution of proteinogenic and non-proteinogenic amino acids by reversed-phase high-performance liquid chromatography using chiral variants of Sanger’s reagent

Four chiral derivatizing reagents (CDR 1–4), namely, FDNP-l-Ala, FDNP-l-Val, FDNP-l-Phe, and FDNP-l-Leu, were synthesized using microwave (MW) irradiation by substituting one of the fluorine atoms in difluoro dinitro benzene (DFDNB) with l-Ala, l-Val, l-Phe, and l-Leu (CDR 1–4). The other set of CDRs, namely, FDNP-l-Phe-NH₂, FDNP-l-Val-NH₂, and FDNP-l-Leu-NH₂, was also prepared. These reagents were used for synthesis of diastereomers of 18 proteinogenic and 08 non-proteinogenic amino acids, which were resolved by reversed-phase high-performance liquid chromatography using C18 column and gradient eluting mixture of aq.TFA and acetonitrile with UV detection at 340 nm. The reagents were used for resolution of a complex mixture of 18 racemic proteinogenic amino acids in a single chromatographic run of 65 min and to determine concentration of the d-amino acid in a solution of dl-amino acid. The resolution (R _S) and selectivity (α) obtained for the two sets of diastereomers were compared among themselves and among the two groups. The method was validated for accuracy, precision, limit of detection (LOD), and limit of quantification. LOD is 0.001% impurity of d-enantiomer.     

Enantiomeric separation in high-performance liquid chromatography using novel β-cyclodextrin derivatives modified by R-configuration groups as chiral stationary phases

Two new chiral stationary phases (CSP) were successfully prepared through bonding β-cyclodextrin (CD) derivatives modified by R-configuration groups (R-CPGCD, R-HMPGCD) to silica gel. Nineteen chiral nitro aromatic alcohol derivatives were separated under the polar organic and the reversed phase modes. Better enantioseparation was obtained in the reversed phase mode. The resolution values of the analytes ranged from 1.98 to 7.57 and from 2.19 to 8.14 on R-CPGCD and R-HMPGCD CSPs, respectively, using a mobile phase composed of methanol/water (v/v, 40/60). Better enantioseparation was obtained on R-HMPGCD CSP than on R-CPGCD CSP because of stronger hydrogen bonding and π-π interactions between the substituents on the cyclodextrin derivatives and the analytes. For different analytes, the increasing electronic density of the benzene ring was found to be favorable to the enantioseparation of the test analytes. The thermodynamic parameters showed that the enantioseparation of analytes was enthalpy-controlled and a lower temperature aided the enantiomeric separation of the solutes on the two CSPs. MD simulations were used to investigate the recognition mechanism between the chiral selectors and the analyte using R-, S-2-naphthalenemethanol and R-CPGCD and R-HMPGCD complexes as examples. S-2-naphthalenemethanol had the stronger interactions with R-CPGCD and R-HMPGCD than the R-isomer. The substituent derivatized on R-CPGCD and the cyclodextrin cavity contributed to the discrimination of the S-isomer, but only the derivatized group on R-HMPGCD was found to play a major role in separating prosess. In addition, the larger free energy deviation of the R- and S-isomers in the R-HMPGCD system brought about a higher resolution value (R_s = 8.14).     

Enantioseparation of dansyl amino acids and dipeptides by chiral ligand exchange capillary electrophoresis based on Zn(II)-l-hydroxyproline complexes coordinating with γ-cyclodextrins

A chiral ligand exchange capillary electrophoresis (CLE-CE) method using Zn(II) as the central ion and l-4-hydroxyproline as the chiral ligand coordinating with γ-cyclodextrin (γ-CD) was developed for the enantioseparation of amino acids (AAs) and dipeptides. The effects of various separation parameters, including the pH of the running buffer, the ratio of Zn(II) to l-4-hydroxyproline, the concentration of complexes and cyclodextrins (CDs) were systematically investigated. After optimization, it has been found that eight pairs of labeled AAs and six pairs of labeled dipeptides could be baseline-separated with a running electrolyte of 100.0 mM boric acid, 5.0 mM ammonium acetate, 3.0 mM Zn(II), 6.0 mM l-hydroxyproline and 4.0 mM γ-CD at pH 8.2. The quantitation of AAs and dipeptides was conducted and good linearity (r² ≥ 0.997) and favorable repeatability (RSD ≤ 3.6%) were obtained. Furthermore, the proposed method was applied in determining the enantiomeric purity of AAs and dipeptides. Meanwhile, the possible enantiorecognition mechanism based on the synergistic effect of chiral metal complexes and γ-CD was explored and discussed briefly.     

Enantioseparation of three kinds of adrenaline by capillary electrophoresis using Ser-AuNPs as chiral selectorEI北大核心CSCD

Au nanoparticlesAuNPswere synthesized by chemical reduction methodand its surface was chiral modified with serine to prepare Ser-AuNPs. The synthesis effect and particle size distribution of AuNPs and Ser-AuNPs were characterized by UV-Vis spectrophotometry and laser particle size analyzer. The Ser-AuNPs was used as a chiral selector for the enantiomeric separation of three adrenaline drugs by capillary zone electrophoresis. The influences of the binding concentration of serinebuffer pHseparation voltage and capillary temperature on the enantioseparation were investigated. The results showed that the enantiomeric separation of isoprenaline adrenaline and noradrenaline could achieve baseline separation at the optimal separation conditions. The corresponding resolutions were 3.192.85 and 3.77 for isoprenalineadrenaline and noradrenalinerespectively. And this method possessed good reproducibility and precision. © 2022, Youke Publishing Co.,Ltd. All rights reserved.     

Enantioseparation of a novel “click” chemistry derived native β-cyclodextrin chiral stationary phase for high-performance liquid chromatography

A novel native β-cyclodextrin chiral stationary phase was prepared via “click” chemistry with cuprous iodide–triphenylphosphine complex as the catalyst and applied for enantioseparation of Dns-amino acids, substituted phenyl and phenoxy group modified propionic acids, flavonoids, and some pharmaceutical compounds such as nimodipine, propranolol, brompheniramine and bendroflumethiazide in reversed-phase high-performance liquid chromatography. The studied analytes could be resolved under different separation conditions. The resolution of Dns-DL-Leu could reach 5.08 using a mobile phase consisting of 1% (w/w) triethylammonium acetate buffer (pH 4.11) and methanol (50:50 v/v). The effects of buffer pH and the content of organic modifier on enantioseparation of Dns-amino acids by this novel chiral phase were being investigated. The separation results demonstrate that click chemistry, a versatile reaction, affords a facile approach towards the preparation of stable chiral stationary phases.     

A facile enantioseparation for amino acids enantiomers using β-cyclodextrins functionalized Fe3O4 nanospheres

Herein is presented a strategy for the enantioseparation of amino acids enantiomers using β-CD functionalized Fe(3)O(4) nanospheres, in which β-CD provides the ability to chirally discriminate amino acids enantiomers, while the Fe(3)O(4) nanoparticles serve as magnetic separators.     

Optimization separation of five phosphoamidothioate enantiomers using a series of silica and chiral column by high-performance liquid chromatography

A computer-assisted method is described for optimization of multi-component,mobile phase selection for separation of five phosphoamidothioate enantiomers with a series of silica and chiral columns in normal phase HPLC.The method is based on the triangular solvent selection concept using a statistical scanning method.The optimization of the separation over the experimental region is based on a special polynomial estimation from seven experimental runs,and resolution (Rs) is used as the selection criterion.Excellent agreement was obtained between predicted and experimental data.     

Enantioselective α-amination of branched aldehydes promoted by simple chiral primary amino acids

A series of simple chiral primary amino acids were first successfully applied to promote the enantioselective α-amination of branched aldehydes with azadicarboxylates and the desired adducts bearing quaternary stereogenic centers were obtained in excellent yields (up to 99%) and enantioselectivities (up to 97% ee).     

β-Cyclodextrin derivative as chiral carrier in membrane transport of some aromatic amino acids

The chiral functionalized β-cyclodextrin (heptakis (2,3,6-tri-o-acetyl)-β-cyclodextrin) was studied as an extraction receptor in liquid–liquid extraction and carrier through bulk liquid membrane for highlighting its ability to enantiomeric discrimination of some native aromatic amino acids (tryptophane, phenylalanine, and tyrosine) and methylester derivatives (tryptophane methylester hydrochloride, phenylalanine methylester hydrochloride, and tyrosine methylester hydrochloride). The receptor exhibited carrier ability for aromatic amino acids transport through chloroform liquid membrane, especially for l-amino acids form aiming at their separation. Active transport of amino acids assisted by the pH gradient was carried out. Additional, the circular dichroism measurements were performed to point out the enantiomeric discrimination of aromatic amino acids by functionalized β-cyclodextrin. The results showed that the structural properties of amino acids are an important parameter in extraction and transport experiments.     

The 3-amino-derivative of γ-cyclodextrin as chiral selector of Dns-amino acids in electrokinetic chromatography

The enantioseparation of the enantiomeric pairs of 10 Dns derivatives of α-amino acids was successfully carried out by using for the first time the 3-amino derivative of the γ-cyclodextrin. The effects of pH and selector concentration on the migration times and the resolutions of analytes were studied in detail. 3-Deoxy-3-amino-2(S),3(R)-γ-cyclodextrin (GCD3AM) shows very good chiral recognition ability even at very low concentrations at all the three investigated values of pH, as shown by the very large values of selectivity and resolution towards several pairs of amino acids. The role played by the cavity, the substitution site and the protonation equilibria on the observed properties of chiral selectivity, on varying the specific amino acid involved, is discussed.     

A comparison of phosphated and sulfated β-cyclodextrins as chiral selectors for capillary electrophoresis

The enantioseparation capabilities of three different functionalized β-cyclodextrins, two sulfated β-cyclodextrins with 4 and 15 nominal degrees of substitution and a phosphated β-cyclodextrin with 8 degrees of substitution, were compared. While anodic detection was used with both sulfated cyclodextrins, the phosphated cyclodextrin required cathodic detection suggesting either lower ionization of the phosphated cyclodextrin or generally lower affinity of the analytes for the phosphated cyclodextrin. The effects of several experimental parameters were evaluated with respect to enantioseparation. The degrees of substitution of the cyclodextrin, pH of the background electrolyte as well as the concentration of the functionalized β-cyclodextrin, each had a significant influence on the successful enantiomeric separation of the chiral drugs investigated.     

Dynamic kinetic resolution of racemic N-phthalyl amino acids using (S)-α-methylpantolactone as the chiral auxiliary

A dynamic kinetic resolution of racemic N-phthalyl amino acids by stereoselective esterification was examined using (S)-α-methylpantolactone as the chiral auxiliary. The reaction of various racemic N-phthalyl amino acids with this chiral alcohol in the presence of both DCC and DMAP afforded predominantly the (S,S)-esters in nearly quantitative yield.     

Quantification of recombinant human erythropoietin by amino acid analysis using isotope dilution liquid chromatography–tandem mass spectrometry

Herein, we describe an accurate method for protein quantification based on conventional acid hydrolysis and an isotope dilution-ultra performance liquid chromatography–tandem mass spectrometry method. The analyte protein, recombinant human erythropoietin (rhEPO), was effectively hydrolyzed by incubation with 8 mol/L hydrochloric acid at 130 °C for 48 h, in which at least 1 μmol/kg of rhEPO was treated to avoid possible degradation of released amino acids during hydrolysis. Prior to hydrolysis, sample solution was subjected to ultrafiltration to eliminate potential interfering substances. In a reversed-phase column, the analytes (phenylalanine, proline, and valine) were separated within 3 min using gradient elution comprising 20 % (v/v) acetonitrile and 10 mmol/L ammonium acetate, both containing 0.3 % (v/v) trifluoroacetic acid. The optimized hydrolysis and analytical conditions in our study were strictly validated in terms of accuracy and precision, and were suitable for the accurate quantification of rhEPO. Certified rhEPO was analyzed using a conventional biochemical assay kit as an additional working calibrant for the quantification of EPO and improved the accuracy. The optimized protocol is suitable for the accurate quantification of rhEPO and satisfactorily serves as a reference analytical procedure for the certification of rhEPO and similar proteins.

Asymmetric synthesis of quaternary α-amino acids using d-ribonolactone acetonide as chiral auxiliary

We describe a new simple methodology to prepare enantiopure α,α-dialkylglycines based on the use of commercially available d-ribonolactone as a chiral auxiliary. Enantiopure α-methyl and α-butyl series are prepared through diastereoselective alkylation and subsequent Schmidt rearrangement of α,α-dialkylacetoacetates of d-ribonolactone acetonide. Absolute configuration was assigned through preparation of enantiopure 4,4-disubstituted 3-methyl-2-pyrazolin-5-ones.     

Enantioseparation of five racemic N-alkyl drugs by reverse phase HPLC using sulfobutylether-β-cyclodextrin as a chiral mobile phase additive

Analytical enantioseparations of five N-alkyl drugs, fluoxetine hydrochloride, labetalol, venlafaxine hydrochloride, trans-paroxol, and atropine sulfate, were investigated by reverse phase high-performance liquid chromatography with sulfobutylether-β-cyclodextrin as chiral mobile phase additive. Effects of various factors such as composition of mobile phase, concentration of cyclodextrins, and column temperature on retention and enantioselectivity were studied. Apparent formation constant between methanol, acetonitrile, and sulfobutylether-β-cyclodextrin were determined to be 2.90 × 10⁻³ and 1.00 × 10⁻⁴ L mmol⁻¹ under 25°C using UV-spectrophotometry. Van't Hoff plots were used to investigate thermodynamic parameters for enantiomers–stationary phase interaction and formation of inclusion complex. Two retention models were employed individually for evaluation of inclusion complexation between five racemates and sulfobutylether-β-cyclodextrin. The second model with complex adsorption was more accord with the retention behavior of fluoxetine hydrochloride, labetalol, and venlafaxine hydrochloride enantiomers, while the first model was more consistent with the retention behaviors of trans-paroxol and atropine sulfate. In the selected mobile phase, stoichiometric ratio for both of inclusion complex was found to be 1:1.     

Asymmetric synthesis of fluorine-containing amines, amino alcohols, α- and β-amino acids mediated by chiral sulfinyl group

This review article provides a critical overview of several different synthetic approaches developed for asymmetric preparation of fluorine-containing amines, amino alcohols, α- and β-amino acids. The common feature of these methods is the application of sulfinyl group as a chiral auxiliary to control the stereochemical outcome of the reactions under study. In particular, the following general methods are critically discussed: diastereoselective methylene transfer from diazomethane to the carbonyl of β-keto-γ-fluoroalkyl sulfoxides as a general approach for preparation of various α-fluoroalkyl α-sulfinylalkyl oxiranes. The resulting compounds were used as true chiral synthons for their further elaboration via oxidative or reductive desulfurization, to numerous fluorine-containing and biologically relevant amino- and hydroxy-containing derivatives. Another general approaches discussed here are asymmetric additions to CN double bond. One of them is addition of chiral sulfoxide stabilized carbon nucleophiles to fluorine-containing imines, leading to convenient preparation of alpha-fluoroalkyl derivatives of alpha amino acids and amines. Another approach is asymmetric Reformatsky reaction between N-sulfinyl imines and ethyl bromodifluoroacetate allowing operationally convenient preparation of α,α-difluoro-β-amino acids in enantiomerically pure form. Finally, structurally similar but mechanistically different addition reactions of diethyl difluoromethylphosphonate to N-sulfinyl imines, as a general approach to asymmetric synthesis of α,α-difluoro-β-aminophosphonates and phosphonic acids, are discussed. Effect of fluorine on the mechanism and stereochemical outcome of these reactions is discussed in detail and compared, where it is possible, with that of the analogous reactions of fluorine-free substrates.