Preparation and enantioseparation of a mixed selector chiral stationary phase derived from benzoylated tartaric acid and 1,2‐diphenylethylenediamine

L ‐Dibenzoyl tartaric acid was mono‐esterified with benzyl alcohol, and then chlorinated with SOCl₂ to give (2S,3S)‐1‐(benzyloxy)‐4‐chloro‐1,4‐dioxobutane‐2,3‐diyl dibenzoate (Selector 1 ). (1R,2R)‐1,2‐Diphenylethylenediamine was mono‐functionalized with phenyl isocyanate and phenylene diisocyanate in sequence to give (1R,2R)‐1,2‐diphenyl‐2‐(3‐phenylureido)ethyl 4‐ isocyanatophenylurea (Selector 2 ). Two brush‐type chiral stationary phases (CSPs) of single selector were prepared by separately immobilizing selectors 1 and 2 on aminated silica gel. Selectors 1 and 2 were simultaneously immobilized on aminated silica gel to give a mixed selector CSP. The enantioseparation ability of these CSPs was studied. The CSP of selector 1 has strongest separation ability, while the enantioseparation ability of the mixed selector CSP is relatively lower. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

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.     

Combining the enantioselectivity of a cyclodextrin and a diamide selector in a mixed binary gas‐chromatographic chiral stationary phase

The present work reports on the investigation of a mixed binary chiral stationary phase (CSP) prepared by simultaneous attachment of permethylated‐β‐cyclodextrin ( D selector) and resorcinarene with pendant l ‐ or d ‐valine diamide groups ( L′ and D′ selectors, respectively) to a polysiloxane matrix via platinum‐catalyzed hydrosilylation. The gas‐chromatographic investigation of a number of racemates on the four different CSPs ( D, D′, DD′, and DL′ ) showed that the enantioselectivity of the individual chiral selectors was retained in the mixed binary CSPs. As a consequence, hydrocarbons, underivatized alcohols, ketones, and almost all proteinogenic amino acid derivatives could be separated simultaneously on each of the mixed CSPs. Matched and mismatched cases of enantioseparation on the mixed binary CSPs were observed but turned out to be of minor importance for enantiomeric separation. In general, more racemates were separated with α ≥ 1.02 on the mixed phases as compared to the single phases. In order to analyze the influence of the presence of the diamide selector on the enantioselectivity of the cyclodextrin selector, a mixed ternary CSP containing the selector D and a racemic mixture of the selectors D′ and L′ [ D ( D′L′ )] was prepared and investigated. Merits and limitations of the approach of mixed binary CSPs are discussed. © 2005 Wiley‐Liss, Inc. Chirality     

Synthesis of dendrimer‐type chiral stationary phases based on the selector of (1S,2R)‐(+)‐2‐Amino‐1,2‐diphenylethanol derivate and their enantioseparation evaluation by HPLC

In our recent work, a series of dendritic chiral stationary phases (CSPs) were synthesized, in which the chiral selector was L‐2‐(p‐toluenesulfonamido)‐3‐phenylpropionyl chloride (selector I), and the CSP derived from three‐generation dendrimer showed the best separation ability. To further investigate the influence of the structures of dendrimer and chiral selector on enantioseparation ability, in this work, another series CSPs ( CSPs 1‐4 ) were prepared by immobilizing (1S,2R)‐1,2‐diphenyl‐2‐(3‐phenylureido)ethyl 4‐isocyanatophenylcarbamate (selector II) on one‐ to four‐generation dendrimers that were prepared in previous work. CSPs 1 and 4 demonstrated the equivalent enantioseparation ability. CSPs 2 and 3 showed the best and poorest enantioseparation ability respectively. Basically, these two series of CSPs exhibited the equivalent enantioseparation ability although the chiral selectors were different. Considering the enantioseparation ability of the CSP derived from aminated silica gel and selector II is much better than that of the one derived from aminated silica gel and selector I, it is believed that the dendrimer conformation essentially impacts enantioseparation. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Chiral recognition mechanisms with macrocyclic glycopeptide selectors

Macrocyclic glycopeptide selectors are naturally occurring antibiotics produced by microorganisms. They were found to be excellent chiral selectors for a wide range of enantiomers, including amino acids. Four selectors are commercialized as chiral stationary phases (CSP) for chromatography. They are ristocetin, teicoplanin, vancomycin, and the teicoplanin aglycone (TAG). The key docking interaction for amino acid recognition was established to be a charge-charge interaction between the anionic carboxylate group of the amino acid and a cationic amine group of the macrocyclic peptidic selector basket. The carbohydrate units are responsible for secondary interactions. However, they hinder somewhat the charge-charge docking interaction. The TAG selector is more effective for amino acid enantioseparations than the other CSPs. The "sugar" units are however useful allowing for chiral recognitions of other analytes, e.g., beta-blockers, not possible with the aglycone. Thermodynamic studies established that normal phase and reversed phase enantioseparations were enthalpy-driven. With polar waterless mobile phases used in the polar ionic mode, some separations were enthalpy-driven and others were entropy-driven. The linear solvation energy method was tentatively used to gain knowledge about the chiral recognition mechanism. It appeared to be a viable approach with neutral molecules but it failed with ionizable solutes. With molecular solutes and the teicoplanin CSP, the study showed a significant role of the surface charge-induced dipole interaction and steric effects. The remarkable complementary enantioselectivity effect observed with the four CSPs is discussed.     

Novel chiral stationary phases based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin combining cinchona alkaloid moiety

Novel chiral selectors based on 3,5-dimethyl phenylcarbamoylated β-cyclodextrin connecting quinine (QN) or quinidine (QD) moiety were synthesized and immobilized on silica gel. Their chromatographic performances were investigated by comparing to the 3,5-dimethyl phenylcarbamoylated β-cyclodextrin (β-CD) chiral stationary phase (CSP) and 9-O-(tert-butylcarbamoyl)-QN-based CSP (QN-AX). Fmoc-protected amino acids, chiral drug cloprostenol (which has been successfully employed in veterinary medicine), and neutral chiral analytes were evaluated on CSPs, and the results showed that the novel CSPs characterized as both enantioseparation capabilities of CD-based CSP and QN/QD-based CSPs have broader application range than β-CD-based CSP or QN/QD-based CSPs. It was found that QN/QD moieties play a dominant role in the overall enantioseparation process of Fmoc-amino acids accompanied by the synergistic effect of β-CD moiety, which lead to the different enantioseparation of β-CD-QN-based CSP and β-CD-QD-based CSP. Furthermore, new CSPs retain extraordinary enantioseparation of cyclodextrin-based CSP for some neutral analytes on normal phase and even exhibit better enantioseparation than the corresponding β-CD-based CSP for certain samples.     

Evaluation of "click" binaphthyl chiral stationary phases by liquid chromatography

Two "click" binaphthyl chiral stationary phases were synthesized and evaluated by liquid chromatography. Their structures incorporate S-(-)-1,1'-binaphthyl moiety as the chiral selector and 1,2,3-triazole ring as the spacer. These chiral stationary phases (CSPs) allowed the efficient resolution for a wide range of racemic BINOL derivatives, particularly for nonpolar diether derivatives and 3-phenyl indolin-2-one analogs. The chromatographic data showed that the π-π interaction was crucial for enantiorecognition of these CSPs. Loss of enantioselectivity observed on CSP3, which are lacking the triazole ring linkage, indicated that the triazole ring linkage took part in the enantioseparation process, although it was remote from the chiral selector of the CSP. The substitution of the phenyl group at 6 and 6' positions can significantly improve the separation ability of the CSP. The chiral recognition mechanism was also investigated by tracking the elution orders and studying the thermodynamic parameters.     

Steric hindrance influence on the enantiorecognition ability of tyrosine-derived chiral stationary phases

Four chiral stationary phases (CSPs) derived from N-(3,5-dinitrobenzoyl)tyrosine have been synthesized. They differ by the substituent nature (methyl, ethyl, isopropyl, tert-butyl) of the aliphatic amide function. The enantiorecognition ability of these CSPs was evaluated with 10 racemates. For the majority of them, the stereoselectivity increases with the steric hindrance of the substituent. The chiral selector enantiomeric separation on the resulting CSPs has evidenced a reversal of elution order only for CS 4 on CSP 4 (tert-butyl substituent), suggesting a change in its conformation.     

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.     

Role of the weak interactions in enantiorecognition of racemic dihydropyrimidinones by novel brush-type chiral stationary phases

The chiral discrimination ability of two recently prepared chiral stationary phases (CSP 1 and CSP 2), based on a leucine derived chiral selector, was tested for the enantiomers of dihydropyrimidone (DHPM) derivatives and compared with the commercially available Hyun-leucine CSP 3 and classical Pirkle-leucine CSP 4. By combining all of these CSPs, the enantiomers of all DHPM derivatives used in this study can be properly resolved. Particularly good enantioresolutions were achieved for thioureide derivatives, such as Monastrol. The results presented show that sulfur-aromatic interactions are meritorious for these very good separations.     

Mechanism of chiral recognition in the enantioseparation of 2-aryloxypropionic acids on new brush-type chiral stationary phases

New brush-type chiral stationary phases (CSP I-IV) comprising N-3,5,6-trichloro-2,4-dicyanophenyl-L-alpha-amino acids (1-4) were prepared by binding of chiral selectors 1-4 to gamma-aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N-3,5,6-trichloro-2,4-dicyanophenyl-L-alanyl-(3-triethoxysilyl)propylamide to unmodified silica gel. The best separation of racemic 2-aryloxypropionic acids (TR-1-13) was obtained with CSP I; the -(-)-S enantiomer were regularly eluted first, as determined by a CD detector. The mechanism of chiral recognition implies a synergistic interaction of carboxylic acid analyte with the chiral selector and achiral free gamma-aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separation ability for 2-aryloxypropionic acids, but a similar enantioselective discrimination of esters TR-19-20, in comparison with CSP I. CSP I-IV retain unaltered separation ability after a few months of continuous work using a large number of various mobile phases.     

Enantiomerization of 3-carbethoxy-l,4-benzodiazepin-2-one: combined chiral HPLC and spectroscopic study.

Recently developed chiral HPLC columns CHIRIS AD1 and CHIRIS AD2 have been demonstrated to separate racemic, configurationally unstable ethyl-7-chloro-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepine-3-carboxylate (1) and its 3-methyl congener 2; fast on-column enantiomerization of configurationally unstable 1 was observed, however. Addition of 0.1% of AcOH to the eluting mixture inhibits enantiomerization, whereas the same percentage of Et(3)N completely precludes enantioseparation, suggesting base-catalysis by free beta-aminoethyl groups, present in low percentage in chiral stationary phase (CSP). When both CSPs were prepared under conditions of nonexhaustive acylation by N-DNB-alpha-aminoacids, no separation of 1 was observed. The rate of enantiomerization on CHIRIS AD2 was determined at 25 degrees C, the mechanism is discussed, and experimental results correlated with calculated relative stabilities of the tautomers la-c. Absolute (3S) configuration of (+) enantiomers of 1 and 2 was determined by comparison of their eluation profile to that of (+/-)-3 and (3S)-(+)-3, taking into account relative (psia or psie) configuration of the prevailing conformer in solution.     

Evaluation of two pairs of chiral stationary phases: effects from the length of the achiral spacers

Two pairs of chiral stationary phases (CSPs) with different C(2)-symmetric central parts were prepared and evaluated by chromatography of a series of structurally different racemates. Within each pair, the selectors on which the CSPs are based had different lengths of their achiral spacers. The CSPs based on selectors with short spacers showed higher enantioselectivity than the phases incorporating long spacers. On one pair of the phases, a study of the influence from different retention modifiers was performed for a series of benzodiazepinones. This demonstrated the importance of the polymer structure formed from the selectors with different spacer lengths for the enantiodiscriminating ability of the CSPs.     

Synthesis and evaluation of two novel “mixed” chiral stationary phases deriving from tyrosine: Csps designed for the resolution of either π-acid or π-basic racemates

The synthesis and chromatographic evaluation of two novel chiral stationary phases (CSPs) deriving from (S)-tyrosine are reported. The chiral graft has been designed in order to bear both π-acid and π-basic sites, each one being connected to a distinct asymmetric centre. An intramolecular π-π interaction may take place within these CSPs, leading to an energetically favoured conformation of the chiral selector (CS). The enantiorecognition ability of these CSPs was investigated for various classes of either π-acid or π-basic racemates. It is shown that these CSPs are able to separate simultaneously π-acid and π-basic racemates. Finally, chiral recognition mechanisms and mobile phase optimization are discussed.     

Synthesis of new C3 symmetric amino acid‐ and aminoalcohol‐containing chiral stationary phases and application to HPLC enantioseparations

We recently reported a new C3‐symmetric (R)‐phenylglycinol N‐1,3,5‐benzenetricarboxylic acid‐derived chiral high‐performance liquid chromatography (HPLC) stationary phase (CSP 1) that demonstrated better results as compared to a previously described N‐3,5‐dintrobenzoyl (DNB) (R)‐phenylglycinol‐derived CSP. Over a decade ago, (S)‐leucinol, (R)‐phenylglycine, and (S)‐leucine derivatives were used as the starting materials of 3,5‐DNB‐based Pirkle‐type CSPs for chiral separation. In this study, three new C3‐symmetric CSPs (CSP 2, 3, and 4) were prepared by combining the ideas and results mentioned above. Here we describe the synthetic procedures and applications of the new C3‐symmetric CSPs (CSP 2–CSP 4).     

Preparation and evaluation of chiral stationary phases based on N, N-2,4-(or 4,6)-disubstituted 4,5-(or 2,5)-dichloro-1, 3-dicyanobenzene

Regioselective functionalization of 2,4,5,6-tetrachloro-1, 3-dicyanobenzene (TCDCB) by nucleophilic substitution of the chlorine at C(4) with L-Ala, L-Phe or L-Pro, followed by amide-bond formation to lipophilic amines containing strong pi-donor group, and by final introduction of the spacer 3-aminopropyltriethoxysilyl (APTES), provided a number of new brush-type chiral selectors in the form of 1-2:1 mixture of 2,4 and 4,6-di(alkyl)amino regioisomers (8/9, 10/11, 12/13, 14/15, 20/21, 23/24). Linking these to silica gel (Nucleosil 100-5) gave new chiral stationary phases for HPLC columns (CSP I-CSP VI). Being strong pi-basic selectors, most of these columns exhibited good resolution properties for pi-acid test racemates (TR 1-TR 9), specifically rac 3, 5-dinitrobenzoyl-alpha-amino acid isopropyl-esters (DNB-AA). CSP V [1,3-dicyano-2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-?[N-butyl]-N'-[(1R)-cyclohexylethyl]-N'-[napht hylmet hyl]acetamido?-aminobenzene] and particularly the dipeptide-containing CSP VI [2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-(3', 5'-dimethylanilido)-L-alanyl-L-prolyl-aminobenzene] proved to have the highest efficiency, comparable with the best commercial brush-type columns with pi-donor properties. Further evidence revealed that multiple hydrogen bonding via the amide group in the chiral environment and pi-pi interaction play a major role in chiral recognition, whereas steric perturbations via nonbonding VDW interactions contribute substantially only to the resolution of CSP III [2,5(5,6)-dichloro-6(2)-(gamma'-silica bound propylamino)-4-N-(cyclohexylamido)-L-alanyl-aminobenzene]. This contribution is minor for the other CSPs.     

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).     

Evaluation of π-acid chiral stationary phases deriving from tyrosine and related amino acids for the chromatographic resolution of racemates: Specific requirements for enantiorecognition ability

Chromatographic applications of three novel chiral stationary phases (CSPs) deriving from (S)-(N)-(3,5-dinitrobenzoyl)tyrosine are reported, under liquid chromatographic (LC) and subscritical fluid chromatographic (SubFC) conditions. Two grafting modes of the chiral moiety have been experimented starting either from γ-mercaptopropyl-silanized (type 1) or γ-aminopropyl-silanized (type 2) silica gels. For type 2 CSPs an evaluation of the stability of the amide linkage was achieved by means of SubFC; the relative contriution of ionic and covalent bindings to the ciral recognitio aility was then outlined. The chromatographic properties of these CSPs were compared with those of the corresponding CSPs deriving from phenylglycine, p-hydroxyphenylglycine, and phenylalanine for the resolution of some tertiary phosphine oxide, naphthoyl amide, and α-methylene γ-lactam enantiomers. Some simple requirements regarding the solute and CSP structures for chiral recognition ability can be inferred from these results. In addition, the resolutio of π-acid α-N-(3,5-dinitrobenzoyl)amino esters was investigated on these π-acid CSPs. An example of preparative scale chromatography is also presented.     

Liquid Chromatographic Resolution of Mexiletine and Its Analogs on Crown Ether‐Based Chiral Stationary Phases

Mexiletine, an effective class IB antiarrhythmic agent, and its analogs were resolved on three different crown ether‐based chiral stationary phases (CSPs), one (CSP 1 ) of which is based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid and the other two (CSP 2 and CSP 3 ) are based on (3,3’‐diphenyl‐1,1’‐binaphthyl)‐20‐crown‐6. Mexiletine was resolved with a resolution (R_S) of greater than 1.00 on CSP 1 and CSP 3 containing residual silanol group‐protecting n‐octyl groups on the silica surface, but with a resolution (R_S) of less than 1.00 on CSP 2 . The chromatographic behaviors for the resolution of mexiletine analogs containing a substituted phenyl group at the chiral center on the three CSPs were quite dependent on the phenoxy group of analytes. Namely, mexiletine analogs containing 2,6‐dimethylphenoxy, 3,4‐dimethylphenoxy, 3‐methylphenoxy, 4‐methylphenoxy, and a simple phenoxy group were resolved very well on the three CSPs even though the chiral recognition efficiencies vary with the CSPs. However, mexiletine analogs containing 2‐methylphenoxy group were not resolved at all or only slightly resolved. Among the three CSPs, CSP 3 was found to show the highest chiral recognition efficiencies for the resolution of mexiletine and its analogs, especially in terms of resolution (R_S). Chirality 26:272–278, 2014. © 2014 Wiley Periodicals, Inc.     

Enantioselective anion exchangers based on cinchona alkaloid‐derived carbamates: Influence of C8/C9 stereochemistry on chiral recognition

Four diastereomeric chiral stationary phases (CSPs) based on quinine, quinidine, epiquinine, and epiquinidine tert‐butyl carbamate selectors were synthesized and evaluated under ion exchange HPLC conditions with a set of racemic N‐acylated and N‐oxycarbonylated α‐amino acids as selectands. The enantioseparation potential of quinine‐ and quinidine‐derived CSPs proved to be far superior to that of their C₉‐epimeric congeners. The absolute configuration of C₉ stereogenic center of the cinchonan backbone of these selectors was identified as the structural feature controlling the elution order. Guided by an X‐ray structure of a most favorable selector–selectand complex and the observed chromatographic enantioseparation data, a chiral recognition model was advanced. The contributions of ion‐pairing, π–π donor–acceptor, hydrogen bonding and steric interactions were established as crucial factors. Chirality 11:522–528, 1999. © 1999 Wiley‐Liss, Inc.