Comparison of the chiral separation of amino-acid derivatives by a teicoplanin and RN-beta-CD CSPs using waterless mobile phases: Factors that enhance resolution

A variety of compounds containing amines (i.e., amino acids, amino alcohols, etc.) were chemically derivatized with a variety of electrophilic tagging reagents to elucidate the chiral recognition sites on a teicoplanin-bonded chiral stationary phase (CSP) and on R-naphthylethylcarbamate-beta-cyclodextrin (RN-beta-CD)-bonded CSP. Solutes were separated under optimum chromatographic conditions on teicoplanin and RN-beta-CD CSPs for comparison using an acetonitrile-based mobile phase. It was noted that the size of the analyte or tagging reagent exerted a greater influence on compounds separated on teicoplanin than on RN-beta-CD when using the polar organic mode. This suggests that chiral recognition on teicoplanin CSP is more sensitive to size and indicates that the hydrophobic pocket of teicoplanin plays a significant role in chiral recognition in this mode. However, the type of functional groups had a greater impact than the size of analyte on separations obtained from RN-beta-CD phase in the polar-organic mode. Specifically, the pi-pi interaction was enhanced by derivatizing the aromatic ring of the tagging reagent with electron-withdrawing groups and thus altered the resolution substantially. For both phases, chiral recognition is most pronounced when the stereogenic center of the analyte is near the tagging moiety and surrounded by functional groups (e.g., carboxylic, etc.) which are favorable for hydrogen bonding.     

Chromatographic optical resolution on trans-1,2-diaminocyclohexane derivatives: Theory and applications

An overall view on some new chiral stationary phases based on (trans)-1,2-diaminocyclohexane is illustrated. The selected chiral moiety, derivatized with different aroyl groups, has been linked to a silica matrix in order to give chiral stationary phases (CSPs) enabling them to be used efficiently in the normal and reverse phase, both for analytical and preparative purposes. In addition new polymeric CSPs have been prepared by using the same selector, suitably modified, as monomer. The new chiral stationary phases have been characterised by physicochemical methods and used for the resolution of various racemic compounds classes such as α-aryloxyacetic acids, alcohols, sulfoxides, selenoxides, phosphinates, tertiaryphosphine oxides, benzodiazepines etc. without prederivatization or as amines, amino acids, amino alcohols, nonsteroidal antiinflammatory agents in a derivatized form. The separated solutes structural variety suggests that multiple interaction sites are involved in the recognition process: some thermodynamic data relative to the CSPs—selectands interactions are also illustrated. © 1992 Wiley-Liss, Inc.     

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     

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.     

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 of poly(phenylacetylene)s containing chiral phenylethyl carbamate residues as coated-type CSPs with high solvent tolerability

Two novel helical poly(phenylacetylene) derivatives containing chiral phenylethyl carbamate residues in the end of each side chain ( PPA-S and PPA-R ) were synthesized by polymerization of the corresponding phenylacetylene monomers using Rh(nbd)BPh₄ as a catalyst in DMF. The enantioseparation properties of the polymers were evaluated as coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Under the same chromatographic conditions, PPA-S and PPA-R showed different enantioseparation properties, indicating that the different interactions between the analytes and the polymers, which result from the different chiral phenylethyl carbamate groups in the end of each side chains. Racemates 1 , 7 , and 8 could be better resolved on PPA-S , while racemate 6 was separated on PPA-R more efficiently. In addition, the coated-type CSPs showed good solvent tolerability and could work without any damage by introducing the polar solvents, such as CHCl₃ and THF, in eluent. Moreover, some racemates could be better resolved on these coated-type CSPs with the addition of THF to the eluent.     

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).     

Solid-phase synthesis of chiral stationary phases based on 2,4,5,6-tetrachloro-1,3-dicyanobenzene derivatives spaced from N-3,5-dinitrobenzoyl alpha-amino acids: comparative study of their resolution efficacy

Two new chiral stationary phases, 3-[5-chloro-1,3-dicyano-2,4-[2'-(N'-1,3-dinitrobenzoyl-D-phenylglycinyl) aminoethyl]aminophen-1-yl] aminopropyl silica (CSP-1) and 3-[5-chloro-1,3-dicyano-2,4-[2'-(N'-1,3-dinitrobenzoyl-L-leucinyl) aminoethyl] aminophen-1-yl] aminopropyl silica (CSP-2), were prepared by solid-phase synthesis. They comprise chiral unit, 3,5-dinitrobenzoyl derivative of the amino acid, D-PhGly or L-Leu, bound via spacer 1,2-diaminoethane to 2,4-positions of the persubstituted benzene ring, derived from compound 1, and possess pseudo-C2 symmetry. Preparation of model compounds 6 and 7 confirmed the structure of chiral selectors, which comprise pi-donor persubsituted aromatic ring and two strong pi-acceptor 3,5-dinitrobenzoyl amido units. CD spectra of model selectors 6 and 7, run in DMSO above 250 nm, exhibit negative exciton coupling (EC) between pi-acceptor and pi-donor chromophores, C(1) symmetric model compound 8 exhibited much weaker EC and 9, devoid of pi-donor unit, does not exhibit any significant CD. Combined pi-donor and pi-acceptor properties enable the new CSPs to separate a broad range of racemates. The columns with CSP-1 and CSP-2 were tested for the separation of 22 racemates by HPLC with two different mobile phase systems and the results are compared with those obtained by using a structurally related commercial column.     

Green chiral HPLC enantiomeric separations using high temperature liquid chromatography and subcritical water on Chiralcel OD and Chiralpak AD

We report here the application of subcritical water in chiral separations on two popular polysaccharide chiral stationary phases (CSPs): Chiralpak AD and Chiralcel OD. The behavior of these two CSPs was studied under reversed phase conditions at room temperature to discover the maximum percentage of water in the mobile phase, which provided the separation of enantiomers of flavanone and benzoin, respectively, in a reasonable time (i.e., less than 1 h). Then, the stability of Chiralpak AD and Chiralcel OD versus temperature was investigated and discussed. Chiralcel OD separation of flavanone racemate was obtained at 120 °C with water and 2-propanol (80/20) as the mobile phase, while benzoin racemate was separated in pure water at 160 °C. Separations of several racemates were also presented, and advantages and limitations of the technique were discussed.     

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.     

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.     

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.     

Chiral separation on various modified amino alcohol‐derived HPLC chiral stationary phases

3,5‐Dinitrobenzoyl chloride was previously used for the preparation of (R)‐phenylglycinol‐ and (S)‐leucinol‐derived chiral stationary phases. In this study, 3,5‐bis(trifluoromethyl)benzoyl chloride, 2‐furoyl chloride, 2‐theonyl chloride, 10,11‐dihydro‐5H‐dibenzo[b,f]azepine‐5‐carbonyl chloride, diphenylcarbamoyl chloride, and 1‐adamantanecarbonyl chloride were used to prepare six new phenylglycinol‐derived chiral stationary phases (CSPs) and five new leucinol‐derived CSPs. Using these 11 CSPs, chiral separation of nine π‐acidic amino acid derivatives and five π‐basic compounds was performed, and the separation results were compared. An adamantyl‐derived CSP showed good separation. Chirality 28:276–281, 2016. © 2016 Wiley Periodicals, Inc.     

A covalently bonded teicoplanin chiral stationary phase for HPLC enantioseparations

A macrocyclic glycopeptide antibiotic containing a hydrophobic “tail” is covalently attached to silica gel via linkage chains. This material is extensively evaluated as a chiral stationary phase (CSP) for HPLC. The relevant structural features of the teicoplanin molecule which make it an effective chiral selector are discussed. The teicoplanin CSP appears to have excellent enantioselectivity for native amino acids, peptides, α-hydroxycarboxylic acids, and a variety of neutral analytes including cyclic amides and amines. Enantio-separations can be achieved in the reversed phase, normal phase, and “polar-organic” modes. This chiral selector is stable and the integrity of the CSP is excellent in all separation modes. Hence it can be considered a highly effective multimodal column. Optimization of these separations is discussed in terms of both selectivity and efficiency. Results indicate that the surface loading of the chiral selector affects all relevant separation parameters. A hypothesis is proposed to explain the enhanced efficiency obtained when using teicoplanin CSPs with higher surface coverage. It appears that teicoplanin is a widely applicable, highly effective chiral selector for HPLC enantioseparations. © 1995 Wiley-Liss, Inc.     

Immobilization of difunctional building blocks on hydroxysuccinimide activated silica: versatile in situ preparation of chiral stationary phases.

Several brush-type chiral stationary phases (CSPs) based on undecanoyl- or butanoyl-bound (R,R)-1,2-diphenylethane-1,2-diamine (DPEDA) as chiral selector were prepared by an innovative, fast, and less expensive kind of preparation. The key to this method is the immobilization of the enantiomeric pure diamine with only one amino function in a simple substitution reaction on hydroxysuccinimide ester-activated silica. No excess chiral material is lost. Loading can be easily monitored analyzing the filtrate. The free second amino function can subsequently be acylated with different acyl halogenides. Examples with benzoyl- and 3,5-dinitrobenzoyl (DNB) amides show that, based on our new approach, a library of differently acylated Pirkle-type CSPs can easily be obtained. A benzoylated analog of the commercially available ULMO CSP is shown to be very effective in separating enantiomers of N-acyl amino acids.     

A novel chiral inorganic mesoporous silica used as a stationary phase in GC

Chiral mesoporous silica (CMS) has attracted widespread attention because of some unique properties, such as high surface area, uniformly structured nanoscale cavities, and excellent chemical and thermal stability. In this work, we report the utilization of a CMS as the stationary phase for the separation of racemates in gas chromatography (GC). A CMS‐coated capillary column was fabricated by a dynamic coating method. Eighteen racemates belonging to different classes of organic compounds were separated on this column, including chiral alcohols, aldehydes, ketones, organic acids, halohydrocarbons, alkenes, alcohol amines, epoxides, and amino acid derivatives. In addition, linear alkanes, alcohols, and aromatic hydrocarbons have also been resolved.     

Chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether for enantiomer separation of amino compounds using a normal mobile phase

In order to apply the excellent chiral recognition ability of chiral pseudo-18-crown-6 ethers that we developed to chiral separation, we prepared a chiral stationary phase (CSP) by immobilizing a chiral pseudo-18-crown-6-type host on 3-aminopropyl silica gel. A chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. A liquid chromatography system using this CSP combined with the detection by mass spectrometry was used for enantiomer separation of amino compounds. A normal mobile phase can be used on this CSP as opposed to conventional dynamic coating-type CSPs. Enantiomers of 18 common natural amino acids were efficiently separated. The chiral separation observed for amino acid methyl esters, amino alcohols, and lipophilic amines was fair using this HPLC system. In view of the correlation between the enantiomer selectivity observed in chromatography and the complexion in solution, the chiral recognition in host-guest interactions might contribute to this enantiomer separation.     

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.     

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.     

Enantioseparation by HPLC using phenylcarbonate, benzoylformate, p-toluenesulfonylcarbamate, and benzoylcarbamates of cellulose and amylose as chiral stationary phases

Phenylcarbonate, benzoylformate, and p-toluenesulfonylcarbamate of cellulose and five new benzoylcarbamate derivatives of both cellulose and amylose were synthesized and their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Cellulose benzoylcarbamate has a higher chiral recognition ability compared to phenylcarbonate, p-toluenesulfonylcarbamate, and benzoylformate of cellulose. The benzoylcarbamate derivatives exhibited a characteristic chiral recognition for the racemates, which bear a hydrogen atom capable of hydrogen bonding to the carbonyl group of the benzoylcarbamates. The structures of the benzoylcarbamates were investigated by CD spectroscopy.