New methods for resolution of hydroxychloroquine by forming diastereomeric salt and adding chiral mobile phase agent on RP-HPLC.

Hydroxychloroquine (HCQ), 2-([4-([7-Chloro-4-quinolyl]amino)pentyl]ethylamino)ethanol, exhibited significant biological activity, while its side effects cannot be overlooked. The RP-HPLC enantio-separation was investigated for cost-effective and convenient optical purity analysis of HCQ. The thermodynamic resolution of Rac-HCQ, driven by enthalpy and entropy, was achieved on the C18 column using Carboxymethyl-ß-cyclodextrin (CM-ß-CD) as the chiral mobile phase agent (CMPA). The effects of CCM-ß-CD, pH, and triethylamine (TEA) V% on the enantio-separation process were explored. Under the optimum conditions at 24°C, the retention times for the two enantiomers were t R 1 = 29.39 min $$ {t}_{R1}=29.39\ \min $$ and t R 2 = 32.42 min $$ {t}_{R2}=32.42\ \min $$ , resulting in R s = 1.87 $$ {R}_s=1.87 $$ . The resolution via diastereomeric salt formation of Rac-HCQ was developed to obtain the active pharmaceutical ingredient of single enantiomer S-HCQ. Di-p-Anisoyl-L-Tartaric Acid (L-DATA) was proved effective as the resolution agent for Rac-HCQ. Surprisingly, it was found that refluxing time was a key fact affecting the resolution efficiency, which meant the kinetic dominate during the process of the resolution. Four factors-solvent volume, refluxing time, filtration temperature, and molar ratio-were optimized using the single-factor method and the response surface method. Two cubic models were established, and the reliability was subsequently verified. Under the optimal conditions, the less soluble salt of 2L-DATA:S-HCQ was obtained with a yield of 96.9% and optical purity of 63.0%. The optical purity of this less soluble salt increases to 99.0% with a yield of 74.2% after three rounds recrystallization.

Enantiomeric separation of tryptophan via novel chiral polyamide composite membrane.

The separation of chiral drugs continues to pose a significant challenge. However, in recent years, the emergence of membrane-based chiral separation has shown promising effectiveness due to its environmentally friendly, energy-efficient, and cost-effective characteristics. In this study, we prepared chiral composite membrane via interfacial polymerization (IP), utilizing ß-cyclodextrin (ß-CD) and piperazine (PIP) as mixed monomers in the aqueous phase. The chiral separation process was facilitated by ß-CD, serving as a chiral selective agent. The resulting membrane were characterized using SEM, FT-IR, and XPS. Subsequently, the chiral separation performance of the membrane for DL-tryptophan (Trp) was investigated. Lastly, the water flux, dye rejection, and stability of the membrane were also examined. The results showed that the optimized chiral PIP0.5ß-CD0.5 membrane achieved an enantiomeric excess percentage (ee%) of 43.0% for D-Trp, with a solute flux of 66.18 nmol·cm-2·h-1, and maintained a good chiral separation stability. Additionally, the membrane demonstrated positive performance in the selective separation of mixed dyes, allowing for steady operation over a long period of time. This study offers fresh insights into membrane-based chiral separations.

Stable and permeable polyion complex vesicles designed as enzymatic nanoreactors.

Polymeric vesicles are perspective vehicles for fabricating enzymatic nanoreactors towards diverse biomedical and catalytic applications, yet the design of stable and permeable vesicles remains challenging. Herein, we developed polyion complex (PIC) vesicles featuring high stability and a permeable membrane for adequate enzyme loading and activation. Our design relies on co-assembly of an anionic diblock copolymer (PSS96-b-PEO113) with cationic branched poly(ethylenimine) (PEI). The polymer combination endows strong electrostatic interaction between the PSS and PEI building blocks, so their assembly can be implemented at a high salt concentration (500 mM NaCl), under which the charge interaction of the enzyme-polymer is inhibited. This control realizes the successful and safe loading of enzymes associated with the formation of stable PIC vesicles with an intrinsic permeable membrane that is favourable for enhancing enzymatic activity. The control factors for vesicle formation and enzyme loading were investigated, and the general application of loading different enzymes for cascade reaction was validated as well. Our study reveals that proper design and combination of polyelectrolytes is a facile strategy for fabricating stable and permeable polymeric PIC vesicles, which exhibit clear advantages for loading and activating enzymes, consequently boosting their diverse applications as enzymatic nanoreactors.

Novel ethylenediamine-ß-cyclodextrin grafted membranes for the chiral separation of mandelic acid and its derivatives.

In the present study, flat cellulose acetate ultrafiltration membranes were prepared first by nonsolvent induced phase separation method. Then chiral membranes for separating the enantiomers were prepared by grafting the ultrafiltration membranes using ethylenediamine-ß-cyclodextrin as the chiral selector and epichlorohydrin as the spacer arm. The pure water permeability of the ultrafiltration membrane was around 115 L·m-2·h-1·bar-1. The properties of the chiral membranes were characterized using infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The chiral membrane performance in enantiomer separation was evaluated with racemates, such as mandelic acid (MA), 2-chloromandelic acid (2-ClMA), 4-chloromandelic acid (4-ClMA), and methyl mandelate (MM). The influence of feed concentration on the separation efficiency was also investigated. The results indicated that the enantiomeric excess percentages (e.e%) of the racemic mixtures for these four chiral compounds were up to 31.8%, 25.4%, 17.8%, and 32.6%, respectively. The binding free energy of the chiral selector with the (S)-enantiomer calculated by molecular dynamics simulations was stronger than that with the (R)-enantiomer, which was consistent with the experimental results (higher concentration of (R)-enantiomer in the permeate). This supports the affinity absorption-separation mechanism.

Resolution of trans-1,2-cyclohexanedicarboxylic acid containing two carboxylic groups by forming diastereomeric salts based on feeding molar ratio control.

To investigate the thermodynamic and molecular self-assembly mechanism of trans-1,2-cyclohexane dicarboxylic acid containing two carboxylic acid groups in the chiral resolution process, (S)-phenylethylamine was used as the chiral resolving agent. Two stoichiometric salts were formed when the raw materials were fed at different molar ratios: cyclohexane dicarboxylate monophenylethylamine salt and cyclohexane dicarboxylate diphenylethylamine salt. When the molar ratio of the (S)-phenylethylamine to trans-1,2-cyclohexane dicarboxylic acid was less than 3:1, trans-(1S,2S)-cyclohexane dicarboxylic acid was obtained with 97 e.e% purity. But when the molar ratio exceeded 3:1, the product was the racemic trans-(1,2)-cyclohexane dicarboxylic acid. In addition, single crystal structures of more soluble mono-salt, less soluble mono-salt, and less soluble di-salt were obtained. The weak intermolecular interactions and the way of the molecules packing in the crystals were analyzed. The hydrogen bond was stronger in the less soluble salt than that in the more soluble salt. And a "lock-and-key" structure in the hydrophobic layers makes it more tightly packed through the van der Waals interaction, which is responsible for the stability of less soluble salts.

Enantioseparation of basic drugs by reverse phase high-performance liquid chromatography system using carboxymethyl-ß-cyclodextrin as chiral mobile phase additive.

A rapid and efficient method was developed for enantioseparation of basic drugs, using carboxymethyl-ß-cyclodextrin (CM-ß-CD) as chiral mobile phase additive, rather than involving costly chiral column in high-performance liquid chromatography (HPLC) system. Four of the six basic drug enantiomers investigated were successfully separated. The highest resolution reaches 2.15 for threo-(1S,2S)-2-amino-l-p-nitrophenyl-1,3-propanediol. The effects of the organic modifier, pH value, concentration of chiral additive, column temperature, and flow rate of mobile phase on the enantioseparation of analytes were researched. The apparent formation constants of inclusion and the thermodynamic parameters were evaluated to explain the mechanism of chiral recognition.

Resolution of Halogenated Mandelic Acids through Enantiospecific Co-Crystallization with Levetiracetam.

The resolution of halogenated mandelic acids using levetiracetam (LEV) as a resolving agent via forming enantiospecific co-crystal was presented. Five halogenated mandelic acids, 2-chloromandelic acid (2-ClMA), 3-chloromandelic acid (3-ClMA), 4-chloromandelic acid (4-ClMA), 4-bromomandelic acid (4-BrMA), and 4-fluoromandelic acid (4-FMA), were selected as racemic compounds. The effects of the equilibrium time, molar ratio of the resolving agent to racemate, amount of solvent, and crystallization temperature on resolution performance were investigated. Under the optimal conditions, the resolution efficiency reached up to 94% and the enantiomeric excess (%e.e.) of (R)-3-chloromandelic acid was 63%e.e. All five halogenated mandelic acids of interest in this study can be successfully separated by LEV via forming enantiospecific co-crystal, but the resolution performance is significantly different. The results showed that LEV selectively co-crystallized with S enantiomers of 2-ClMA, 3-ClMA, 4-ClMA, and 4-BrMA, while it co-crystallized with R enantiomers of 4-FMA. This indicates that the position and type of substituents of racemic compounds not only affect the co-crystal configuration, but also greatly affect the efficiency of co-crystal resolution.

Diastereomeric resolution of 3-chloromandelic acid with threo-(1S,2S)-2-amino-l-p-nitrophenyl-1,3-propanediol.

An optical resolution of 3-chloromandelic acid (3-ClMA) using threo-(1S,2S)-2-amino-l-p-nitrophenyl-1,3-propanediol ([S,S]-SA) as a resolving agent was presented. The effects of the type of solvents, the amount of solvent, molar ratio of the resolving agent to racemate and filtration temperature on resolution were investigated. Under the optimal resolution conditions, the content of less soluble salt reached 98%, and the resolution efficiency was as high as 94%. The weak intermolecular interactions (such as hydrogen bond, halogen bond, CH/π and van der Waals interactions) and molecular packing mode in crystal structure of the less soluble salt (R)-3-ClMA(S,S)-SA were investigated. A wall-like 2-D hydrogen-bonding network and hydrophobic structure between hydrogen-bonding walls were revealed. (S,S)-SA was also used to resolve 2-ClMA and 4-ClMA respectively and the corresponding less soluble salts (R)-2-ClMA·(R,R)-SA and (R)-4-ClMA·(R,R)-SA were obtained using threo-(1R,2R)-2-amino-l-p-nitrophenyl-1,3-propanediol ((R,R)-SA) as a resolving agent. In addition, two other resolving agents, (R)-ɑ-phenethylamine ((R)-PEA) and (R)-N-benzyl phenethylamine ((R)-BPA) reported in the literature for the resolution of 3-ClMA were examined along with the newly proposed resolving agent, (S,S)-SA. The crystal structures of the resulting less soluble salts (R)-3-ClMA·(S,S)-SA, (R)-3-ClMA·(R)-PEA and (R)-3-ClMA·(R)-BPA were compared and examined.

Improved Resolution of 4-Chloromandelic Acid and the Effect of Chlorine Interactions Using (R)-(+)-Benzyl-1-Phenylethylamine as a Resolving Agent.

In order to avoid the disadvantage of commonly used resolving agent 1-phenylethylamine (hereafter: PEA), which is soluble in water, (R)-(+)-benzyl-1-phenylethylamine ((R)-(+)-BPA) was used to resolve 4-chloromandelic acid (4-ClMA) in this study. The optimal resolution conditions were determined: absolute ethanol as a solvent, the molar ratio of 4-ClMA to (R)-(+)-BPA as 1:1, the filtration temperature as 15 °C, and the amount of solvent as 1.6 mL/1 mmol 4-ClMA. Thermophysical properties, such as melting point, heat of fusion, and solubility, exhibited significant differences between the less and more soluble salts. The single crystals for the pair of diastereomeric salts were cultivated and their crystal structures were examined thoroughly. In addition to commonly observed interactions like hydrogen bonding and CH/π interactions. The chlorine…chlorine interaction was observed in the less soluble salt presenting as Cl…Cl between adjacent hydrogen network columns, while the Cl/π interaction was observed in the more soluble salt. It was found that halogen interactions played an important role in chiral recognition of 4-ClMA by (R)-(+)-BPA.

Enantioselective resolution of 4-chloromandelic acid by liquid-liquid extraction using 2-chloro-N-carbobenzyloxy-L-amino acid.

A liquid-liquid extraction resolution of 4-chloro-mandelic acid (4-ClMA) was studied by using 2-chloro-N-carbobenzyloxy-L-amino acid (2-Cl-Z-AA) as a chiral extractant. Important factors affecting the extraction efficiency were investigated, including the type of chiral extractant, pH value of aqueous phase, initial concentration of chiral extractant in organic phase, initial concentration of 4-ClMA in aqueous phase, and resolution temperature. It was observed that the concentration of (R)-4-ClMA was much higher than that of (S)-4-ClMA in organic phase due to a higher stability of the complex formed between (R)-4-ClMA and 2-Cl-Z-AA. A separation factor (α) of 3.05 was obtained at 0.02 mol/L 2-Cl-Z-Valine dissolved in dichloromethane, pH of 2.0, concentration of 4-ClMA of 0.11 mmol/Land T of 296.7K.

Enantioseparation of Citalopram by RP-HPLC, Using Sulfobutyl Ether-ß-Cyclodextrin as a Chiral Mobile Phase Additive.

Enantiomeric separation of citalopram (CIT) was developed using a reversed phase HPLC (RP-HPLC) with sulfobutylether-ß-cyclodextrin (SBE-ß-CD) as a chiral mobile phase additive. The effects of the pH value of aqueous buffer, concentration of chiral additive, composition of mobile phase, and column temperature on the enantioseparation of CIT were investigated on the Hedera ODS-2 C18 column (250 mm × 4.6 mm × 5.0 um). A satisfactory resolution was achieved at 25°C using a mobile phase consisting of a mixture of aqueous buffer (pH of 2.5, 5 mM sodium dihydrogen phosphate, and 12 mM SBE-ß-CD), methanol, and acetonitrile with a volumetric ratio of 21 : 3 : 1 and flow rate of 1.0 mL/min. This analytical method was evaluated by examining the precision (lower than 3.0%), linearity (regression coefficients close to 1), limit of detection (0.070 µg/mL for (R)-CIT and 0.076 µg/mL for (S)-CIT), and limit of quantitation (0.235 µg/mL for (R)-CIT and 0.254 µg/mL for (S)-CIT).

Crystal structure of (R)-N-benzyl-1-phenylethanaminium (R)-4-chloro-mandelate.

The absolute configuration of the title mol-ecular salt, C15H18N(+)·C8H6ClO3 (-), has been confirmed by resonant scattering. In the (R)-N-benzyl-1-phenyl-ethyl-ammonium cation, the phenyl rings are inclined to one another by 44.65 (7)°. In the crystal, the (R)-4-chloro-mandelate anions are linked via O-H⋯O hydrogen bonds and bridged by N-H⋯O hydrogen bonds involving the cations, forming chains along [010]. There are C-H⋯O hydrogen bonds present within the chains, which are linked via C-H⋯π inter-actions and a short Cl⋯Cl inter-action [3.193 (1) Å] forming a three-dimensional framework. The structure was refined as a two-component inversion twin giving a Flack parameter of 0.05 (4).

Enantioselective liquid-liquid extraction of zopiclone with mandelic acid ester derivatives.

Enantioselective liquid-liquid extraction of zopiclone was conducted by employing a series of (R)-mandelic acid esters as chiral extractants. The effects of concentration of extractant, concentration of zopiclone, type of organic solvent, pH value, and temperature on the extraction efficiency were investigated. (R)-o-chloromandelic acid propyl ester was demonstrated to be an efficient chiral extractant for zopiclone resolution with a maximum enantioselectivity of 1.6.

Resolution of 2-chloromandelic acid with (R)-(+)-N-benzyl-1-phenylethylamine: chiral discrimination mechanism.

During the resolution of 2-chloromandelic acid with (R)-(+)-N-benzyl-1-phenylethylamine, the crystals of the less soluble salt were grown, and their structure were determined and presented. The chiral discrimination mechanism was investigated by examining the weak intermolecular interactions (such as hydrogen bond, CH/π, and van der Waals interactions) and molecular packing mode in crystal structure of the less soluble diastereomeric salt. A one-dimensional double-chain hydrogen-bonding network and a "lock-and-key" supramolecular packing mode are disclosed. The investigation demonstrates that hydrophobic layers with corrugated surfaces can fit into the grooves of one another to realize a compact packing, when the molecular structure of resolving agent is much larger than that of the racemate. This "lock-and-key" assembly is recognized to be another characteristic of molecular packing contributing to the chiral discrimination, in addition to the well-known sandwich-like packing by hydrophobic layers with planar boundary surfaces.

Diastereomeric resolution of p-chloromandelic acid with (R)-phenylethylamine.

The optical resolution of p-chloromandelic acid using (R)-alpha-phenylethylamine as resolving agent was presented. The effect of solvents, molar ratio of racemate to the resolving agent, filtration temperature as well as the amount of solvent on resolution was investigated by orthogonal experimentation. The binary melting point phase diagram and crystal structure analysis of diastereomeric salts rationalized the success of the resolution.