Enantiomeric separation of α-phenylethylamine and its analogs by mixed chiral and achiral stationary phase

A mixed stationary phase of modified β-cyclodextrin, 2,6-di-O-butyl-3-O-butyryl-β-cyclodextrin (phase A), and SE-54 (phase B) was used for enantiomeric separation of α-phenylethylami-ne, o,m,p-methyl and o,m,p-methoxy-substituted analogs. The composition of mixed phase was selected by comparison of each calculated amin(= (γm,i+1)/(γm,i))> the relative retention values of the most adjacent peaks, and γm,last, the relative retention values of the last eluting peak at each preselected ratio. Values of γm,i,α calculated by derived equations were in good agreement with the experimental results obtained with two specified mixed phases. All solutes investigated were almost baseline separated at a predicted composition of phase A and phase B in a single run within 18 minutes.     

Restricted variation in the glycosylation of human α1-acid glycoprotein

Summary The glycosylation patterns of multiglycosylated proteins reflect both the cellular control of glycosyl transferase activity and the local control of the transfer event. Little information is available concerning these mechanisms. Changes in glycosylation occur in the disease state and provide a convenient way of examining the control mechanism(s) operating at individual glycosylation sites. Chromatographic methods have been applied to the human orosomucoid (OMD) isolated from seventeen different pathologies. Based on the distribution of the glycans at the individual sites it is clear that the major protein-glycan interactions that restrict glycosylation in normal OMD remain intact in the disease state in the presence of changing glycosyl transferase activity.     

LFER and CoMFA studies on optical resolution of α-alkyl α-aryloxy acetic acid methyl esters on DACH-DNB chiral stationary phase

Summary The HPLC resolution of a series of racemic -substituted -aryloxy acetic acid methyl esters I on a -acid chiral stationary phase containing N,N-(3,5-dinitrobenzoyl)-trans-1,2-diaminocyclohexane as chiral selector was modelled by linear free energy-related (LFER) equations and comparative molecular field analysis (CoMFA). Our results indicate that the retention process mainly depends on solute lipophilicity and steric properties, whereas enantioselectivity is primarily influenced by electrostatic and steric interactions. CoMFA provided additional information with respect to the LFER study, allowed the mixing of different subsets of I and led to a quantitative 3D model of steric and electrostatic factors responsible for chiral recognition.     

Comparative separation of biologically active components inRhizoma chuanxiong by affinity chromatography with α1-acid glycoprotein and human serum albumin as stationary phasesglycoprotein and human serum albumin as stationary phases

Summary Affinity chromatography with α₁-acid glycoprotein (AGP) and human serum albumin (HSA) stationary phases was applied to screen and analyze the biologically active components ofRhizoma chuanxiong. Five major peaks and a number of small peaks were resolved based on their affinity for AGP and HSA, respectively, and three of them, identified as ferulic acid, chuanxiongzine and ligustilide via standard compounds, are regarded as effective components. The effects of acetonitrile concentration, pH, inorganic salt concentration and temperature on the retention behaviors of five major components on the two stationary phases were also investigated. It was observed that hydrophobicity is the major contributor to retention on both stationary phases, and ferulic acid has a weak electrostatic interaction with HSA. It demonstrated that the chromatograms ofRhizoma chuanxiong on the two stationary phases have concise and different fingerprinting characteristics. The amount of ferulic acid, chuanxiongzine and ligustilide inRhizoma chuanxiong determined using the AGP column are as much as 0.064%, 0.021% and 2.00% by weight.     

Evaluation of chiral separation based on bovine serum albumin–conjugated carbon nanotubes as stationary phase in capillary electrochromatography

In this work, we utilized adsorbed BSA and multiwalled carbon nanoparticles (BSA/MWCNTs) as a stationary phase in open tubular (OT) capillary for separation of chiral drugs. (3-Aminopropyl)triethoxysilane was used to assist fabrication of BSA/MWCNTs-coated OT column by covalent bonding. Incorporation of MWCNTs nanomaterials into a polymer matrix could increase the phase ratio and take advantage of the easy preparation of an open tubular CEC column. SEM was carried out to characterize the BSA/MWCNTs OT columns. The electrochromatographic performance of the OT columns was evaluated by separation of ketoprofen, ibuprofen, uniconazole, and hesperidin. The effects of MWCNTs concentration, background solution pH and concentration, and applied voltage on separation were investigated. Chiral separations of ketoprofen, ibuprofen, uniconazole, and hesperidin were achieved using the BSA/MWCNTs-coated OT column with resolutions of 24.20, 12.81, 1.50, and 1.85, respectively. Their optimas were found in the 30 mM phosphate buffers at pH 5.0, 6.5, 7.0, and 6.5, respectively. In addition, the columns demonstrated good repeatability and stability with the run-to-run, day-to-day, and batch-to-batch RSDs of migration times less than 3.5%.     

Synthesis and evaluation of a chiral stationary phase based on quinine: enantioresolution of dinitrophenyl derivatives of α-amino acids

The natural alkaloid quinine (QN) was immobilized on porous silica particles, and part of the material was subsequently endcapped with n-hexyl hydrocarbon chains. Two synthetic strategies for silanization of the support were first compared. These columns were thoroughly evaluated in order to study the influence of endcapping in the enantiorecognition features. Enantioseparations of twenty N-derivatized 2,4-dinitrophenyl α-amino acids (DNP-amino acids) were studied by changing mobile phase pH, buffer concentration, type of organic solvent in the mobile phase, and column temperature. Maximum retention factors were observed at pH ≈6, at this intermediate pH the tertiary amine of the quinine is protonated to a high degree and therefore available for strong electrostatic interactions with unprotonated anionic DNP-amino acids. The enantioselectivity factors, however, increased as the pH did in the range between 5 and 7. The increase in ionic strength had influence on retention, but not on enantioselectivity, allowing the use of this variable for optimization of retention factors. Finally, the thermodynamic transfer parameters of the enantiomers from the mobile to both CSPs (with and without endcapping, QN-CSP(EC) and QN-CSP, respectively) were estimated from van't Hoff plots within the range of 10-40 °C. Thus, the differences in the transfer enthalpy, Δ(ΔH°), and transfer entropy, Δ(ΔS°), enabled an investigation of the origin of the differences in interaction energies.     

The separation of the enantiomers of some potassium channel activators on α1-acid glycoprotein: the effect of solute conformationglycoprotein: the effect of solute conformation

Summary The amide conformers of two compounds and their enantiomers have been separated by liquid chromatography on an ₁-acid glycoprotein column. The effects of organic modifier and pH on the separations obtained were investigated. The conformers of one of the compounds were separated micro-preparatively at low temperature on the same column using a D₂O-based eluent; D₂O had no deleterious effects on the chromatography obtained. Some preliminary competition experiments on one of the amides and a close analogue are presented.     

Measurement and modeling of the equilibrium behavior of the Tröger's base enantiomers on an amylose-based chiral stationary phase

The binary isotherms of the two enantiomers of Tr?ger's base were measured on a system made of pure 2-propanol as the mobile phase and of Chiralpak AD, a chiral stationary phase (CSP) based on amylose tri-(3,5-dimethylphenyl carbamate). The experimental data were acquired using both frontal analysis and the perturbation method. The results obtained are most unusual. The adsorption of the more-retained (-)-enantiomer is not competitive: the amount adsorbed onto the CSP at equilibrium with a constant concentration of the (-)-enantiomer is independent of the concentration of the (+) enantiomer. On the other hand, the adsorption of the less-retained enantiomer is cooperative: the amount of this (+)-enantiomer adsorbed by the CSP at equilibrium with a constant concentration of this enantiomer increases with increasing concentration of the (-)-enantiomer. Such a phenomenon has hardly ever been reported. A model equation is proposed that accounts well for all these isotherm data.     

A β-cyclodextrin covalent organic framework used as a chiral stationary phase for chiral separation in gas chromatography

Chiral covalent organic frameworks(CCOFs) featuring chirality, stability, and good porosity have attracted a considerable amount of attention due to their important applications, such as asymmetric catalysis, chiral separation, and chiral recognition. In this study, a β-cyclodextrin(β-CD) covalent organic framework(β-CD-COF) diluted with polysiloxane OV-1701 was explored as a novel chiral stationary phase(CSP) for gas chromatography(GC) separation of racemates. The β-CD-COF coated capillary colu...     

Separation of chiral phosphorus compounds on the substituted P-cyclodextrin stationary phase in normal-phase liquid chromatog-raphy

The separation of enantiomers of a series of eighteen novel nitrogen mustard linked phosphoryl diamide derivatives was investigated on the prepared phenyl carbamate derivative β-cy-clodextrin bonded phase in normal-phase HPLC. Some of the enantiomers could be separated in baseline. The chiral recognition mechanism was also suggested for the separation of chiral phosphorus organic compounds.     

Enantioselective synthesis of α-amino acids in chiral reverse micelles

Through alkylation of ethyl 2-phthalimidoacetate in chiral reverse micelles formed from chiral surfactants, followed by hydrazinolysis and hydrolysis of the resulting products, optically active α-amino acids were synthesized. The highest enantioselectivity was 59.5%. Meanwhile, we have found that the asymmetric induction depends on the reaction temperature, the alkyl chain length of surfactant and the strucure of the surfactants.     

Optical isomer separation of flavanones and flavanone glycosides by nano-liquid chromatography using a phenyl-carbamate-propyl-β-cyclodextrin chiral stationary phase

In this paper a phenyl-carbamate-propyl-β-cyclodextrin stationary phase was employed for the enantioseparation of several flavonoids, including flavanones and methoxyflavanones by using nano-liquid chromatography (nano-LC). The same stationary phase was also used for the diastereoisomeric separation of two flavanone glycosides. The compounds: flavanone, 2′-hydroxyflavanone, 4′-hydroxyflavanone, 6-hydroxyflavanone, 7-hydroxyflavanone, 4′-methoxyflavanone, 6-methoxyflavanone, 7-methoxyflavanone, hesperetin, hesperidin, naringenin and naringin were studied using reversed, polar organic and normal elution modes. The effect of the nature and composition of the mobile phase (organic modifier type, buffer and water content in the reversed phase mode) on the enantioresolution (R_s), retention factor (k) and enantioselectivity (α) were investigated. Baseline resolution of all studied flavonoids, with the exception of 2′-hydroxyflavanone and naringin, was achieved in reversed phase mode using a mixture of MeOH/H₂O at different ratios as mobile phase. Good results, in terms of peak efficiency and short analysis time, were obtained adding 1% triethylammonium acetate pH 4.5 buffer to MeOH/H₂O mixture. The separation of the studied compounds was also performed in polar organic mode. By using 100% of MeOH as mobile phase, the resolution was achieved for the studied analytes, except for 7-hydroxyflavanone, 2′-hydroxyflavanone, naringenin, hesperidin and naringin. Normal mode was tested employing a mixture of EtOH/hexane/TFA as mobile phase achieving the enantiomeric and diastereomeric separation of only hesperetin and hesperidin, respectively. The use of nano-LC technique for the resolution of flavanones optical isomers allowed to achieve good resolutions in shorter analysis time compared to the results reported in literature with conventional HPLC.     

Enantioselective α-hydrazination of α-fluoro-β-ketoesters catalyzed by chiral nickel complexes

The catalytic enantioselective electrophilic α-hydrazination promoted by chiral nickel complexes is described. Treatment of α-fluoro-β-ketoesters with azodicarboxylates as electrophilic amination reagents under mild reaction conditions afforded the corresponding α-amino α-fluoro-β-ketoesters with high yields (80-96%) and enantioselectivities (up to 78% ee).     

Study of the capillary electrophoresis profile of intact α-1-acid glycoprotein isoforms as a biomarker of atherothrombosis

α-1-Acid glycoprotein (AGP) is a serum glycoprotein that presents several isoforms. Changes in the isoforms of AGP have been related to different pathological states including cardiovascular diseases (CVDs) such as acute myocardial infarction. However, to our knowledge, the role of variations of AGP isoforms as a potential biomarker of atherothrombosis has not been addressed. In this work, a preliminary study about differences in the capillary zone electrophoresis (CZE) profile of intact (non-hydrolyzed) AGP isoforms between healthy individuals and patients with atherothrombosis, specifically abdominal aortic aneurysm (AAA) and carotid atherosclerosis (CTA), has been performed. Biological samples (plasmas and sera) were analyzed by CZE after immunoaffinity chromatography purification. Up to 13 peaks corresponding to groups of isoforms of intact AGP from plasma samples were detected by CZE-UV. Electrophoretic profiles were aligned, peaks assigned, and linear discriminant analysis (LDA) of percentage of the corrected areas of AGP peaks was employed to discriminate and classify the CZE profiles of AGP samples. LDA enabled to accomplish 92.9% of correct classification of the AGP samples when the three groups of samples were considered. Besides, the LDA model showed high predictive power in the groups healthy vs. sick, healthy vs. AAA, and healthy vs. CTA. The described method was a successful approach to study the potential of AGP isoforms profile as a biomarker of atherothrombosis. To the best of our knowledge this has been the first time that a possible role of the CZE profile of intact AGP isoforms as a biomarker of vascular diseases has been demonstrated.     

Diastereo-enantioseparation of novel γ-lactamic derivatives on cellulose chiral stationary phases

Summary This paper describes the enantiorecognition of 1-methyl-3-hydroxy-5-aryl-2-pyrrolidinonic systems by high-performance liquid chromatography using two chiral derivatized cellulose stationary phases (CSPs) operated in the normal phase mode. According our results, the tris-(3,5-dimethylphenyl carbamate) cellulose, Chiralcel^? OD, is more suitable than the tris-(4-methyl-phenylbenzoate), Chiralcel^? OJ. On the first column, the resolution of the pyrrolidinonic compounds depends on the alcoholic modifier percentage. A possible solute-stationary phase recognition mechanism is discussed. The temperature and mobile phase composition have been considered to explain the different contribution for the enantiomeric resolution.     

Enantioselective reaction of α-lithiated thiazolidines as new chiral formyl anion equivalents

The reaction of lithiated N-Boc-thiazolidine and N-Boc-benzothiazolidine with benzophenone in the presence of (−)-sparteine afforded the products with up to 97% ee and 93% ee, respectively. The reaction with various aromatic and aliphatic aldehydes also afforded the products with high enantioselectivity and moderate diastereoselectivity. Each diastereomer could be converted to optically active diols. Consequently, lithiated N-Boc-thiazolidine and N-Boc-benzothiazolidine serve as chiral formyl anion equivalents. The reaction was confirmed to proceed through a dynamic thermodynamic resolution pathway.     

Enantioselective α-alkylation of unsaturated carboxylic acids using a chiral lithium amide

The regio- and stereochemistry of the alkylation of dienediolates from unsaturated carboxylic acids with benzylic halides, which often results in mixtures of isomers, can be controlled by means of changes in the lithium amide, allowing the α-regioisomer to be obtained as the major diastereoisomer. In addition, when chiral amines are used, moderate enantiomeric excesses can be attained.     

Enantioselective α-hydroxylation of β-keto esters catalyzed by chiral S-timolol derivatives

A screen of aryloxy aminopropanol organocatalysts derived from the β-blocker inhibitor S-timolol determined the most active catalyst of asymmetric α-hydroxylation of β-keto esters. (R)-1-(tert-butylamino)-3-(3,4,5-trimethoxyphenoxy) propan-2-ol (3k) was the most effective derivative, enantioselectively catalyzing α-hydroxylation of β-keto esters using tert-butyl hydroperoxide as the oxidant in hexane to afford the corresponding products in excellent yield and with good enantioselectivity (up to 96% yield, 88% ee).     

Preparation and chromatographic evaluation of a chiral stationary phase based on carboxymethyl-β-cyclodextrin for high-performance liquid chromatography

A novel chiral stationary phase (CSP) was prepared by chemically bonding carboxymethyl-β-cyclodextrin (CM-β-CD) onto 3-aminopropyl silica gel and showed excellent enantioseparation abilities for a broad range of chiral compounds and drugs.