Gas chromatographic enantioseparation of unfunctionalized chiral alkanes: a challenge in separation science (overview, state of the art, and perspectives)

The chromatographic enantioseparation of small unfunctionalized chiral alkanes C*HR(1)R(2)R(3) (R = alkyl) represents a challenge in separation science. Because of the lack of any functional groups, enantiorecognition in the presence of a chiral selector is solely based upon weak enantioselective Van der Waals forces. Racemic alkanes containing seven and eight carbon atoms, i.e. 3-methylhexane (C7), 2,3-dimethylpentane (C7), 3-methylheptane (C8), 3,4-dimethylhexane (C8), 2,4-dimethylhexane (C8), 2,3-dimethylhexane (C8), and 2,2,3-trimethylpentane (C8) have been gas chromatographically enantioseparated on different modified cyclodextrins. The substitution pattern and cavity size of the cyclodextrin selectors have a pronounced effect on the degree of enantiorecognition observed. Thermodynamic parameters of enantiorecognition between four chiral alkanes and octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) have been determined. The possible role of molecular inclusion is indicated by the complete loss of enantioselectivity when the cyclodextrins are replaced by the corresponding linear dextrins ("acyclodextrins"). The enantioseparations of all seven chiral C7-C8 alkanes, six of them simultaneously, has been achieved on mixed binary selector systems whereby two different modified cyclodextrins are present in one gas chromatographic column. The smallest chiral (nonisotopically labeled) allene, i.e., 2,3-pentadiene, has been resolved gas chromatographically on a cyclodextrin selector.     

Evaluation of newly synthesized and commercially available charged cyclomaltooligosaccharides (cyclodextrins) for capillary electrokinetic chromatography

A highly new charged cyclodextrin (CD) derivatives, (6-O-carboxymethyl-2,3-di-O-methyl)cyclomaltoheptaoses (CDM-beta-CDs), was synthesized and characterized as anionic reagents for capillary electrophoresis (CE) in an electrokinetic chromatography mode of separation. Substitution with dimethyl groups at the secondary hydroxyl sites of the CD is aimed at influencing the magnitude and selectivity of analyte-CD interactions, while substitution by carboxymethyl groups at the primary hydroxyl sites provides for high charge and electrophoretic mobility. Full regioselective methylation at the secondary hydroxyl sites was achieved in this work, while substitution at the primary hydroxyl sites generated a mixture of multiply charged products. The separation performance of CDM-beta-CD was evaluated using a variety of analyte mixtures. The results obtained from commercially available negatively charged cyclodextrins, heptakis(2,3-di-O-methyl-6-O-sulfo)cyclomaltoheptaose (HDMS-beta-CD) and O-(carboxymethyl)cyclomaltoheptaose (CM-beta-CD) with an average degree of substitution one (DS 1), were compared to CDM-beta-CD using a sample composed of eight positional isomers of dihydroxynaphthalene. Four hydroxylated polychlorobiphenyl derivatives, a group of chiral and isomeric catchecins, and chiral binaphthyl compounds were also separated with CDM-beta-CD. The effect of adding neutral beta-cyclodextrin (beta-CD) into the running buffer containing charged cyclodextrins was investigated and provided evidence of significant inter-CD interactions. Under certain running buffer conditions, the charged cyclodextrins also appear to adsorb to the capillary walls to various degrees.     

Capillary electrophoretic chiral resolution of vicinal diols by complexation with borate and cyclodextrin: Comparative studies on different cyclodextrin derivatives

Capillary electrophoretic enantioseparation of compounds containing vicinal diol groups have been investigated using different β-cyclodextrin (CD) derivatives and borate as a background electrolyte. Both native β-CD and several β-CD derivatives are examined. Chiral recognition is attributed to both enantioselective inclusion of the analyte into the chiral cavity of the CD and complexation with borate. The influence of concentration of the chiral selector, pH, and organic modifiers on the resolution was studied. Four diols were baseline separated. Chirality 9:153–156, 1997. © 1997 Wiley-Liss, Inc.     

Enantioselective potential of polysaccharide‐based chiral stationary phases in supercritical fluid chromatography

The enantioselective potential of two polysaccharide‐based chiral stationary phases for analysis of chiral structurally diverse biologically active compounds was evaluated in supercritical fluid chromatography using a set of 52 analytes. The chiral selectors immobilized on 2.5 μm silica particles were tris‐(3,5‐dimethylphenylcarmabate) derivatives of cellulose or amylose. The influence of the polysaccharide backbone, different organic modifiers, and different mobile phase additives on retention and enantioseparation was monitored. Conditions for fast baseline enantioseparation were found for the majority of the compounds. The success rate of baseline and partial enantioseparation with cellulose‐based chiral stationary phase was 51.9% and 15.4%, respectively. Using amylose‐based chiral stationary phase we obtained 76.9% of baseline enantioseparations and 9.6% of partial enantioseparations of the tested compounds. The best results on cellulose‐based chiral stationary phase were achieved particularly with propane‐2‐ol and a mixture of isopropylamine and trifluoroacetic acid as organic modifier and additive to CO₂, respectively. Methanol and basic additive isopropylamine were preferred on amylose‐based chiral stationary phase. The complementary enantioselectivity of the cellulose‐ and amylose‐based chiral stationary phases allows separation of the majority of the tested structurally different compounds. Separation systems were found to be directly applicable for analyses of biologically active compounds of interest.     

Enantioseparation and molecular modeling study of chiral amines as three naphthaldimine derivatives using amylose or cellulose trisphenylcarbamate chiral stationary phases

This study describes the enantioseparation of three chiral amines as naphthaldimine derivatives, using normal phase HPLC with amylose and cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phases (CSPs). Three chiral amines were derivatized using three structurally similar naphthaldehyde derivatizing agents, and the enantioselectivity of the CSPs toward the derivatives was examined. The degree of enantioseparation and resolution was affected by the amylose or cellulose-derived CSPs and aromatic moieties as well as a kind of chiral amine. Especially, efficient enantiomer separation was observed for 2-hydroxynapthaldimine derivatives on cellulose-derived CSPs. Molecular docking studies of three naphthaldimine derivatives of leucinol on cellulose tris(3,5-dimethylphenylcarbamate) were performed to estimate the binding energies and conformations of the CSP–analyte complexes. The obtained binding energies were in good agreement with the experimentally determined enantioseparation and elution order.     

Capillary electrophoretic enantiomeric separations using the glycopeptide antibiotic,teicoplanin

Teicoplanin is the third in a series of macrocyclic glycopeptide antibiotics that has been evaluated as a chiral selector in capillary electrophoresis (CE). It was used to resolve over 100 anionic racemates at low selector concentrations. Like the other related glycopeptide antibiotics, its enantioselectivity tends to be opposite to that of the ansa-type antibiotics which prefers cationic compounds, particularly amines. Factors that affect teicoplanin-based enantioseparations include the selector concentration, pH, and the concentration of the organic modifier. The temperature and the nature and strength of the buffer are also known-to affect the stability of the chiral selector as well as the enantioseparation. Teicoplanin exhibited some features that were not noted with the other glycopeptide antibiotics. For example, it aggregates (forms micelles) in aqueous solutions and this influences its enantioselectivity. Unlike the other studied glycopeptides, teicoplanin precipitates in alcohol-water mixtures. It also binds less to the capillary wall than vancomycin as evidenced by the faster electroosmotic flow velocity. The micellization of teicoplanin is pH dependent so that the effect of pH on enantiorecognition is more complex for teicoplanin than for other chiral selectors. Also it is shown that the simple model proposed to explain the role of organic modifiers in cyclodextrin-based CE enantioseparations may not apply to these and other systems. © 1996 Wiley-Liss, Inc.     

Direct and indirect separations of five isomers of Brivanib Alaninate using chiral high-performance liquid chromatography

Brivanib Alaninate is a novel chiral prodrug possessing two stereogenic centers. Simultaneous HPLC separation of five isomers of Brivanib Alaninate was systematically investigated on a wide variety of polysaccharide-based chiral stationary phases (CSPs) using underivatization and pre-column derivatization methods. The influence of derivatizing groups and mobile phase composition on the enantioseparation and retention behavior of Brivanib Alaninate compounds was studied. To better understand the chiral recognition mechanism, the temperature effect was also evaluated. The results of these studies led to the first complete HPLC resolution of all five isomers of Brivanib Alaninate as carbobenzyloxy (CBZ) derivatives on a cellulose benzoate CSP (OJ-H).     

Optimization of lambda-carrageenan as a chiral selector in capillary electrophoresis separations

Lambda-carrageenan, a linear high molecular weight sulfated polysaccharide, was employed as a chiral selector in capillary electrophoresis for the separation of enantiomers of weakly basic pharmaceutical compounds. In order to improve the utility of the chiral selector, the purity and concentration of the lambda-carrageenan and other important capillary electrophoresis method parameters were investigated. The results indicated that the purity and concentration of the lambda-carrageenan, ionic strength of the buffer, and temperature were critical to successful enantioseparation. These new method conditions were then applied to previously investigated beta-blockers (such as propranolol HCl and pindolol) and racemic tryptophan derivatives. These studies were successful in identifying important method conditions for the improved enantioselectivity with lambda-carrageenan.     

Enantioseparation on 4-halogen-substituted phenylcarbamates of amylose as chiral stationary phases for high-performance liquid chromatography

Four 4-halogen-substituted phenylcarbamate derivatives of amylose were prepared and their chiral recognition abilities as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC) were evaluated and compared with those of the corresponding cellulose derivatives. The amylose derivatives with fluoro, chloro, bromo, or iodo group at the four-position on the phenyl group were found to show higher chiral resolving ability than the corresponding cellulose derivatives. Among four amylose derivatives 4-fluoro- and 4-chlorophenylcarbamates showed an excellent chiral recognition ability. Especially, amylose tris(4-chlorophenylcarbamate) resolved (±)-1,2,2,2-tetraphenylethanol with a very high α value (α = 8.29). In order to obtain useful information concerning the chiral recognition mechanism of this resolution, we also performed enantioseparation of a variety of analogous racemic alcohols, and found that both the hydroxy and bulky triphenylmethyl groups of the racemate are essential for the effective chiral recognition. Chirality 9:63–68, 1997. © 1997 Wiley-Liss, Inc.     

Immobilization and chiral recognition of 3,5‐dimethylphenylcarbamates of cellulose and amylose bearing 4‐(trimethoxysilyl)phenylcarbamate groups

A small amount of 4‐(trimethoxysilyl)phenyl groups was randomly introduced onto the 3,5‐dimethylphenylcarbamates of cellulose and amylose by a one‐pot method. The obtained derivatives were then effectively immobilized onto silica gel as chiral packing materials (CPMs) for high‐performance liquid chromatography through intermolecular polycondensation of the trimethoxysilyl groups. The effects of the amount of 4‐(trimethoxysilyl)phenyl groups on immobilization and enantioseparation were investigated. Also, the solvent durability of the immobilized‐type CPMs was examined with the eluents containing chloroform and tetrahydrofuran. When these eluents were used, the chiral recognition abilities of the CPMs for most of the tested racemates were improved to some extent depending on the compounds. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Synthesis and HPLC chiral recognition of regioselectively carbamoylated cellulose derivatives

Four regioselective-carbamoylated cellulose derivatives having two different substituents at 2-, 3-, and 6-position were prepared and evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography. Investigations showed that the nature and arrangement of the substituents significantly influenced the chiral recognition abilities of the heterosubstituted cellulose derivatives and each derivative exhibited characteristic enantioseparation. Some racemates were better resolved on these derivatives than the corresponding homogeneously substituted cellulose derivatives including a commercial CSP, Chiralcel OD. Racemic compounds shown in this study were most effectively discriminated on cellulose 2,3-(3-chloro-4-methylphenylcarbamate)-6-(3,5-dimethylphenylcarbamate) and 2,3-(3,5-dimethylphenylcarbamate)-6-(3-chloro-4-methylphenylcarbamate).     

Preparation and evaluation of regioselectively substituted amylose derivatives for chiral separations

Six novel regioselectively substituted amylose derivatives with a benzoate at 2‐position and two different phenylcarbamates at 3‐ and 6‐positions were synthesized and their structures were characterized by ¹H nuclear magnetic resonance (NMR) spectroscopy. Their enantioseparation abilities were then examined as chiral stationary phases (CSPs) for high‐performance liquid chromatography (HPLC) after they were coated on 3‐aminopropyl silica gels. Investigations indicated that the substituents at the 3‐ and 6‐positions played an important role in chiral recognition of these amylose 2‐benzoate serial derivatives. The derivatives demonstrated characteristic enantioseparation and some racemates were better resolved on these derivatives than on Chiralpak AD, which is one of the most efficient CSPs, utilizing coated amylose tris(3,5‐dimethylphenylcarbamate) as the chiral selector. Among the derivatives prepared, amylose 2‐benzoate‐3‐(phenylcarbamate/4‐methylphenylcarbamate)‐6‐(3,5‐dimethylphenylcarbamate) exhibited chiral recognition abilities comparable to that of Chiralpak AD and may be useful CSPs in the future. The effect of mobile phase on chiral recognition was also studied. In general, with the decreased concentration of 2‐propanol, better resolutions were obtained with longer retention times. Moreover, when ethanol was used instead of 2‐propanol, poorer resolutions were often achieved. However, in some cases, improved enantioselectivity was achieved with ethanol rather than 2‐propanol as the mobile phase modifier.     

Maltodextrins as Chiral Selectors in CE: Molecular Structure Effect of Basic Chiral Compounds on the Enantioseparation

Prediction of chiral separation for a compound using a chiral selector is an interesting and debatable work. For this purpose, in this study 23 chiral basic drugs with different chemical structures were selected as model solutes and the influence of their chemical structures on the enantioseparation in the presence of maltodextrin (MD) as chiral selector was investigated. For chiral separation, a 100‐mM phosphate buffer solution (pH 3.0) containing 10% (w/v) MD with dextrose equivalent (DE) of 4‐7 as chiral selector at the temperature of 25°C and voltage of 20 kV was used. Under this condition, baseline separation was achieved for nine chiral compounds and partial separation was obtained for another six chiral compounds while no enantioseparation was obtained for the remaining eight compounds. The results showed that the existence of at least two aromatic rings or cycloalkanes and an oxygen or nitrogen atom or –CN group directly bonded to the chiral center are necessary for baseline separation. With the obtained results in this study, chiral separation of a chiral compound can be estimated with MD‐modified capillary electrophoresis before analysis. This prediction will minimize the number of preliminary experiments required to resolve enantiomers and will save time and cost. Chirality 26:620–628, 2014. © 2014 Wiley Periodicals, Inc.     

Semipreparative HPLC enantioseparation,chiroptical properties,and absolute configuration of two novel cyclooxygenase‐2 inhibitors

A direct semipreparative HPLC enantioseparation of two chiral thiazolidinone derivatives having cyclooxygenase‐2 inhibition activity was performed on the Chiralpak IA chiral stationary phase. Semipreparative amounts of enantiopure forms were collected using acetonitrile‐ethanol‐trifluoroacetic acid mixtures as mobile phase. The absolute configuration of both compounds was unequivocally established by single‐crystal X‐ray diffraction method and correlated to the chiroptical properties of isolated enantiomers. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Immobilization of (1S,2R)-(+)-2-amino-1,2-diphenylethanol derivates on aminated silica gel with different linkages as chiral stationary phases and their enantioseparation evaluation by HPLC

A chiral selector was prepared through the reaction between (1S,2R)-(+)-2-amino-1,2-diphenylethanol and phenyl isocyanate. This selector was immobilized on aminated silica gel, respectively, with bifunctional group linkers of 1,4-phenylene diisocyanate, methylene-di-p-phenyl diisocyanate, and terephthaloyl chloride to produce corresponding three chiral stationary phases. The prepared compounds and chiral stationary phases were characterized by FT-IR, elemental analysis, (1)H NMR, and solid-state (1)H NMR. The enantioseparation ability of these chiral stationary phases was evaluated with structurally various chiral compounds. The chiral stationary phase prepared with 1,4-phenylene diisocyanate as linker showed excellent enantioseparation ability. The influence of different linkages on the enantioseparation was discussed.     

New chiral stationary phases for liquid chromatography based on small molecules: Development,enantioresolution evaluation and chiral recognition mechanisms

Recently, we reported the development of new chiral stationary phases (CSPs) for liquid chromatography (LC) based on chiral derivatives of xanthones (CDXs). Based on the most promising CDX selectors, 12 new CSPs were successfully prepared starting from suitable functionalized small molecules including xanthone and benzophenone derivatives. The chiral selectors comprising one, two, three, or four chiral moieties were covalently bonded to a chromatographic support and further packed into LC stainless-steel columns (150 × 2.1 mm I.D.). The enantioselective performance of the new CSPs was evaluated by LC using different classes of chiral compounds. Specificity for enantioseparation of some CDXs was observed in the evaluation of the new CSPs. Besides, assessment of chiral recognition mechanisms was performed by computational studies using molecular docking approach, which are in accordance with the chromatographic parameters. X-Ray analysis was used to establish a chiral selector 3D structure.     

A mixed stationary phase containing two versatile cyclodextrin-based selectors for the simultaneous gas chromatographic enantioseparation of racemic alkanes and racemic alpha-amino acid derivatives

In an effort to simultaneously enantioseparate racemic unfunctionalized alkanes and racemic alpha-amino acid derivatives by gas chromatography (GC) in forthcoming experiments related to the search for extraterrestrial homochirality, the two versatile modified cyclodextrin (CD) selectors octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin were dissolved in a polysiloxane and the mixed binary chiral selector system was coated onto a 50m x 0.25 mm i.d. fused silica capillary column. Whereas the former CD selector enantioseparates racemic unfunctionalized alkanes the latter CD selector preferentially resolves N-(O,S)-trifluoroacetyl-alpha-amino acid alkyl esters. With both CD selectors employed as mixed binary chiral selector system present in one chiral stationary phase (CSP), the simultaneous gas chromatographic enantioseparation of racemic alkanes and of racemic derivatized alpha-amino acids is achieved in a single temperature-programmed run. Also for other classes of racemic compounds, the scope of enantioseparation could be extended as compared to the conventional use of the single CD selectors in GC.     

Synthesis of polymer-type chiral stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two new chiral polymers of different molecular weights were synthesized by the copolymerization of (1R,2R)-(+)-1,2-diphenylethylenediamine, phenyl diisocyanate and terephthaloyl chloride. The polymers were immobilized on aminated silica gel to afford two chiral stationary phases. The polymers and the corresponding chiral stationary phases were characterized by Fourier transform-IR, elemental analysis, 1H and 13C NMR. The surface coverages of chiral structural units on the chiral stationary phases were estimated as 0.27 and 0.39 mmol/g, respectively. The enantioseparation ability of these chiral stationary phases was evaluated with a variety of chiral compounds by high-performance liquid chromatography. The effects of the organic additives, the composition of mobile phases, and the injection amount of sample on enantioseparation were investigated. A comparison of enantioseparation ability between these two chiral stationary phases was made. It was believed that the chain length of polymeric chiral selector significantly affected the enantioseparation ability of corresponding chiral stationary phase.     

Détermination des sites d'oxydation des dérivés du α-D-glucofuranose utilisés comme donneurs

The sites of oxidation, by catalytic transfer of H, of derivatives of 1,2-O-isopropylidene-α-D-glucofuranose suggest a regiospecific reaction. Compounds having vicinal hydroxyl groups at C-5 and C-6, or at C-3 and C-5, are oxidized at OH-5, whereas compounds having two hydroxyl groups at C-3 and C-6 or three hydroxyl groups give first aldehydes and then lactones.     

Semipreparative enantioseparation of Tröger base derivatives by HPLC

We describe the enantioseparation of functionalized derivatives of the Tr?ger base by HPLC on commercially available chiral stationary phases. Cellulose-derived Chiralcel OJ and brush-type Whelk O1 are demonstrated to be complementary to each other in their scope. On the basis of the results obtained, the separation of selected compounds was successfully transferred onto semipreparative columns. We believe that the availability of enantiopure functionalized derivatives of the Tr?ger base will stimulate the further use of this interesting molecular scaffold in molecular recognition, asymmetric catalysis, and related areas of research and technology.