Chiral separations in normal-phase liquid chromatography: Enantioselectivity of recently commercialized polysaccharide-based selectors. Part II. Optimization of enantioselectivity

Variation of Activation Volume as an Indicator of the Difference in Clusterization Phenomenon Induced by H-Bonding and F-Π Stacking Interactions in Enantiomers and a Racemate of Flurbiprofen

In this paper, X-ray diffraction (XRD), differential scanning calorimetry (DSC), broadband dielectric (BDS), and Fourier transform infrared (FTIR) spectroscopy supported by molecular dynamics (MD) simulations and quantum chemical computations were applied to investigate the structural and thermal properties, molecular dynamics, and H-bonding pattern of R-, S-, and RS-flurbiprofen (FLP). Experimental data indicated various spatial molecular arrangements in crystalline forms of examined systems, which seemed to disappear in the liquid state. Surprisingly, deeper analysis of high-pressure dielectric data revealed unexpected variation in the activation volume of pure enantiomers and a racemate. MD simulations showed that it is an effect of the clusterization phenomenon and a higher population of small associates in the former samples. Moreover, theoretical consideration exposed the particular role of unspecific F-Π interactions as a driving force underlying local molecular arrangements of molecules in the liquid and the crystal lattice of R-, S-, and RS-FLP.

Experimental design optimization of simultaneous enantiomeric separation of atenolol and chlorthalidone binary mixture by high-performance liquid chromatography using polysaccharide-based stationary phases

In this work, enantiomeric separation of a drug combination of two chiral drugs, namely, atenolol and chlorthalidone, is described. Prior investigation of the effect of different variables on the resolution of the enantiomers' peaks and the total run time represented by the retention time of the last eluted peak was conducted using face-centered composite design. Twenty-two experiments were carried out by varying the chiral stationary phase type as a categorical factor and mobile phase composition including the percentage of ethanol and percentage of diethylamine as continuous factors. According to the optimization process, a mobile phase consisting of hexane:ethanol:DEA:TFA (60:40:0.2:0.1%, v/v/v/v) pumped at flow rate 1 ml min^-1 onto Lux-Cellulose 2 stationary phase was applied for the chiral separation and quantification of the drug combination at 230 nm. Application of the developed method to the pharmaceutical formulation of this combination was successfully performed, and satisfactory percentage of recoveries was obtained. The method was also fully validated following International Conference on Harmonization (ICH) guidelines. This method could be of high value and relevance for application in quality control laboratories.

Stereoselective HPLC separation of alvimopan on cellulose-based immobilized polysaccharide as a chiral stationary phase

Chiral separation by normal phase high performance liquid chromatography is one of the most powerful technique to quantify the chiral purity of the compounds. In this study, a novel, simple, and specific analytical method was proposed to ascertain the chiral purity of alvimopan (ALV). The normal phase HPLC method was developed based on cellulose tris (3,5-dichlorophenylcarbamate) stationary phase. The separation of ALV isomers achieved by using column CHIRALPAK IC (250 × 4.6 mm, 5 μm), mobile phase n-hexane: isopropyl alcohol: ethanol: diethylamine (650:200:150:5 v/v), column oven temperature 30°C, flow rate 1.0 mL min-1 , injection volume was 10 μL, chromatographic response monitored at 273 nm. The developed method was validated as per the ICH guidelines and found precise, accurate, and linear. The advantage of the method is a good separation of ALV isomers within 35 minutes of the analysis time. Therefore, this method is suitable for routine determination of chiral purity of ALV active pharmaceutical ingredient.

Liquid Chromatographic Separation of Novel 4-Amino-Flavanes Series Diastereomers on a Polysaccharide-Type Chiral Stationary Phase

Two broad approaches for the syntheses of a series of 4-aminoflavanes are used in this study, and they have been prepared in 30-99% overall yields using the reductive condensation of flavanone with primary amine, as a key step. By this methodology, the formyl derivatives of several secondary amines were obtained in good to excellent yields. The structures of all new products have been confirmed by spectral experiences (IR, ¹H NMR and ¹³C NMR). However, the present non-stereoselective synthesis results in a mixture of 2-7: diastereomers, which differ from the configuration of the flavanone atom asymmetric center. Since each diastereomer may have different biological activity and pharmacokinetic profile, analytical methods have to be developed for their separation. The 4-aminoflavanes diastereomers were separated using polysaccharide chiral stationary phases columns consisting of cellulose (Chiralcel^® OD-H and Chiralcel^®OJ) by high-performance liquid chromatography; the separation was affected by the nature and concentration of the alcohol modifiers in the mobile phase. Separations were carried out under normal phase mode on the Chiralcel^®OJ column. This method can properly separate the two diastereoisomers (Rs > 2) within an analysis time of <50 min.

High enantioselective Novozym 435-catalyzed esterification of (R,S)-flurbiprofen monitored with a chiral stationary phase

Lipases form Candida rugosa and Candida antarctica were tested for their application in the enzymatic kinetic resolution of (R,S)-flurbiprofen by enantioselective esterification. Successful chromatographic separation with well-resolved peaks of (R)- and (S)-flurbiprofen and their esters was achieved in one run on chiral stationary phases by high-performance liquid chromatography (HPLC). In this study screening of enzymes was performed, and Novozym 435 was selected as an optimal catalyst for obtaining products with high enantiopurity. Additionally, the influence of organic solvents (dichloromethane, dichloroethane, dichloropropane, and methyl tert-butyl ether), primary alcohols (methanol, ethanol, n-propanol, and n-butanol), reaction time, and temperature on the enantiomeric ratio and conversion was tested. The high values of enantiomeric ratio (E in the range of 51.3-90.5) of the esterification of (R,S)-flurbiprofen were obtained for all tested alcohols using Novozym 435, which have a great significance in the field of biotechnological synthesis of drugs. The optimal temperature range for the performed reactions was from 37 to 45 °C. As a result of the optimization, (R)-flurbiprofen methyl ester was obtained with a high optical purity, eep = 96.3 %, after 96 h of incubation. The enantiomeric ratio of the reaction was E = 90.5 and conversion was C = 35.7 %.

Chiral recognition of l-tryptophan with beta-cyclodextrin-modified biomimetic single nanochannel

We realized the chiral recognition of an essential amino acid with a biomimetic nanochannel system for the first time.

Synthesis of cellulose-2,3-bis(3,5-dimethylphenylcarbamate) in an ionic liquid and its chiral separation efficiency as stationary phase

A chiral selector of cellulose-2,3-bis(3,5-dimethylphenylcarbamate) (CBDMPC) was synthesized by reacting 3,5-dimethylphenyl isocyanate with microcrystalline cellulose dissolved in an ionic liquid of 1-allyl-3-methyl-imidazolium chloride (AMIMCl). The obtained chiral selector was effectively characterized by infrared spectroscopy, elemental analysis and 1H NMR. The selector was reacted with 3-aminopropylsilanized silica gel and the CBDMPC bonded chiral stationary phase (CSP) was obtained. Chromatographic evaluation of the prepared CSPs was conducted by high performance liquid chromatographic (HPLC) and baseline separation of three typical fungicides including hexaconazole, metalaxyl and myclobutanil was achieved using n-hexane/isopropanol as the mobile phase with a flow rate 1.0 mL/min. Experimental results also showed that AMIMCl could be recycled easily and reused in the preparation of CSPs as an effective reaction media.