(RS)-Propranolol: enantioseparation by HPLC using newly synthesized (S)-levofloxacin-based reagent, absolute configuration of diastereomers and recovery of native enantiomers by detagging

'Ab Ovo' Chiral Phases and Chiral Reagents for Liquid Chromatographic Separation and Isolation of Enantiomers

The de-novo approach of mixing chirally pure reagents or Cu(II)-L-amino acid complexes in the slurry of silica gel for preparing TLC plates was reported from author's laboratory and was successful for separation and isolation of enantiomers. Using high molar absorptivity molecules, e. g., 1,5-difluoro-2,4-dinitrobenzene and cyanuric chloride, more than 38 new chiral derivatizing reagents were synthesized in our laboratory by straightforward nucleophilic substitution with simple chiral auxiliaries. Besides, (S)-naproxen, (S)-ketoprofen, and (S)-levofloxacin were used as chiral platforms. A conceptual approach using both achiral phases in chromatography for enantioseparation was also adopted. ¹ H NMR and DFT based software were used to explain structures of non-covalent and covalent diastereomeric pairs and determination of configuration and separation mechanism. The methods can be easily used to determine and control enantiomeric purity with advantages over a variety of commercial chiral phases.

Micellar liquid chromatographic green enantioseparation of racemic amino alcohols and determination of elution order

A micellar liquid chromatographic method was developed for the green enantioseparation of racemic amino alcohols using an aqueous solution of the mixed surfactants as an alternative for organic solvents. In this study, the derivatives of the amino alcohols were synthesized using highly reactive chiral esters of (S)-levofloxacin (Lfx) under microwave conditions, and an aqueous solution of the surfactants (Brij-35 and SDS) was used for the enantioseparation of the synthesized diastereomeric derivatives (DDs) of amino alcohols using reversed-phase HPLC. The activated ester of Lfx was synthesized by reacting with N-hydroxybenzotriazole and characterized using UV, IR, ¹ H NMR, high-resolution mass spectrometry, and elemental analysis. The DDs of racemic amino alcohols were separated on a C₁₈ column using micellar LC. Chromatographic conditions were optimized by varying the concentration of the surfactants in aqueous solution and by varying the concentration and pH of the buffer. The green assessment score was calculated for the developed method (score: 82, an excellent green method). In addition, the density functional theory calculations were performed to develop the lowest energy-optimized structures of DDs. The method was validated according to the International Conference of Harmonization guidelines, and the retention factor (k), selectivity factor (α), resolution factor (R_S ), limit of detection (0.198 ng mL^-1 or 0.291 pM mL^-1 ), and limit of quantification (0.594 ng mL^-1 or 0.873 pM mL^-1 ) were calculated.

An efficient method for the determination of enantiomeric purity of racemic amino acids using micellar chromatography, a green approach

A simple, sensitive and green micellar liquid chromatographic method (RP-HPLC) was developed for enantioseparation of four racemic amino acids, namely, (RS)-selenomethionine, (RS)-methionine, (RS)-cysteine and (RS)-penicillamine. An aqueous solution of sodium dodecyl sulfate and Brij-35 was prepared and used as mobile phase for HPLC analysis. Activated esters of (S)-ibuprofen, (S)-ketoprofen and (S)-levofloxacin were synthesized by reacting them with N-hydroxybenzotriazole. These esters were characterized by UV, IR, ¹ HNMR, HRMS and elemental analysis. These chiral reagents (activated esters) were used for the synthesis of diastereomeric derivatives of the chosen amino acids. The diastereomeric derivatives were separated on a C₁₈ column by micellar liquid chromatography. Chromatographic conditions were optimized by varying concentration of surfactant in aqueous solution, and by varying the concentration and pH of the buffer. The green assessment score was calculated for the developed method (78, an excellent green method score). In addition, density functional theory calculations were performed, using Gaussian 09 rev. A.02 and hybrid density functional B3LYP with a 6-31G* basis set program, in order to develop lowest energy optimized structures of diastereomeric derivatives. The method was validated according to International Conference on Harmonization guidelines and the retention factor (k), selectivity factor (α), resolution factor (R_S ) and limit of detection (0.295 ng ml^-1 ) and limit of quantification (0.896 ng ml^-1 ) were calculated.

Differential potential ratiometric sensing platform for enantiorecognition of chiral drugs

A versatile, robust and efficient differential potential ratiometric sensing platform was developed for enantioselective recognition of dual chiral targets based on a composite membrane of molecularly imprinted polymers (MIPs) and reduced graphene oxide (rGO) modified glassy carbon electrode (GCE). The functional chitosan-based MIPs and rGO were compatibly immobilized on the GCE with high selectivity and efficient signal amplification. Moreover, via the systematic optimization of series conditions, a distinct potential difference (PD), reaching 135 mV, was obtained between the R-/S-prop based on the MIPs/rGO/GCE. In a controllable concentration range from 50 μM to 1000 μM, different ratios of R-/S-prop were linearly related to the peak potentials (Eps) in the racemic mixture. Using this low-cost reversible electrochemical platform, both Prop enantiomers were simultaneously identified with high repeatability and time-based stability. This novel semi-quantitative electrochemical sensing platform was established to rapidly quantify the ratio of S-/R-prop by Ep for the chiral drug recognition with great potential for practical applications in fields of pharmacological detection and clinical analysis.

A novel coating method for CE capillary using carboxymethyl-β-cyclodextrin-modified magnetic microparticles as stationary for electrochromatography enantioseparation

Magnetic microparticles (MMPs) have been extensively studied and aroused considerable interest in separation science owing to their superior characteristics. In this paper, a novel coated capillary with carboxymethyl-β-cyclodextrin-functionalized magnetic microparticles (CD-MMPs) as stationary phase was constructed and then applied to establish an open-tubular capillary electrochromatography enantioseparation system. The preparation of the CD-MMP-coated open-tubular column was very convenient because the coating of the magnetic microparticles onto the capillary column could be easily manipulated by an external magnetic field. The preparation conditions of the coated capillary such as magnetic field intensity and coating time are discussed in detail. The new constructed CD-MMP capillary system was applied to separate enantiomers of several racemic drugs. Compared to the uncoated capillary system, obviously preferable separations of tested enantiomers were obtained. Several important parameters affecting the enantioseparation, such as CM-β-CD concentration, running buffer pH, organic solvent, and applied voltage, were systematically optimized. Furthermore, satisfactory repeatability and chemical stability of this new CD-MMP capillary system were achieved in the experiment. Graphical abstract ᅟ.

Evaluation of the enantioselectivity of capillary electrokinetic chromatography using ethanediamine-bonded poly (glycidyl methacrylate) microspheres as the pseudostationary phases

In this work, a new capillary electrokinetic chromatography (EKC) approach using ethanediamine-bonded poly (glycidyl methacrylate) (Ami-PGMA) microspheres as pseudostationary phases (PSPs) for enantioseparation with a polysaccharide, chondroitin sulfate E (CSE), as the chiral selector. The CSE@Ami-PGMA EKC system was applied to enantioseparate basic drugs, and distinct improved separations of tested enantiomers were obtained while comparing with the single CSE system (the resolution increased from 0.41 to 1.26 for nefopam, from 1.24 to 2.15 for laudanosine, and from 0.92 to 2.36 for amlodipine). The Ami-PGMA microspheres were fully characterized by scanning electron microscopy (SEM) and Fourier Transform Infrared (FT-IR) spectroscopy, and the results showed Ami-PGMA microspheres were uniform and spherical in size (1 μm). Several principal parameters were systematically investigated, and the optimal chiral separations were obtained with Tris/H₃ PO₄ (20 mM, pH 2.4, and 3.4 for NEF) containing 2.5% (w/v) CSE and 20-μg Ami-PGMA microspheres in 20°C. Subsequently, the concentrations of Ami-PGMA microspheres and CSE were proved to be the dominant factors for the separation in the CSE@Ami-PGMA EKC system by Statistical Product and Service Solutions (SPSS).

RP-HPLC enantioseparation of β-adrenolytics using micellar mobile phase without organic solvents

Enantioseparation of a few commonly administered racemic β-adrenolytics (namely, carvedilol, betaxolol, salbutamol and bisoprolol) has been achieved using a water micellar mobile phase containing surfactants (sodium dodecyl sulphate and Brij-35) without organic solvents as a new approach in RP-HPLC. Two chiral derivatizing reagents based on enantiomerically pure (S)-(-)-levofloxacin were synthesized using N-hydroxysuccinimide and N-hydroxybenzotriazole as the activation auxiliaries. Diastereomeric derivatives of the chosen β-adrenolytics were synthesized under microwave irradiation in a very short reaction time. The (S)-(-)-levofloxacin moiety enhanced molar absorbance of the diastereomeric derivatives resulting in very low limit of detection (1.618 and 4.902 ng/mL, respectively, for diastereomeric derivatives of (RS)-betaxolol and better resolution with lower retention times (for all the analytes), in comparison to literature reports. There was 15-20 times less consumption of mobile phase because of lower retention time.

Synthesis of diastereomeric anhydrides of (RS)-ketorolac and (RS)-etodolac, semi-preparative HPLC enantioseparation, establishment of molecular asymmetry and recovery of pure enantiomers

Herein, enantioseparation of two anti-inflammatory drugs, namely, (RS)-ketorolac and (RS)-etodolac, commonly marketed and administered as racemates, was achieved by RP-HPLC.