Stereoselective separation of spiroindoline phytoalexins on R-naphthylethyl cyclofructan 6-based chiral stationary phase

Impact of cyclofructan derivatives as efficient chiral selector in chiral analysis: An overview

The development of chiral selectors for the separation and analysis of chiral molecules has been an evolving process happening over three decades, since the introduction of the first chiral stationary phase (CSP) in 1938. The main impetus for designing new chiral selectors is to get to most promising one which has a broad chiral recognition property, separation capability for a wide range of chiral analytes, and the cost-effective CSP, which is also a major concern. Today, we have more than 100 commercially available CSPs, and these are prepared by coating or immobilizing the classical chiral selectors on to the chromatographic support, normally, silica gel. The purpose of this review is to look at progress and the impact of cyclofructan derivatives, a novel chiral selector introduced recently, for performing chiral analysis.

Liquid chromatographic chiral recognition of phytoalexins on immobilized polysaccharides chiral stationary phases. Unusual temperature behavior

Three immobilized polysaccharide chiral stationary phases, Chiralpak IA, Chiralpak IB and Chiralpak IC, were used for the study of enantioseparation of 36 derivatives of natural indole phytoalexins, in most cases bioactive, including racemic spirobrassinin, 1-methoxyspirobrassinin and 1-methoxyspirobrassinol methyl ether. Almost all analytes were baseline resolved at least on two different polysaccharide columns in normal phase mode. The effects of mobile phase composition, the analyte structure and the column temperature on the retention and enantioseparation were investigated. Evaluation of the corresponding thermodynamic parameters using van´t Hoff plots (ln k versus 1/T) in the temperature range -15 to 50 °C indicated that separations were enthalpy controlled in most cases, but some entropy controlled separations were also observed. Moreover, unusual phenomenon, an increase retention with increasing temperature accompanied with increased resolution was observed on the Chiralpak IC column. The elution order of enantiomers was determined in some cases and reversed elution order was also observed.

Fast enantioseparation of indole phytoalexins in additive free supercritical fluid chromatography

A series of chiral indole phytoalexins with potential anticancer and antimicrobial activity were enantioseparated in supercritical fluid chromatography. Two polysaccharide-based chiral stationary phases composed of tris-(3,5-dimethylphenylcarbamate) derivatives of amylose or cellulose coated on 2.5 μm silica particles were successfully used. The influences of the polysaccharide backbone, co-solvent type and co-solvent content in the mobile phase on retention, enantioselectivity and enantioresolution of indole phytoalexins were investigated. Fast baseline separations were achieved for 26 from 27 tested compounds. Amylose-based chiral stationary phase provided higher number of baseline resolutions of the indole phytoalexins than the cellulose-based one. However, certain complementary enantioresolution results towards the studied compounds were observed between the investigated columns. The relationship between structure of the indole phytoalexins and their chromatographic behavior in supercritical fluid chromatography was discussed.

Cyclofructans as Chiral Selectors: An Overview

Cyclofructans are cyclic oligosaccharides made of β-2,1-linked fructofuranose units. They have been utilized as chiral selectors, usually after derivatization, with high-performance liquid chromatography (HPLC), gas chromatography (GC), capillary electrophoresis (CE), and supercritical fluid chromatography (SFC). The focus herein will be directed to their development and applications as chiral selectors in various chiral separation techniques. Discussion of their use in hydrophilic liquid interaction chromatography (HILIC) will be limited. Their use in liquid chromatography, especially their improvements with the use of superficially porous particles (SPPs) will be emphasized. Method parameters and future directions are also discussed.

Recent development trends for chiral stationary phases based on chitosan derivatives, cyclofructan derivatives and chiral porous materials in high performance liquid chromatography

The separation of enantiomers by chromatographic methods, such as gas chromatography, high-performance liquid chromatography and capillary electrochromatography, has become an increasingly significant challenge over the past few decades due to the demand of pharmaceutical, agrochemical, and food analysis. Among these chromatographic resolution methods, high-performance liquid chromatography based on chiral stationary phases has become the most popular and effective method used for the analytical and preparative separation of optically active compounds. This review mainly focuses on the recent development trends for novel chiral stationary phases based on chitosan derivatives, cyclofructan derivatives, and chiral porous materials that include metal-organic frameworks and covalent organic frameworks in high-performance liquid chromatography. The enantioseparation performance and chiral recognition mechanisms of these newly developed chiral selectors toward enantiomers are discussed in detail.

Enantiomeric separation of new phytoalexin analogs with cyclofructan chiral stationary phases in normal-phase mode

For the first time, three different derivatized cyclofructan chiral stationary phases were used for the direct high-performance liquid chromatographic enantiomeric separation of 11 new racemic analogs of a natural indole phytoalexin. This class of compounds is known to have significant antiproliferative activity and other potentially useful pharmacological properties. The effect of various experimental factors was investigated to optimize the separations in the normal-phase mode. It was found that the nature of polar modifier and additive in the mobile phase have significant impact on the enantioseparations. Better chiral recognition of analyzed compounds was achieved on (R)-naphthylethyl carbamate cyclofructan 6 than on isopropyl carbamate cyclofructan 6 and dimethylphenyl carbamate cyclofructan 7. The thermodynamic parameters showed that the chiral separation was enthalpy controlled in all cases.

Enantiomeric separations of ruthenium (II) polypyridyl complexes using HPLC with cyclofructan chiral stationary phases

The enantiomeric separation of 21 ruthenium (II) polypyridyl complexes was achieved with a novel class of cyclofructan-based chiral stationary phases (CSPs) in the polar organic mode. Aromatic derivatives on the chiral selectors proved to be essential for enantioselectivity. The R-napthylethyl carbamate functionalized cyclofructan 6 (LARIHC CF6-RN) column proved to be the most effective overall, while the dimethylphenyl carbamate cyclofructan 7 (LARIHC CF7-DMP) showed complementary selectivity. A combination of acid and base additives was necessary for optimal separations. The retention factor vs. acetonitrile/methanol ratio plot showed a U-shaped retention curve, indicating that different interactions take place at different polar organic solvent compositions. The separation results indicated that π-π interactions, steric effects, and hydrogen bonding contribute to the enantiomeric separation of ruthenium (II) polypyridyl complexes with cyclofructan chiral stationary phases in the polar organic mode.

Enantiomeric separation of biaryl atropisomers using cyclofructan based chiral stationary phases

Normal phase chiral HPLC methods are presented for the enantiomeric separation of 30 biaryl atropisomers including 18 new compounds recently produced via a novel synthetic approach. Three new cyclofructan based chiral stationary phases were evaluated. Separations were achieved for all but six analytes and the LARIHC™ CF6-P alone provided 15 baseline separations. Effects of polar modifiers and temperature effects also were studied. Apparent thermodynamic parameters were determined by van't Hoff plots. Preparative scale methods were developed and employed resulting in the first ever isolation of these novel atropisomers in their pure enantiomeric form. Insights into the mechanism of retention and chiral discrimination are presented.