Recent developments on polysaccharide-based chiral stationary phases for liquid-phase separation of enantiomers

Stereoselective determination of 2-benzamidomethyl-3-oxobutanoate and methyl-2-benzoylamide-3-hydroxybutanoate by chiral high-performance liquid chromatography in biotransformation

(2S, 3R)-methyl-2-benzamidomethyl-3-hydroxybutyrate (MBHB) is a key intermediate in the synthesis of 4-aceoxyazetidinone, a building block for the production of penems and carbapenems. More attentions have been paid to screen biocatalysts achieving asymmetric preparation of (2S, 3R)-MBHB. In this study, an improved chiral high-performance liquid chromatographic (HPLC) method was developed for the stereoselective determination of 2-benzamidomethyl-3-oxobutanoate (BMOB) and MBHB, and further employed into the biotransformation of BMOB. Chiral separation was achieved within 12 min on Chiralpak AY-H column, which was faster and more suitable for screening biocatalysts exhibited reduction activity and (2S, 3R)-stereospecificity toward BMOB than on other columns. Ultimately, a new strain, Burkholderia gladioli ZJB-12126 capable of reducing BMOB to (2S, 3R)-MBHB was successfully isolated based on this newly constructed HPLC method. Samples were prepared by liquid-liquid extraction system using ethyl acetate as the extractor solvent. The extraction recoveries of BMOB and MBHB isomers ranged from 91.6 to 94.1% with relative standard deviation (RSD) below 10%. Linear calibration curves were obtained in the concentration range of 50-5000 μg/mL for both BMOB and MBHB isomers, respectively. Intra-day and inter-day precisions and accuracy were below 15% for all isomers evaluated by RSDs and relative errors (REs), respectively. This novel method was demonstrated to be suitable for assessing the biotransformation process of BMOB.

Enantiomer separation of propranolol and tryptophan using bovine serum albumin functionalized silica nanoparticles as adsorbents

The immobilization of popular chiral selectors on the surface of nanomaterials to prepare new chiral adsorbents for preparative chiral separation is a research hotspot in separation science nowadays.

Liquid chromatographic resolution of fendiline and its analogues on a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid

Fendiline, an effective anti-anginal drug for the treatment of coronary heart diseases, and its sixteen analogues were resolved on a CSP based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. Fendiline was resolved quite well with the separation factor (α) of 1.25 and resolution (RS) of 1.55 when a mobile phase consisting of methanol-acetonitrile-trifluoroacetic acid-triethylamine at a ratio of 80/20/0.1/0.5 (v/v/v/v) was used. The comparison of the chromatographic behaviors for the resolution of fendiline and its analogues indicated that the 3,3-diphenylpropyl group bonded to the secondary amino group of fendiline is important in the chiral recognition and the difference in the steric bulkiness between the phenyl group and the methyl group at the chiral center of fendiline is also important in the chiral recognition.

High-performance liquid chromatography enantioseparation of polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases under multimodal elution

An investigation on the high-performance liquid chromatography enantioseparation of 12 polyhalogenated 4,4'-bipyridines on polysaccharide-based chiral stationary phases is described. The overall study was directed toward the generation of efficient separations in order to obtain pure atropisomers that will serve as ligands for building homochiral metal organic frameworks. Four coated columns--namely, Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-4, and Lux Amylose-2--and two immobilized columns--namely, Chiralpak IC and IA--were used under normal, polar organic, and reversed-phase elution modes. Moreover, Chiralcel OJ was considered under normal-phase and polar organic conditions. The effect of the chiral selector and mobile phase composition on the enantioseparation, the enantiomer elution order and the beneficial effect of nonstandard solvents were studied. The effect of water in the mobile phase on the enantioselectivity and retention was investigated and retention profiles typical of hydrophilic interaction liquid chromatography were observed. Interesting phenomena of solvent-induced enantiomer elution order reversal occurred under normal-phase mode. All the considered 4,4'-bipyridines were enantioseparated at the multimilligram level.

Enantioresolution, stereochemical characterization and biological activity of a chiral large-conductance calcium-activated potassium channel opener

A number of large-conductance calcium-activated potassium (BK) channel openers based on the 2-aryl-1,4-benzothiazine scaffold were previously synthesized, and 2-(5-bromo-2-methoxyphenyl)-6-trifluoromethyl-2H-1,4-benzothiazin-3(4H)-one (1) was identified as the most active compound. Since a stereoselective activation of BK channels was demonstrated for arylindolone derivatives, the effect of the absolute configuration at the C-2 position on the vasorelaxing potency of 2-aryl-1,4-benzothiazines is investigated in this article. Compound 1 was initially evaluated as a racemate: subsequently, the "racemic approach" was used to isolate its enantiomers. The excellent enantioresolution obtained using the Sepapak-4 column (CSP 4, cellulose tris(4-chloro-3-metylphenylcarbamate); RS=8.36; α=2.03) allowed to collect highly pure enantiomeric fractions, with enantiomeric excess (e.e.) values higher than 97% and 98% for the first- and second-eluted enantiomer, respectively. Electronic circular dichroism (ECD) studies on the two isolated enantiomers, combined with time-dependent density functional theory (TD-DFT) calculations allowed to characterize the configuration of the enantiomers and determine a (R), (S) elution order. Results from biological assays indicated that the racemate and the isolated enantiomers are endowed with comparable vasorelaxing potency.

Direct chromatographic enantioresolution of fully constrained β-amino acids: exploring the use of high-molecular weight chiral selectors

To the best of our knowledge enantioselective chromatographic protocols on β-amino acids with polysaccharide-based chiral stationary phases (CSPs) have not yet appeared in the literature. Therefore, the primary objective of this work was the development of chromatographic methods based on the use of an amylose derivative CSP (Lux Amylose-2), enabling the direct normal-phase (NP) enantioresolution of four fully constrained β-amino acids. Also, the results obtained with the glycopeptide-type Chirobiotic T column employed in the usual polar-ionic (PI) mode of elution are compared with those achieved with the polysaccharide-based phase. The Lux Amylose-2 column, in combination with alkyl sulfonic acid containing NP eluent systems, prevailed over the Chirobiotic T one, when used under the PI mode of elution, and hence can be considered as the elective choice for the enantioseparation of this class of rigid β-amino acids. Moreover, the extraordinarily high α (up to 4.60) and R S (up to 10.60) values provided by the polysaccharidic polymer, especially when used with camphor sulfonic acid containing eluent systems, make it also suitable for preparative-scale enantioisolations.

Cationic β-cyclodextrin-modified hybrid magnetic microspheres as chiral selectors for selective chiral absorption of dansyl amino acids

Chiral selective magnetic microspheres based on imidazolium cationic-modified β-CD derivatives have been developed for direct enantioseparation of dansyl amino acids.