Effective enantiomeric separations of racemic primary amines by the isopropyl carbamate-cyclofructan6 chiral stationary phase

A Real-Time Strategy for Chiroptical Sensing and Enantiomeric Excess Determination of Primary Amines via an Acid-Base Reaction

Two achiral aromatic carboxylic acids that included the 1,8-naphthalimide group and an imidazolium cation were synthesized and exploited as chiroptical sensors. These compounds showed the real-time discrimination and enantiomeric excess determination of chiral amines and amino alcohols via an acid-base interaction, especially for UV-silent chiral compounds.

Synthesis and application of N-3,5-dinitrobenzoyl and C3 symmetric diastereomeric chiral stationary phases

Three diastereomeric chiral compounds, namely, (R,R)-(+)-2-amino-1,2-diphenylethanol, (1S,2R)-(+)-2-amino-1,2-diphenylethanol, and (1R,2R)-(+)-1,2-diphenylethylenediamine were used as starting materials for preparing three N-3,5-dinitrobenzoyl derivative chiral stationary phases (CSPs) (CSP 1, 2, and 3) and three C₃ symmetric CSPs (CSP 4, 5, and 6). The six newly prepared CSPs were applied to the chiral separation of 44 chiral samples by HPLC. Most samples were isolated on CSP 6, with the highest average separation factor among the six newly prepared CSPs.

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 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.

Chiral resolution and absolute configuration determination of new metal-based photodynamic therapy antitumor agents

The advent of cisplatin as a cancer drug in the late 1960s generated considerable interest in the use of transition metal complexes as cancer therapy agents. Despite enhanced research in this area, there has yet to be any non-platinum-based transition metal complex cancer drugs approved by the Food and Drug Administration (FDA). Recently a Ru(II) metal-organic dyad (TLD1433) has provided promising results as a photodynamic therapy (PDT) agent for some types of cancer. This particularly effective PDT compound has an oligothiophene chain appended to an imidazophenanthroline ligand which chelates Ru(II). The entire complex is chiral and is synthesized as a racemate. Five such chiral Ru(II) and Os(II) PDT agents were synthesized and their enantiomers separated for the first time. The enantiomers of these compounds are not easily crystalized. However, preparative LC provided sufficient amounts of these novel PDT agents to determine their absolute configurations by vibrational circular dichroism (VCD). The synthesis, separation and absolute configuration determinations are described and discussed in detail.

Enantiomeric Separation of New Chiral Azole Compounds

Twelve new azole compounds were synthesized through an ene reaction involving methylidene heterocycles and phenylmaleimide, producing four oxazoles, five thiazoles, and one pyridine derivative, and ethyl glyoxylate for an oxazole and a thiazole compound. The twelve azoles have a stereogenic center in their structure. Hence, a method to separate the enantiomeric pairs, must be provided if any further study of chemical and pharmacological importance of these compounds is to be accomplished. Six chiral stationary phases were assayed: four were based on macrocyclic glycopeptide selectors and two on linear carbohydrates, i.e., derivatized maltodextrin and amylose. The enantiomers of the entire set of new chiral azole compounds were separated using three different mobile phase elution modes: normal phase, polar organic, and reversed phase. The most effective chiral stationary phase was the MaltoShell column, which was able to separate ten of the twelve compounds in one elution mode or another. Structural similarities in the newly synthesized oxazoles provided some insights into possible chiral recognition mechanisms.

Analysis of Enantiomers in Products of Food Interest

The separation of enantiomers has been started in the past and continues to be a topic of great interest in various fields of research, mainly because these compounds could be involved in biological processes such as, for example, those related to human health. Great attention has been devoted to studies for the analysis of enantiomers present in food products in order to assess authenticity and safety. The separation of these compounds can be carried out utilizing analytical techniques such as gas chromatography, high-performance liquid chromatography, supercritical fluid chromatography, and other methods. The separation is performed mainly employing chromatographic columns containing particles modified with chiral selectors (CS). Among the CS used, modified polysaccharides, glycopeptide antibiotics, and cyclodextrins are currently applied.

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.

HPLC separation of panthenol enantiomers on different types of chiral stationary phases

Panthenol is a biologically active compound closely related to vitamin B5 (pantothenic acid). This work deals with the separation of panthenol enantiomers using high performance liquid chromatography. Different types of chiral stationary phases (β-cyclodextrin, isopropyl carbamate cyclofructan 6, amylose tris(3,5-dimethylphenylcarbamate)) were tested in normal phase separation mode. Effect of mobile phase composition on the resolution and retention of enantiomers was studied. Two types of detectors, low-wavelength UV and polarimetric, were used. The optimal chromatographic system includes a chiral stationary phase based on amylose and a mobile phase of hexane/ethanol (60/40, v/v) where the resolution of enantiomers reached the value R s = 2.49. Suitable chromatographic conditions were applied for the determination of panthenol enantiomers in samples of pharmaceutical preparations with the obtained recovery of more than 92 %. Linearity of the high performance liquid chromatography method with spectrophotometric detection was from 1.0 × 10−3 to 1.3 mg mL−1 (R 2 = 0.998), with the limit of detection of 0.3 × 10−3 mg mL−1 for both enantiomers.

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.

Effective methodologies for enantiomeric separations of 150 pharmacology and toxicology related 1°, 2°, and 3° amines with core-shell chiral stationary phases

Core-shell particles (superficially porous particles, SPPs) have been proven to provide high-throughput and effective separations of a variety of chiral molecules. However, due to their limited commercialization, many separations have not been reported with these stationary phases. In this study, four SPP chiral stationary phases (CSPs) were utilized for the enantiomeric separation of 150 chiral amines. These amines encompass a variety of structural and drug classes, which are particularly important to the pharmaceutical industry and in forensics. This comprehensive evaluation demonstrates the power of these CSPs and the ease of method development and optimization. The CSPs used in this study included the macrocyclic glycopeptide-based CSPs (VancoShell and NicoShell), the cyclodextrin-based CSP (CDShell-RSP), and the cyclofructan-based CSP (LarihcShell-P). These CSPs offered versatility for a variety of applications and worked in a complementary fashion to baseline separate all 150 amines. The LarihcShell-P was highly effective for separating primary amines. VancoShell, NicoShell, and CDShell-RSP were useful for separating all types of amines. These CSPs are multi-modal and can be utilized with mass spectrometry compatible solvents. Eighteen racemic controlled substances were simultaneously baseline separated in a single liquid chromatography-mass spectrometry (LC-MS) analysis. Details in high-performance liquid chromatography (HPLC) parameters will be discussed as well as the improved chromatographic performance afforded by the SPP CSPs.

High performance affinity chromatography and related separation methods for the analysis of biological and pharmaceutical agents

The last few decades have witnessed the development of many high-performance separation methods that use biologically related binding agents. The combination of HPLC with these binding agents results in a technique known as high performance affinity chromatography (HPAC). This review will discuss the general principles of HPAC and related techniques, with an emphasis on their use for the analysis of biological compounds and pharmaceutical agents. Various types of binding agents for these methods will be considered, including antibodies, immunoglobulin-binding proteins, aptamers, enzymes, lectins, transport proteins, lipids, and carbohydrates. Formats that will be discussed for these methods range from the direct detection of an analyte to indirect detection based on chromatographic immunoassays, as well as schemes based on analyte extraction or depletion, post-column detection, and multi-column systems. The use of biological agents in HPLC for chiral separations will also be considered, along with the use of HPAC as a tool to screen or study biological interactions. Various examples will be presented to illustrate these approaches and their applications in fields such as biochemistry, clinical chemistry, and pharmaceutical research.

Enantioseparation of isoxazolines with functionalized perphenylcarbamate cyclodextrin clicked chiral stationary phases in HPLC

The enantioseparations of 12 isoxazoline racemates were explored with four perphenylcarbamate cyclodextrin (CD) clicked chiral stationary phases (CSPs) in high performance liquid chromatography (HPLC). The results demonstrated that the functionalities on phenylcarbamate moiety greatly determined the chiral separation ability of CD clicked CSPs. Among of them, per(3-chloro-4-methylphenylcarbamate) CD clicked CSP (CCC3M4-CSP) exhibited the best enantioseparation ability, affording 4ClPh-OPr with a chiral resolution over 20 in ternary eluent mobile phases. The optimization of CSPs structures provided wide platform for their chiral separations towards multi-mode HPLC.

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.

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.

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.

Gone in seconds: praxis, performance, and peculiarities of ultrafast chiral liquid chromatography with superficially porous particles

A variety of brush-type chiral stationary phases (CSPs) were developed using superficially porous particles (SPPs). Given their high efficiencies and relatively low back pressures, columns containing these particles were particularly advantageous for ultrafast "chiral" separations in the 4-40 s range. Further, they were used in all mobile phase modes and with high flow rates and pressures to separate over 60 pairs of enantiomers. When operating under these conditions, both instrumentation and column packing must be modified or optimized so as not to limit separation performance and quality. Further, frictional heating results in axial thermal gradients of up to 16 °C and radial temperature gradients up to 8 °C, which can produce interesting secondary effects in enantiomeric separations. It is shown that the kinetic behavior of various CSPs can differ from one another as much as they differ from the well-studied C18 reversed phase media. Three additional interesting aspects of this work are (a) the first kinetic evidence of two different chiral recognition mechanisms, (b) a demonstration of increased efficiencies at higher flow rates for specific separations, and

Separation of therapeutic peptides with cyclofructan and glycopeptide based columns in hydrophilic interaction liquid chromatography

Three cyclofructan-based, two glycopeptide-based, and one zwitterionic column used in the HILIC mode were assessed within a graphical framework based on different functional characteristics contributing to selectivity. The characteristics of these six HILIC columns are put in the perspective of 33 columns evaluated previously. The isopropyl carbamate modified cyclofructan 6 (CF6) stationary phase, Larihc P, showed reduced component contributions for hydrophilicity and hydrogen bonding relative to the native cyclofructan 6 column (Frulic N). Both Frulic N and Larihc P exhibited cation exchange attributed primarily to deprotonation of residual unsubstituted silica with the greater exchange ascribed to the reduced loading of CF6 observed for Larihc P. The cyclofructan 6 column with a polymeric styrene divinylbenzene support (MCI GEL™ CRS100) showed distinct selectivities consistent with its decreased cation exchange attributable to its nonionic core. The Chirobiotic T, Chirobiotic V, and ZI-DPPS columns displayed hydrophilicity and ion exchange selectivities similar to other zwitterionic stationary phases. All of the more hydrophilic columns showed excellent separation for the four classes of therapeutic peptides investigated: microbial secondary metabolites used as immune suppressants, synthetic gonadotropin hormones, synthetic cyclic disulfide-linked hormone-regulating hormones, and non-ribosomally derived polycyclic antibiotics. Resolution provided by these columns and ZIC-HILIC is compared for each class of peptide. Frulic N is primarily suitable for use in the HILIC mode whereas Chirobiotic T, because of its increased efficiency and selectivity, can be useful in both HILIC and reverse phase modes. In some Chirobiotic T applications, addition of low levels of a strong additive (trifluoroacetic acid, formic acid, etc.) to the mobile phase can be beneficial. In these peptide analyses, a relative weakening of the often-dominant ionic interaction between analyte and residual charge on the stationary phase improved resolution and selectivity.

PREPARATION AND EVALUATION OF HPLC CHIRAL STATIONARY PHASES BASED ON CATIONIC/BASIC DERIVATIVES OF CYCLOFRUCTAN 6

The cyclofructan 6 (CF6) macrocyclic-oligosaccharide was derivatized with five different substituents able to bear positive charges: propyl imidazole (IM) methyl benzimidazole (BIM), dimethyl aminopropyl (AP), pyridine (PY) and dimethyl aminophenyl (DMAP). The derivatized cyclofructans were reacted with triethoxysilyl-propylisocyanate as a linker to bond them to 5 μm spherical silica gel particles and then used to prepare HPLC columns. The bonded silica particles were analyzed to establish the bonding densities. A set of 34 chiral compounds including acids, neutral compounds and bases was tested with nine different mobile phase compositions including two reverse phase (RP) acetonitrile/pH 4 buffer, three polar organic (PO) acetonitrile/methanol and four normal phase (NP) heptane/ethanol mobile phases. No compounds could be separated in the RP mode. Eight compounds only could be enantioseparated in the PO mode and 21 compounds in the NP mode. The most effective chiral stationary phase was the propyl imidazole derivatized CF6 phase, provided that no more than six imidazole substituents and two linkers are attached per CF6 unit.

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.

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.

Superficially porous particles vs. fully porous particles for bonded high performance liquid chromatographic chiral stationary phases: isopropyl cyclofructan 6

This work reports a comparison of HPLC separations of enantiomers with chiral stationary phases (CSPs) prepared by chemically bonding cyclofructan-6, functionalized with isopropyl carbamate groups on fully and superficially porous particles (SPPs). The chromatographic performance of the superficially porous CSP based column was compared with columns packed with 5 μm and 3 μm fully porous particles (FPPs). At a flow rate of 3.0 mL/min the number of plates on column afforded by the SPP column was ∼7× greater than the number of plates on column (same length) obtained when using the 5 μm FPP based column. The flow rate providing the highest efficiency separation was ∼1.0 mL/min for the SPP column while it was ∼0.5 mL/min for both FPP columns. It was found that the selectivity and resolution of the separations were comparable between fully porous and superficially porous based columns (under constant mobile phase conditions), even though the SPP column contained lower absolute amounts of chiral selector. When tested under constant retention conditions, the SPP based CSP greatly improved resolution compared to the FPP based columns. At high flow rates the efficiency gained by using superficially porous CSP was accentuated. The advantages of columns based on SPPs become more obvious from the viewpoint of plate numbers and resolution per analysis time.

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.

Enantiomeric separations of illicit drugs and controlled substances using cyclofructan-based (LARIHC) and cyclobond I 2000 RSP HPLC chiral stationary phases

Recently a novel class of chiral stationary phases (CSPs) based on cyclofructan (CF) has been developed. Cyclofructans are cyclic oligosaccharides that possess a crown ether core and pendent fructofuranose moieties. Herein, we evaluate the applicability of these novel CSPs for the enantiomeric separation of chiral illicit drugs and controlled substances directly without any derivatization. A set of 20 racemic compounds were used to evaluate these columns including 8 primary amines, 5 secondary amines, and 7 tertiary amines. Of the new cyclofructan-based LARIHC columns, 14 enantiomeric separations were obtained including 7 baseline and 7 partial separations. The LARIHC CF6-P column proved to be the most useful in separating illicit drugs and controlled substances accounting for 11 of the 14 optimized separations. The polar organic mode containing small amounts of methanol in acetonitrile was the most useful solvent system for the LARIHC CF6-P CSP. Furthermore, the LARIHC CF7-DMP CSP proved to be valuable for the separation of the tested chiral drugs resulting in four of the optimized enantiomeric separations, whereas the CF6-RN did not yield any optimum separations. The broad selectivity of the LARIHC CF7-DMP CSP is evident as it separated primary, secondary and tertiary amine containing chiral drugs. The compounds that were partially or un-separated using the cyclofructan based columns were screened with a Cyclobond I 2000 RSP column. This CSP provided three baseline and six partial separations.