Chiral amorphous metal–organic polyhedra used as the stationary phase for high-resolution gas chromatography separations

Herein, we describe a new chiral amorphous metal–organic polyhedra used as the stationary phase for high-resolution gas chromatography (GC). The chiral stationary phase was coated onto a capillary column via a dynamic coating process and investigated for a variety of compounds. The experimental results showed that the chiral stationary phase exhibits good selectivity for linear alkanes, linear alcohols, polycyclic aromatic hydrocarbons, isomers, and chiral compounds. In addition, the column has the advantages of high column efficiency and short analysis time. The present work indicated that amorphous metal–organic polyhedra have great potential for application as a new type of stationary phase for GC.     

Benzoyl cellulose beads in the pure polymeric form as a new powerful sorbent for the chromatographic resolution of racemates

Cellulose-based stationary phases are known to be very efficient and versatile chiral sorbents for the chromatographic resolution of racemates. Except for microcrystalline cellulose triacetate (CTA I), basically all other cellulose-based phases have been prepared by coating of ca. 20% weight polymer on a wide pore silica gel used as a carrier. In this work we describe the preparation of benzoylcellulose (TBC) beads in the pure polymeric form (without inorganic carrier) from an emulsion of the organic polymer. The new material has been fully characterized and used as a chiral stationary phase for the resolution of various classes of racemic compounds such as benzylic alcohols or acetate derivatives of aliphatic alcohols and diols. The structural variety of the separated solutes as well as the irrational influence of the aromatic substituent in different classes of aryl compounds suggest that multiple interaction sites are involved in the complexation, making a prediction of the separation difficult. The benzoyl cellulose beads exhibit a very high loading capacity, which is particularly useful for preparative purposes as demonstrated for selected examples.     

Influence of uncharged mobile phase additives,pH, and structure differences on retention and enantioselectivity of chiral hexa- and octa-hydrobenzo(g) quinolines on an ovomucoid glycoprotein column

The enantiomeric resolution of six closely related hexa- and octa-hydrobenzo(g) quinoline racemates by HPLC on an ovomucoid column (Ultron ES-OVM) is described. First, the influence of uncharged mobile phase additives (with different hydrogen bonding properties) and pH on retention and enantioselectivity is investigated. It is shown that an optimum pH of around 6 for the given separations is much more important than the choice of modifier. Second, the influence on enantioselectivity of small differences in molecular structure is examined. In one case a large difference is obtained for diastereomeric racemates depending on the cis- or trans-configuration of the quinoline skeleton. Higher flexibility of the solute seen in the cis enantiomers seems to improve enantioselectivity. © 1993 Wiley-Liss, Inc.     

A 3D Homochiral MOF [Cd2(d‐cam)3]•2Hdma•4dma for HPLC Chromatographic Enantioseparation

Up to now, some chiral metal‐organic frameworks (MOFs) have been reported for enantioseparation in liquid chromatography. Here we report a homochiral MOF, [Cd2(d‐cam)3]·2Hdma·4dma, used as a new chiral stationary phase for high‐performance liquid chromatographic enantioseparation. Nine racemates of alcohol, naphthol, ketone, and base compounds were used as analytes for evaluating the separation properties of the chiral MOF packed column. Moreover, some effects such as mobile phase composition, column temperature, and analytes mass for separations on this chiral column also were investigated. The relative standard deviations for the resolution values of run‐to‐run and column‐to‐column were less than 2.1% and 3.2%, respectively. The experimental results indicate that the homochiral MOF offered good recognition ability, which promotes the application of chiral MOFs use as stationary phase for enantioseparation. Chirality 28:340–346, 2016. © 2016 Wiley Periodicals, Inc.     

HPLC enantioseparation on cellulose tris(3,5-dimethylphenylcarbamate) as a chiral stationary phase: Influences of pore size of silica gel,coating amount,coating solvent,and column temperature on chiral discrimination

Cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC) was coated on large-pore silica gels and used as a chiral stationary phase (CSP) for high-performance liquid chromatographic separation of enantiomers. The influences of pore size of silica gel, coating amount of CDMPC, coating solvent, and column temperature on chiral discrimination were investigated. CSPs prepared with a large-pore silica gel having a small surface area showed higher chiral recognition. The amount of CDMPC adsorbed on the silica gel influenced the chiral recognition of some racemates. Loading capacity of racemates increased with an increase of the amount of CDMPC supported on the silica gel, and a CSP coated with 45% CDMPC by weight can be used for both analytical and semi-preparative scale separations. The CDMPC, coated using acetone as the coating solvent, exhibited, in many cases, higher enantioselectivity than that obtained with tetrahydrofuran F as the coating solvent. © 1996 Wiley-Liss, Inc.     

Direct HPLC enantioseparation of chiral aptazepine derivatives on coated and immobilized polysaccharide-based chiral stationary phases

The direct HPLC enantioseparation of Mianserin and a series of aptazepine derivatives is accomplished on polysaccharide-based chiral stationary phases (CSPs). The resolutions are performed on the coated-type Chiralcel OD and Chiralpak AD CSPs and on the first commercially available immobilized-type Chiralpak IA CSP, in normal-phase and polar-organic modes. The complete separation of enantiomers of all racemates investigated was successfully achieved under at least one of CSP/eluent combinations employed. Pure alcohols such ethanol or 2-propanol, with a fixed percentage of DEA added, serve as valuable alternatives to the more common n-hexane-based normal-phase eluents in resolution of Mianserin on the AD CSP. In order to study the chiroptical properties of aptazepine derivatives, chromatographic resolutions are carried out at semipreparative scale using Chiralpak AD and Chiralpak IA as CSPs. Nonconventional dichloromethane-based eluents have permitted to expand the chiral resolving ability of the immobilized Chiralpak IA CSP and to perform mg-scale enantioseparations with an analytical-size column. Assignment of the absolute configuration of the separated enantiomers is empirically established by comparing their chiroptical data with those of structurally related Mianserin.     

Application of lipases in kinetic resolution of racemates

Lipases have been well established as valuable catalysts in organic synthesis. This review article focuses on some of the recent developments in the rapidly growing field of lipase-catalyzed kinetic resolution of racemates as a versatile method for the separation of enantiomers. The literature search dates back to the last five years and covers some comprehensive examples. The main emphasis is on the use of lipases in organic solvents.     

Evaluation of dalbavancin as chiral selector for HPLC and comparison with teicoplanin‐based chiral stationary phases

Dalbavancin is a new compound of the macrocyclic glycopeptide family. It was covalently linked to 5 μm silica particles using two different binding chemistries. Approximately 250 racemates including (a) heterocyclic compounds, (b) chiral acids, (c) chiral amines, (d) chiral alcohols, (e) chiral sulfoxides and sulfilimines, (f) amino acids and amino acid derivatives, and (g) other chiral compounds were tested on the two new chiral stationary phases (CSPs) using three different mobile phases. As dalbavancin is structurally related to teicoplanin, the same set of chiral compounds was screened on two commercially available teicoplanin CSPs for comparison. The dalbavancin CSPs were able to separate some enantiomers that were not separated by the teicoplanin CSPs and also showed improved separations for many racemates. However, there were other compounds only separated or better separated on teicoplanin CSPs. Therefore, the dalbavancin CSPs are complementary to the teicoplanin CSPs. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Chiral investigation of midodrine, a long-acting alpha-adrenergic stimulating agent

Midodrine hydrochloride is a peripheral alpha(1)-adrenoreceptor agonist that induces venous and arterial vasoconstriction. Midodrine, after oral or intravenous administration, undergoes enzymatic hydrolysis and releases deglymidodrine, a pharmacologically active metabolite. Midodrine and deglymidodrine have a chiral carbon in the 2-position. To investigate the bioactivity of racemates and enantiomers of the drug and metabolite, three chromatographic chiral stationary phases, Chiralcel OD-H, Chiralcel OD-R, and alpha(1)-AGP, were evaluated for enantiomeric resolution. Good enantioseparation of midodrine racemate was obtained using the Chiralcel OD-H column. This stationary phase was then used to collect separately the midodrine enantiomers. By alkaline hydrolysis of rac-midodrine and each separated enantiomer, rac-deglymidodrine and its enantiomers were prepared. The control of the enantiomeric purity was carried out by alpha(1)-AGP stationary phase, while the hydrolysis of rac-midodrine and its enantiomers was controlled by capillary electrophoresis using trimethyl-beta-cyclodextrin as chiral selector. The pharmacological activity of the two racemates and the two enantiomeric pairs was tested in vitro on a strip of rabbit descending thoracic aorta. The tests continued that the activity of the drug and metabolite is due only to the (-)-enantiomer because neither of the (+)-enantiomers is active.     

Enantiomeric separation of imidazolinone herbicides using chiral high-performance liquid chromatography

Chiral high-performance liquid chromatography (HPLC) is one of the most powerful tools to prepare enantiopure standards of chiral compounds. In this study, the enantiomeric separation of imidazolinone herbicides, i.e., imazethapyr, imazapyr, and imazaquin, was investigated using chiral HPLC. The enantioselectivity of Chiralpak AS, Chiralpak AD, Chiralcel OD, and Chiralcel OJ columns for the three analytes was compared under similar chromatographic conditions. Chiralcel OJ column showed the best chiral resolving capacity among the test columns. The resolved enantiomers were distinguished by their signs of circular dichroism detected at 275 nm and their structures confirmed with LC-mass spectrometric analysis. Factors affecting the chiral separation of imidazolinones on Chiralcel OJ column were characterized. Ethanol acted as a better polar modifier than the other alcohols including 2-propanol, 1-butanol, and 1-pentanol. Although the acidic modifier in the mobile phase did not influence chiral recognition, it was necessary for reducing the retention time of enantiomers and suppressing their peak tailing. Thermodynamic evaluation suggests that enantiomeric separation of imidazolinones on Chiralcel OJ column is an enthalpy-driven process from 10 to 40 degrees C. This study also shows that small amounts of pure enantiomers of imidazolinones may be obtained by using the analytical chiral HPLC approach.     

Influence of the nature and substitution of chiral 2,3-epoxy alcohol derivatives on the enantiomeric elution order on chiralcel OD column

A number of 2,3-epoxy alcohol derivatives (1–16), obtained either as racemates or through the Sharpless asymmetric epoxidation reaction, were studied on a Chiralcel OD column. Nearly all compounds exhibit good enantioselective resolution on this chiral support. The order of elution of enantiomers is reversed between nerol and geraniol compounds. For 2,3-epoxy alcohols bearing a remote alkoxy (or silyloxy) group, the order of the enantiomeric elution alternates with the number n (n = 1–3) of methylenic groups present between the epoxide ring and the terminal OR (R = p − BrBn or OSitBuPh₂) functionality. In the case of trans 2,3-epoxy alcohols for the same number n, the order of elution is reversed when changing the terminal group −OSi to −OR. The latter group greatly improves the separation of the two enantiomers. Chirality 10:804–807, 1998. © 1998 Wiley-Liss, Inc.     

Enantiomeric separation of mineralocorticoid receptor (hMR) antagonists using the Chiralcel OJ-H HPLC column with novel polar cosolvent eluent systems

This study demonstrates the increased versatility of the Chiralcel OJ-H stationary phase when using various alcohol/acetonitrile mobile phases. This chiral stationary phase has traditionally been employed in the normal phase mode and more recently with neat alcohols as eluents. Selected isomeric human mineralocorticoid receptor (hMR) antagonist pharmaceutical candidates and synthetic intermediates were separated using the Chiralcel OJ-H HPLC column with novel polar cosolvent eluent systems. The capacity factors, resolution, and selectivity of the chiral separations were assessed while varying the alcohol/acetonitrile composition and alcohol identity. The mixed polar eluents provide separations that are nearly always superior to both the traditional hexane-rich and single-alcohol "polar organic" eluents for the compounds tested in this article.     

Excavations in drug chirality: 1. Cyclothiazide

There is a great deal of current interest in the role and importance of chirality in the development of new drugs, but little attention is being paid to the stereochemistry of older drugs. Indeed, many older chiral drugs were introduced without adequate information on their stereochemical identity or composition. We have examined one such drug, the antihypertensive diuretic agent cyclothiazide. Standard sources of drug information and the research literature do not provide data on the stereochemical composition of clinically used cyclothiazide, although scattered reports indicate that the drug may consist of "several stereoisomers." Inspection of the chemical structure of the drug, 6-chloro-3,4-dihydro-3-(5-norbornen-2-yl)-2H-1,2,4-benzothiadiazin e-7- sulfonamide 1,1-dioxide, shows that it can exist as eight stereoisomers that may form four racemates. Using synthesis, fast-atom-bombardment mass spectrometry, gas-liquid chromatography, chiral and nonchiral high-performance liquid chromatography, and nuclear magnetic resonance spectroscopy, we determined that pharmaceutical cyclothiazide is in fact a mixture of the eight stereoisomers in the form of the four racemates. The two racemates with endo configuration at the norbornene moiety predominate over the exo racemates, and small but significant differences in isomer distribution between different batches of the drug were observed. We urge that in studies of older drugs the stereochemical details be considered.     

Predictability of Enantiomeric Chromatographic Behavior on Various Chiral Stationary Phases Using Typical Reversed Phase Modeling Software

Pharmaceutical companies worldwide tend to apply chiral chromatographic separation techniques in their mass production strategy rather than asymmetric synthesis. The present work aims to investigate the predictability of chromatographic behavior of enantiomers using DryLab HPLC method development software, which is typically used to predict the effect of changing various chromatographic parameters on resolution in the reversed phase mode. Three different types of chiral stationary phases were tested for predictability: macrocyclic antibiotics‐based columns (Chirobiotic V and T), polysaccharide‐based chiral column (Chiralpak AD‐RH), and protein‐based chiral column (Ultron ES‐OVM). Preliminary basic runs were implemented, then exported to DryLab after peak tracking was accomplished. Prediction of the effect of % organic mobile phase on separation was possible for separations on Chirobiotic V for several probes: racemic propranolol with 97.80% accuracy; mixture of racemates of propranolol and terbutaline sulphate, as well as, racemates of propranolol and salbutamol sulphate with average 90.46% accuracy for the effect of percent organic mobile phase and average 98.39% for the effect of pH; and racemic warfarin with 93.45% accuracy for the effect of percent organic mobile phase and average 99.64% for the effect of pH. It can be concluded that Chirobiotic V reversed phase retention mechanism follows the solvophobic theory. Chirality 25:506–513, 2013. © 2013 Wiley Periodicals, Inc.     

Advances in asymmetric oxidative kinetic resolution of racemic secondary alcohols catalyzed by chiral Mn(III) salen complexes

Enantiomerically pure secondary alcohols are essential compounds in organic synthesis and are used as chiral auxiliaries and synthetic intermediates in the pharmaceutical, agrochemical, and fine chemical industries. One of the attractive and practical approaches to achieving optically pure secondary alcohols is oxidative kinetic resolution of racemic secondary alcohols using chiral Mn(III) salen complexes. In the last decade, several chiral Mn(III) salen complexes have been reported with excellent enantioselectivity and activity in the homogeneous and heterogeneous catalysis of the oxidative kinetic resolution of racemic secondary alcohols. This review article is an overview of the literature on the recent development of chiral Mn(III) salen complexes for oxidative kinetic resolution of racemic secondary alcohols. The catalytic activity of monomeric, dimeric, macrocyclic, polymeric, and silica/resin supported chiral Mn(III) salen complexes is discussed in detail.     

HPLC with cellulose Tris (3,5‐DimethylPhenylcarbamate) chiral stationary phase: Influence of coating times and coating amount on chiral discrimination

Coating cellulose tris (3,5‐dimethylphenylcarbamate) (CDMPC) on silica gels with large pores have been demonstrated as an efficient way for the preparation of chiral stationary phase (CSP) for high‐performance liquid chromatography (HPLC). During the process, a number of parameters, including the type of coating solvent, amount of coating, and the method for subsequent solvent removing, have been proved to affect the performance of the resultant CSPs. Coating times and the concentration of coating solution, however, also makes a difference to CSPs' performance by changing the arrangement of cellulose derivatives while remaining the coating amount constant, have much less been studied before, and thereby, were systematically investigated in this work. Results showed that CSPs with more coating times exhibited higher chiral recognition and column efficiency, suggesting that resolution was determined by column efficiency herein. Afterwards, we also investigated the effect of coating amount on the performance of CSPs, and it was shown that the ability of enantio‐recognition did not increase all the time as the coating amount; and four of seven racemates achieved best resolution when the coating amount reached to 18.37%. At the end, the reproducibility of CDMPC‐coated CSPs were further confirmed by two methods, ie, reprepared the CSP‐0.15‐3 and reevaluated the effect of coating times.     

A new class of network-polymeric chiral stationary phases

A strategy based on the use of homo bi- and multifunctional building blocks for the synthesis of a new class of network-polymeric chiral stationary phases has been evaluated. The key steps comprise acylation of N,N′-diallyl-L-tartardiamide (DATD) and reaction with a multifunctional hydrosilane, yielding a network polymer incorporating the bifunctional C₂-symmetric chiral selector. Covalent bonding to a functionalized silica takes place during the latter process. Many of these chiral sorbents show interesting enantioselective properties toward a wide variety of racemic solutes under normal-phase (hexane-based) conditions. The retention is mainly caused by the hydrogen-bonding ability of the analyte, which is regulated by mobile phase additives like alcohol or ether cosolvents. The most interesting chiral stationary phases, in terms of broad enantioselectivity, were obtained from O,O′-diaryol-DATD-derivatives, particularly those containing the 3,5-dimethylbenzoyl and the 4-(tert-butyl)benzoyl moieties. Since high column efficiencies can be obtained with these chiral sorbents, an α-value of ca. 1.2 is usually sufficient to produce baseline separation. A large number of neutral as well as acidic or basic drug racemates are resolved without derivatization. © 1995 Wiley-Liss, Inc.     

Chiral separations on polysaccharide stationary phases using polar organic mobile phases

About 30% of a chemically diverse set of compounds were found to separate on four polysaccharide chiral stationary phases using polar organic mobile phases. No structural features appeared to correlate to successful separations. Titrations between normal and polar organic mobile phases suggested that separation mechanisms do not differ between these mobile phases. Attempts made to control retention met with varying degrees of success. Addition of hexane to alcohols had minor effects on retention although this was occasionally beneficial. Addition of water to alcohols increased retention. Addition of water to acetonitrile decreased retention. Addition of alcohol to acetonitrile also proved beneficial to the separation of some compounds. Loading studies performed to mimic preparative separations indicated that the benefits of polar organic mobile phases are largely due to increased solubility.     

Chiral 3D Open‐Framework Material Ni(D‐cam)(H2O)2 Used as GC Stationary Phase

Metal‐organic frameworks (MOFs) have been explored for analytical applications because of their outstanding properties such as high surface areas, flexibility and specific structure features, especially for chromatography application in recent years. In this work, a chiral MOF Ni(D‐cam)(H₂O)₂ with unusual integration of molecular chirality, absolute helicity, and 3‐D intrinsic chiral net was chosen as stationary phase to prepare Ni(D‐cam)(H₂O)₂‐coated open tubular columns for high‐resolution gas chromatographic (GC) separation. Two fused‐silica open tubular columns with different inner diameters and lengths, including column A (30 m × 250 µm i.d.) and column B (2 m × 75 µm i.d.), were prepared via a dynamic coating method. The chromatographic properties of the two columns were investigated using n‐dodecane as the analyte at 120 °C. The number of theoretical plates (plates/m) of the two metal–organic framework (MOF) columns was 1300 and 2750, respectively. The racemates, isomer and linear alkanes mixture were used as analytes for evaluating the separation properties of Ni(D‐cam)(H₂O)₂‐coated open tubular columns. The results showed that the columns offered good separations of isomer and linear alkanes mixture, especially racemates. Chirality 26:27–32, 2013. © 2013 Wiley Periodicals, Inc.     

Synthesis of regioselectively substituted cellulose derivatives and applications in chiral chromatography

Various cellulose-2,3-bis-arylcarbamate-6-O-arylesters and cellulose-2,3-bis-arylester-6-O-arylcarbamates, designed to test the possible combined effects of the known tris-arylcarbamate and tris-arylester classes, were synthesized with high regioselectivity at O-C(6), and their use as CSP s in liquid chromatography for enantiomeric separations was investigated. The separations obtained with the synthesized CSP s were compared to the separations achieved on a self-packed reference column, consisting of cellulose-tris-(3,5-dimethylphenyl-carbamate) as CSP standard. Among the synthesized, regioselectively substituted cellulose derivatives, 2,3-bis-O-(3,5-dimethylphenylcarbamate)-6-O-benzoate-cellulose and 2,3-bis-O-(benzoate)-6-O-(3,5-dichlorophenylcarbamate)-cellulose gave the best CSP s for the separation of the test racemates. CSP s from regioselectively substituted cellulose derivatives seem to exhibit higher selectivities than cellulose-tris-(3,5-dimethylphenylcarbamate) for certain classes of racemic compounds. Chirality 10:294–306, 1998. © 1998 Wiley-Liss, Inc.