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

Enantioseparation and molecular docking study of selected chiral pharmaceuticals on a commercialized phenylcarbamate-β-cyclodextrin column using polar organic mode

The chiral separation capability of Chiral-CD-Ph column, containing phenylcarbamate-β-cyclodextrin as the chiral selector in polar organic mode was investigated. A total of twenty-five compounds with different structures and acid-base properties were evaluated, and twenty of them were separated using acetonitrile or methanol as eluent. The effects of various chromatographic parameters, such as the type and proportion of organic modifier, flow rate, and column temperature were analyzed in detail in relation to chromatographic performance. A U-shape retention curve was observed when a mixture of acetonitrile and methanol was used as the eluent, indicating different types of interactions in different solvent mixtures. Van 't Hoff analysis was used for calculation of thermodynamic parameters which revealed that the enantioseparation is mainly enthalpy controlled; however, entropic control was also observed. The enantiomer recognition ability at the atomic level was also investigated through a molecular docking study, which revealed surface binding in polar organic mode instead of inclusion complexation. Our work proves that the phenylcarbamate-β-cyclodextrin-based chiral stationary phase can be effectively used in polar organic mode for the chiral separation of structurally diverse compounds. Furthermore, it is important to note that our study demonstrated that surface binding is responsible for the formation of supramolecular complexes in certain cyclodextrin derivatives.

Insights into enantioselective separations of ionic metal complexes by sub/supercritical fluid chromatography

Sub/supercritical fluid chromatography (SFC) is a green separation technique that has been used to separate a wide variety of compounds and is proven to be immensely useful for chiral separations. However, SFC is currently not thought to be applicable for ionic compounds due to their low solubility in CO2, even with additives and organic modifiers. Recently, a large amount of research has been centered on octahedral complexes of Ru(II) and Os(II) with bidentate polypyridyl ligands due to their ability to serve in cancer treatment and other biological activities. These compounds exist as the delta (Δ) and lambda (Λ) enantiomers. Previously, similar compounds have been enantiomerically separated using HPLC and capillary electrophoresis, but never with SFC. Cyclofructan-6 (CF6) derivatized with (R)-naphthyl ethyl (RN) groups has been proven to be an effective chiral stationary phase for these separations in HPLC. This column chemistry was expanded to SFC to provide the first chiral separation of a wide variety (23 complexes in total) of ionic octahedral polypyridyl complexes. Unexpected behavior for mixing methanol and acetonitrile as the organic modifier will be discussed, along with the effects of additives. Enantioselectivity on CF6-RN chemistry is shown to be dependent on the conjugation level and rigidity of the metal complexes. Mass transfer kinetic behavior is also shown, and high-efficiency baseline resolved rapid separations are shown for fast screening or quantitation of representative coordination complexes.

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.

Comparison of cyclofructan-, cyclodextrin-, and polysaccharide-based chiral stationary phases for the separation of pharmaceuticals

In this study, cyclofructan (CF)-, cyclodextrin (CD)-, and polysaccharide-based chiral stationary phases (CSPs) were exploited in high-performance liquid chromatography (HPLC) for the chiral separations of different clinically and pharmaceutically important compounds. In particular, R-naphthylethyl carbamate CF6 (RN-CF6), 3,5-dimethylphenyl carbamate CF7 (DMP-CF7), neutral beta cyclodextrin (β-CD), 3,5-dimethylphenyl carbamate β-CD (DMP-β-CD), and cellulose tris-(3,5-dimethylphenylcarbamate) (Cellulose-Tris DMP) columns were utilized under isocratic elution. The performance of these CSPs as chiral separation media was evaluated by use of nine analytes: acidic, basic, and amphiprotic. A possible correlation between the functional groups of these analytes and the chiral-recognition ability of each chiral column was also examined. The enantioseparations were optimized by varying different parameters, such as mobile phase additives, column temperature, and flow rate. Finally, a comparison was made between all CSPs, and it was expressed in terms of resolution (R_S), efficiency (N), selectivity (α), retention factors (k₁', k₂') and analysis time (t_R1, t_R2). It was observed that RN-CF6 was the most suitable and efficient CSP for the chiral separation of various types of analytes, including acids, primary and tertiary amines, alcohols, and many neutral compounds. It was the only CSP that provided baseline enantioseparation of thyroxine (R_S = 1.6) and cetirizine (R_S = 2.0).

Liquid Chromatographic Enantioseparations Utilizing Chiral Stationary Phases Based on Crown Ethers and Cyclofructans

Natural compounds can exist in different forms, where molecules possessing chirality play an essential role in living organisms. Currently, one of the most important tasks of modern analytical chemistry is the enantioseparation of chiral compounds, in particular, the enantiomers of compounds having biological and/or pharmaceutical activity. Whether the task is to analyze environmental or food samples or to develop an assay for drug control, well-reproducible, highly sensitive, stereoselective, and robust methods are required. High-performance liquid chromatography best meets these conditions. Nevertheless, in many cases, gas chromatography, supercritical fluid chromatography, or capillary electrophoresis can also offer a suitable solution. Amino acids, proteins, cyclodextrins, derivatized polysaccharides, macrocyclic glycopeptides, and ion exchangers can serve as efficient selectors in liquid chromatography, and they are quite frequently applied and reviewed. Crown ethers and cyclofructans possessing similar structural characteristics and selectivity in the enantiodiscrimination of different amine compounds are discussed less frequently. This review collects information on enantioseparations achieved recently with the use of chiral stationary phases based on crown ethers or cyclofructans, focusing on liquid chromatographic applications.

Advances in Chiral Separations at Nano Level

Background::

Nano level chiral separation is necessary and demanding in the development of the drug, genomic, proteomic, and other chemical and the environmental sciences. Few drugs exist in human body cells for some days at nano level concentrations, that are out of the jurisdiction of the detection by standard separation techniques. Likewise, the separation and identification of xenobiotics and other environmental contaminants (at nano or low levels) are necessary for our healthiness.

Discussion:

Conclusion:

This article will be beneficial for chiral chromatographers, academicians, pharmaceutical industries, environmental researchers and Government regulation authorities.

Triple Helical Ir(ppy)3 Phenylene Cage Prepared by Diol-Mediated Benzannulation: Synthesis, Resolution, Absolute Stereochemistry and Photophysical Properties

Cyclometalation of a triple helical N-doped phenylene cage prepared by ruthenium(0)-catalyzed diol-diene benzannulation delivers a chiral, conformationally constrained Ir(ppy)₃ analogue. Like the parent complex, fac-Ir(ppy)₃ , the iridium-containing PAH-cage is phosphorescent, but displays enhanced resistance to oxygen quenching.

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.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

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.

Determination of the interconversion energy barrier of three novel pentahelicene derivative enantiomers by dynamic high resolution liquid chromatography

Dynamic high resolution liquid chromatography (DHPLC) was used to determine the kinetic and thermodynamic activation parameters of interconversion of three novel pentahelicene derivatives {3,5-bis(trifluoromethyl)benzo[i]pentahelicene, naphtho[1,2-i]pentahelicene and 4-methoxybenzo[i]pentahelicene}. DHPLC was performed on a chiral isopropyl - carbamate cyclofructan 6 (LARIHC CF6-P) column under normal phase conditions. Variation of the column temperature and flow rate was used to study the interconversion process. A computer assisted deconvolution method was employed to determine the individual peak areas and the retention times required for the calculation of apparent enantiomerization energy barriers, enthalphy and entropy of the interconvertion of above defined pentahelicene derivative enantiomers. An ab initio quantum chemistry method was used to estimate theoretical kinetic and thermodynamic interconversion parameters and to evaluate experimental data of these three novel pentahelicene derivative enantiomers.

Screening primary racemic amines for enantioseparation by derivatized polysaccharide and cyclofructan columns

It is a challenge to separate the enantiomers of native chiral amines prone to deleterious silanol interactions. A set of 39 underivatized chiral primary amines was screened for enantiomeric separation. Seven recently introduced commercial chiral columns were tested. They included six polysaccharide based chiral stationary phases (CSP) with bonded derivatives, ChiralPak® IA, IB, IC, ID, IE and IF columns and a cyclofructan derivatized CSP, Larihc® CF6-P column. Both the normal phase (NP) mode with heptane/alcohol mobile phases and the polar organic (PO) mode with acetonitrile/alcohol were evaluated. It was found that the cyclofructan based CSP demonstrated the highest success rate in separating primary amines in the PO mode with only one chiral amine not resolved. It is shown that, when screening the columns, there is no standard optimal condition; an excellent mobile phase composition for one column may be poorly suited to another one. Although butylamine was a good mobile phase additive for the polysaccharide columns in both PO and NP modes, it was detrimental to the enantio-recognition capability of the cyclofructan column. Triethylamine was the appropriate silanol screening agent for this latter column.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.

Enantiomeric separations of α-aryl ketones with cyclofructan chiral stationary phases via high performance liquid chromatography and supercritical fluid chromatography

Normal phase chiral HPLC and SFC methods are presented for the enantiomeric separation of 21 α-aryl ketones with a unique class of chiral stationary phases (CSPs) based on cyclofructans (CFs). Separations were achieved for all but 2 analytes, with 17 compounds attaining baseline separation having resolution values up to 4.0. Most separations obtained in HPLC could be transferred to SFC, but the HPLC resolutions were generally better due to greater enantiomeric selectivity values. A structure-separation relationship (SSR) was developed to identify important structural features for separation of this class of compounds using CF-based CSPs. Preliminary studies are also presented that demonstrate the utility of the CF CSPs to investigate the base-induced enantiomerization of α-aryl ketones. It was demonstrated that even small amounts of base (0.01%v/v) in the mobile phase results in rapid, on-column, enantiomerization. Lastly, CSPs composed of superficially porous particles were used to achieve comparable separations of this class of chiral compounds, but at a fraction of the analysis time compared to CSPs composed of fully porous particles.