A study of chiral recognition for NBD-derivatives on a Pirkle-type chiral stationary phase.

A chiral stationary phase (CSP 1) derived from an (S)-N-3,5-dinitrobenzoyl-1-naphthylglycine showed excellent enantiomeric separation for amino acid derivatives with a fluorogenic reagent, 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), in high-performance liquid chromatography (HPLC). We compared elution profiles (separation factor and elution order) of NBD-amino acids and their analogs on HPLC, to determine the diastereomeric complex between the chiral moiety of CSP 1 and NBD-amino acid, which is responsible for the chiral recognition. (1)H-NMR studies of a mixture of model compound of CSP 1 and NBD-Ala suggest that the diastereomeric complex is composed of two hydrogen bonding sites at the amino proton and oxygen atom, and a pi-pi interaction by the benzofurazan structure (2,1,3-benzoxadiazole) of NBD-amino acid. Furthermore CSP 1 was able to separate esters, amides and alpha-methyl amino acids derivatized with NBD-F.     

Chiral recognition model for the resolution of ephedrine and related alpha,beta-aminoalcohols as enantiomeric oxazolidine derivatives

The mechanism of chiral recognition has been investigated for a series of enantiomeric cis-oxazolidines on a commercially available high-performance liquid chromatographic chiral stationary phase (HPLC-CSP). The oxazolidine molecules were synthesized through the condensation of ephedrine and ephedrine-related molecules with aromatic aldehydes. The resulting molecules are rigid five-membered rings whose configuration has been determined by proton magnetic resonance and single-crystal X-ray diffraction. The oxazolidines derived from the condensation of ephedrine and aldehydes containing a pi-basic moiety such as naphthaldehyde were resolved on the HPLC-CSP as were those oxazolidines synthesized by using a pi-acidic aldehyde such as p-nitrobenzaldehyde. However, there was a reversal in the elution order for the two types of oxazolidines. Oxazolidines resulting from the condensation of ephedrine and a pi-neutral aldehyde such as benzaldehyde were not resolved. The results of this study suggest a chiral recognition model based on the formation of diastereomeric solute-CSP complexes through a single attractive interaction and chiral discrimination resulting from the difference in steric fit.     

Comparison of the chiral resolution on cis-trans isomeric chiral stationary phases derived from (S)-1-(1-naphthyl)ethylamine

A pair of cis-trans isomeric chiral stationary phases (CSPs) derived from (S)-1-(1-naphtyl)ethylamine was prepared. The chromatographic behaviours on both CSPs with regard to the resolution of enantiomeric amino acids, amino alcohols, amines, and carboxylic acid were studied. According to separation factors, the trans-CSP showed better chiral recognition ability for the separation of most analytes chosen in this study. Three homologous series of the alkyl esters of racemic amino acids were resolved on both CSPs using n-hexane-2-propanol and n-hexane-dichloromethane as mobile phases. The trans-CSP also showed better enantioselectivity for the resolution of homologues. A reverse of elution order was observed for the resolution of the homologous series of phenylglycine alkyl esters on both CSPs. It was found that the relationship between the separation factor and the alkyl chain length of the ester homologous series depended upon the components of mobile phase. A higher magnitude of difference between the two CSPs in enantioselectivity for the resolution of a given homologue was obtained when n-hexane-dichloromethane was used as a mobile phase. A chiral recognition process, in which steric repulsion, face-to-face π-π interaction, face-to-edge π-π interaction and hydrogen bonding interaction were involved, was also suggested to describe the separation of enantiomeric homologues on both CSPs. This study clearly indicates that the chiral resolution is influenced by the geometry of the double bond in a CSP.     

流动相对百树菊酯高效液相色谱手性分离的影响

在一种由N－3,5二硝基苯甲酸苯基甘氨酸衍生而成的Pirkle1-A型手性固定相上分离百树菊酯的8个立体异构体,通过选用不同的醇作为流动相的强溶剂,研究二元体系中流动相组分对保留值和选择性的影响。结果表明：对同一种组分的流动相,增加流动相的强度会缩短溶质的保留时间,且对非对映体的选择性影响比较大,而对对映体的选择性影响却不大,作为强溶剂的醇的结构不同会对异构体的分高度及峰形产生影响,二级醇或三级醇作强溶剂时的分高度好于一级醇,小分子醇作强溶剂时的峰形好于较大分子醇。     

Molecular Recognition and Enantiomer Separations on a novel chiral stationary phase based on a 9,9′-spirobi[9H-fluorene]-derived molecular cleft

An optically active molecular cleft incorporating a 9,9′-spirobi[9H-fluorene] spacer and two N-(5,7-dimethyl-1,8-naphthyridin-2-yl)carboxamide: (CONH(naphthyr)) moieties as H-bonding sites was covalently bound to silica gel to provide the new chiral stationary phase (CSP) (R)- 16 (Scheme 2). Previous solution-binding studies in CDCl₃ had shown that the anchored molecular cleft was capable of complexing optically active dicarboxylic acids with differences in free energy of the formed diastereoisomeric complexes (Δ(ΔG⁰)) between 0.5 and 1.6 kcal mol^?1 (T = 300 K). The optical resolution of racemic dicarboxylic acids, that are bound with a high degree of enantioselectivity in the liquid phase, was now achieved by HPLC on the CSP (R)- 16. The order of enantiomer elution was as predicted from the solution studies, and the separation factor α varied between 1.18 and 1.24. A series of 1,1′-binaphthalene-2,2′-diol derivatives were also resolved on the new CSP, in some cases with baseline separation. The order of enantiomer elution under normal-phase chromatographic conditions was rationalized by computer modeling of the association between the solute enantiomers and the immobilized molecular cleft. HPLC Separations with eluents of different polarity suggested that the attractive interactions between solute and immobilized chiral selector are a combination of H-bonding, which prevails in apolar eluents, and aromatic π--π stacking, which dominates in polar eluents.     

α-苯乙醇对映体在涂敷三苯甲酸纤维素酯手性固定相上的高效液相色谱法分离──洗脱液中醇的影响

在涂敷质量分数为15％的三苯甲酸纤维素酯手性柱上,考察了洗脱液中乙醇、正丙醇、异丙醇、正丁醇体积分数在色谱分离对映体性能方面的影响。初步认为在分离过程中,洗脱液中的醇与固定相的C＝O形成氢键作用,该作用和对映体与固定相的C＝O形成的氢键作用相竞争;洗脱液中醇的结构不同之所以会影响对映体的分离效果,与洗脱液中醇改变固定相中手性空穴的立体环境有关,醇的结构不同,造成固定相中手性空穴的立体环境不同。     

α-苯乙醇对映体在涂敷三苯甲酸纤维素酯手性固定相上的高效液相色谱法分离──洗脱液中醇的影响

 在涂敷质量分数为15％的三苯甲酸纤维素酯手性柱上,考察了洗脱液中乙醇、正丙醇、异丙醇、正丁醇体积分数在色谱分离对映体性能方面的影响。初步认为在分离过程中,洗脱液中的醇与固定相的C＝O形成氢键作用,该作用和对映体与固定相的C＝O形成的氢键作用相竞争;洗脱液中醇的结构不同之所以会影响对映体的分离效果,与洗脱液中醇改变固定相中手性空穴的立体环境有关,醇的结构不同,造成固定相中手性空穴的立体环境不同。     

Preparation and evaluation of new Pirkle type chiral stationary phases with long alkyl chains for the separation of amino acid enantiomers derivatized with NBD-F.

In order to improve the high-performance liquid chromatographic separation of alpha-amino acids derivatized with the fluorogenic reagent 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) on commercially available chiral stationary phases (CSPs) such as SUMICHIRAL OA-2500(S) (CSP 1) and OA-4700 (CSP 3), the preparation of two new CSPs (CSP 2 and CSP 4) having 11-aminoundecanoic acid between the aminopropyl silica gel support and the chiral moiety in CSP 1 and CSP 3 is described. CSP 2 and CSP 4 improved both the mutual and enantiomeric separation of NBD-amino acids compared with CSP 1 and CSP 3. Thus, 17 pairs of NBD-amino acid enantiomers and NBD-glycine were separated on CSP 2 except for six NBD-amino acids (D-Asn, D-Ser, D-Gln, L-Pro, L-Ser and Gly). CSP 2 and CSP 4 also showed better enantiomeric separation of NBD-amino acid esters and amides than CSP 1 and CSP 3. It was considered that the achiral long alkyl chains in the CSPs might form a hydrophobic space which assisted the stereoselective interaction of analytes with the chiral moiety by changing the environment around the chiral moiety. On CSP 1 and CSP 2, NBD-beta-amino acid was also enantiomerically separated.     

Convenient synthesis of pi-acceptor chiral stationary phases for high-performance liquid chromatography from halogen-substituted 3,5-dinitrobenzoylamides

A convenient method for the "in column" synthesis of chiral stationary phases for high-performance liquid chromatography was elaborated. It involves preparation of chiral amides of 2-bromo- or 4-chloro-substituted 3,5-dinitrobenzoic acids followed by nucleophilic substitution of the halogen in the aromatic moiety with 3-aminopropyl groups of silanized silica gel at ambient temperature. A series of pi-donor compounds, such as amides and alkyl aryl carbinols, were chromatographed on the prepared chiral stationary phases. The results were compared with data reported for chiral separations of the same substrates on similar (R)-N-(3,5-dinitrobenzoyl)-alpha-phenylglycine-derived CSP. An example of indirect enantioseparation of racemic alpha-phenylethylamine was also described using (R)-2-(2-bromo-3,5-dinitrobenzoylamino)-2-phenylethanol as a chiral derivatizing reagent.     

测定手性有机酸对映体纯度的新试剂

用NMR技术测定微量手性化合物的对映体纯度,已有多种方法。其中,加手性镧化物位移试剂法适用面最广,但对有机酸,结果常不理想。另一种使用较广的方法是加手性试剂,将样品中两对映体转变成非对映异构体,然后比较非对映基因NMR信号的强度,确定原样品中对映体的组成比例。国外已出售的手性试     

Effect of the residual silanol group protection on the liquid chromatographic resolution of racemic primary amino compounds on a chiral stationary phase based on optically active (3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6

A liquid chromatographic chiral stationary phase (CSP) based on (3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6, which has been utilized in the resolution of alpha-amino acids, amines and amino alcohols, was treated with excess of n-octyltriethoxysilane to prepare a new improved CSP. The residual silanol groups of the original CSP were protected by n-octyl groups in the new CSP. The chiral recognition ability of the new CSP was superior to that of the original CSP in the resolution of alpha-amino acids, amines and amino alcohols. Retention factors (k1) for the resolution of alpha-amino acids were lower on the new CSP than on the original CSP while those for the resolution of amines and amino alcohols were higher on the new CSP than on the original CSP. The improved chiral recognition ability of the new CSP and the retention behaviors of the two enantiomers on the new CSP have been rationalized to stem from the removal of the non-enantioselective interactions between the analytes and the residual silanol groups of the original CSP and the improved lipophilicity of the CSP.     

Novel urea-linked cinchona-calixarene hybrid-type receptors for efficient chromatographic enantiomer separation of carbamate-protected cyclic amino acids

Two novel diastereomeric cinchona-calixarene hybrid-type receptors (SOs) were synthesized by inter-linking 9-amino(9-deoxy)-quinine (AQN)/9-amino(9-deoxy)-epiquinine (eAQN) and a calix[4]arene scaffold via an urea functional unit. Silica-supported chiral stationary phases (CSPs) derived from these SOs revealed, for N-protected amino acids, complementary chiral recognition profiles in terms of elution order and substrate specificity. The AQN-derived CSP showed narrow-scoped enantioselectivity for open-chained amino acids bearing pi-acidic aromatic protecting groups, preferentially binding the (S)-enantiomers. In contrast, the eAQN congener exhibited broad chiral recognition capacity for open-chained as well as cyclic amino acids, and preferential binding of the (R)-enantiomers. Exceedingly strong retention due to nonenantioselective hydrophobic analyte-calixarene interactions observed with hydro-organic mobile phases could be largely suppressed with organic mobile phases containing small amounts of acetic acid as acidic modifier. With the eAQN-calixarene hybrid-type CSP particularly high levels of enantioselectivity could be achieved for tert-butoxycarbonyl (Boc)-, benzyloxycarbonyl (Z)- and fluorenylmethoxycarbonyl (Fmoc)-protected cyclic amino acids using chloroform as mobile phase, e.g. an enantioselectivty factor alpha >5.0 for Boc-proline. Increasing amounts of acetic acid compromised enantioselectivity, indicating the crucial contributions of hydrogen bonding to chiral recognition. Comparison of the performance characteristics of the urea-linked eAQN-calixarene hybrid-type CSP with those of structurally closely related mutants provided evidence for the active involvement of the urea and calixarene units in the chiral recognition process. The urea linker motif was shown to contribute to analyte binding via multiple hydrogen bonding interactions, while the calixarene module is believed to support stereodiscrimination by enhancing the shape complementarity of the SO binding site.     

Evaluation of the enantiomeric selectivity in the chiral ligand-exchange chromatography of amino acids by a computational model

The chromatographic resolution of enantiomeric amino acids is accomplished on a reversed phase column using aqueous mobile phase containing the chiral reagent N,N-dimethyl-S-phenylalanine-Cu(II). The separation is a result of the whole interaction between the diastereomeric complex surface and the mixed stationary phase realized by the dynamic coating of the RP-18 carbon chains layer. The elution order seems to be related to the different water coordination capability on copper ion in the formation of the mixed ternary complexes.     

Chromatography of B prostaglandins on beta-cyclodextrin silica: application to analysis of major E prostaglandins in human seminal fluid

A new chiral stationary phase (CSP) was developed for the direct optical resolution of enantiomeric amino acid derivatives. The CSP was readily prepared by a three-step reaction carried out in a pre-packed aminopropylsilyl silica gel column. In the first step, a solution of disuccinimido carbonate (DSC) was delivered through the pre-packed column to give a succinimido carbamyl aminopropylsilyl-bonded, activated-carbamate type silica gel (ACsil) column. Through the column was then delivered a solution of pentaethylenehexamine to afford a polyamine-bonded column. Finally, a solution of optically active succinimido (S)- or (R)-naphthylethyl carbamate was delivered through the polyamine column, to give a naphthylethylurea multiple-bonded CSP. p-Bromophenylcarbamyl derivatives of enantiomeric protein amino acids were resolved on these CSPs by elution with an aqueous mobile phase. Simultaneous analysis of these amino acid enantiomers by means of gradient elution was also accomplished.     

Unprecedented chiral molecular recognition in a C3-symmetric environment.

The enantiomeric recognition of alpha-chiral primary ammonium ions has been studied with benzene-based tripodal tris(oxazoline) receptors. Contrary to the literature and our expectation, a good level of chiral discrimination is observed with one of the tripodal receptors, which provides a C3-symmetric chiral environment on guest binding. The chiral discrimination has been found to be general in the case of alpha-aryl substituted guests, suggesting pi-pi interactions as an important factor. This result raises a question with respect to the origin of the chiral discrimination since little steric or electronic difference is expected between the diastereomeric inclusion complexes. Binding studies by NMR titration and isothermal titration calorimetry show that the chiral discrimination results from the different thermodynamic stabilities between the diastereomeric complexes and that the host-guest complex formation is driven by favorable enthalpy changes with a minor negative contribution by entropy changes. The X-ray crystal structures for both of the diastereomeric inclusion complexes are resolved, which unambiguously show the binding mode and provide clues on the origin of the chiral discrimination. Bond angle analyses indicate that the minor complex experiences a larger steric strain, which is discernible when it is viewed from "three-body" interactions between the host and the guest. The guest and oxazoline phenyl rings are well stacked, indicating interplay of the pi-pi interactions. The pi-pi interactions are believed to stabilize host-guest complexes, thereby endowing the highly flexible receptors with a substantial enantio-discrimination.     

Enantiomeric impurities in chiral synthons,catalysts, and auxiliaries: Part 3

The enantiomeric excess of chiral reagents used in asymmetric syntheses directly affects the reaction selectivity and product purity. In this work, 84 of the more recently available chiral compounds were evaluated to determine their actual enantiomeric composition. These compounds are widely used in asymmetric syntheses as chiral synthons, catalysts, and auxiliaries. These include chiral alcohols, amines, amino alcohols, amides, carboxylic acids, epoxides, esters, ketones, and oxolanes among other classes of compounds. All enantiomeric test results were categorized within five impurity levels (i.e., <0.01%, 0.01–0.1%, 0.1–1%, 1–10%, and >10%). The majority of the reagents tested were determined to have enantiomeric impurities over 0.01%, and two of them were found to contain enantiomeric impurities exceeding the 10% level. The most effective enantioselective analysis method was a GC approach using a Chiraldex GTA chiral stationary phase (CSP). This method worked exceedingly well with chiral amines and alcohols.     

Chiral separations by host-guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis

Summary Capillary zone electrophoresis using cyclodextrins and a chiral crown ether as buffer constituents was studied for the enantiomeric separation of drugs and amino acids. Based on results obtained from separation of racemic -amino acids both chiral selectors are compared with respect to resolution, efficiency and retention time. For (±)-Quinagolide effects of buffer composition and temperature are examined using -cyclodextrin as chiral agent. Optimum conditions were pH 2.5 at 30 mmol L^–1 -cyclodextrin. A linear dependence of retention on -cyclodextrin concentration allowed calculation of formation constants of the host-guest complexes. Buffer concentration and temperature also influence resolution. The application of a chiral crown ether to the separation of optical isomers in capillary zone electrophoresis is described for the first time. Chiral recognition of solutes depends on the formation of protonated alkyl amines and separation is attributed to the formation of diastereomeric host-guest complexes with different interactions for each enantiomer. The effects of crown ether concentration on resolution are presented.     

The use of MPA amide for the assignment of absolute configuration of a sterically hindered cyclic secondary amine by 'mix and shake' NMR method

We present here a new method using methoxyphenylacetic acid (MPA) as the chiral derivatizing agent (CDA) for the assignment of absolute configuration of cyclic secondary amines. The MPA amides were prepared using the purification-free 'mix and shake' method. A detailed conformational analysis for the two diastereomeric amides was conducted by 2D NMR experiments and molecular mechanics calculations. We have established that, in the most stable conformation of each syn rotamer of MPA amides, the H-alpha in the MPA moiety is oriented toward the bulky substituent group at the asymmetric carbon in the chiral amine, presumably to avoid steric and/or electrostatic interactions. The observed NMR data were correlated with the conformational model to allow unambiguous assignment of absolute configuration of secondary amines. The results demonstrate that the MPA can be used as a useful CDA in the case of sterically crowded cyclic secondary amines from which the MTPA amides are usually difficult to make.     

Development of novel brush-type chiral stationary phases based on terpenoid selectors: HPLC evaluation and theoretical investigation of enantioselective binding interactions

The terpenoid chiral selectors dehydroabietic acid, 12,14-dinitrodehydroabietic acid and friedelin have been covalently linked to silica gel yielding three chiral stationary phases CSP 1, CSP 2 and CSP 3, respectively. The enantiodiscriminating capability of each one of these phases was evaluated by HPLC with four families of chiral aromatic compounds composed of alcohols, amines, phenylalanine and tryptophan amino acid derivatives and β-lactams. The CSP 3 phase, containing a selector with a large friedelane backbone is particularly suitable for resolving free alcohols and their derivatives bearing fluorine substituents, while CSP 2 with a dehydroabietic architecture is the only phase that efficiently discriminates 1,1′-binaphthol atropisomers. CSP 3 also gives efficient resolution of the free amines. All three phases resolve well the racemates of N-trifluoracetyl and N-3,5-dinitrobenzoyl phenylalanine amino acid ester derivatives. Good enantioseparation of β-lactams and N-benzoyl tryptophan amino acid derivatives was achieved on CSP 1.In order to understand the structural factors that govern the chiral molecular recognition ability of these phases, molecular dynamics simulations were carried out in the gas phase with binary diastereomeric complexes formed by the selectors of CSP 1 and CSP 2 and several amino acid derivatives. Decomposition of molecular mechanics energies shows that van der Waals interactions dominate the formation of the diastereomeric transient complexes while the electrostatic binding interactions are primarily responsible for the enantioselective binding of the (R)- and (S)-analytes. Analysis of the hydrogen bonds shows that electrostatic interactions are mainly associated with the formation of N–H⋯OC enantioselective hydrogen bonds between the amide binding sites from the selectors and the carbonyl groups of the analytes. The role of mobile phase polarity, a mixture of n-hexane and propan-2-ol in different ratios, was also evaluated through molecular dynamics simulations in explicit solvent.     

High-throughput purification of combinatorial libraries II: Automated separation of single diastereomers from a 4-amido-pyrrolidone library containing intentional diastereomer pairs

A 4-amido-pyrrolidone library that was intentionally synthesized as pairs of diastereomers was produced by solution-phase parallel syntheses and purified by an automated high-throughput purification system. A total of 2592 4-amido-pyrrolidinones were ultimately isolated as single diastereomers from a matrix of 1920 syntheses. After the four-step synthesis and HPLC purification, the average yield of a single diastereomer was 36.6%. The average chemical purity was >90%, and the average diastereomeric purity was >87%. The choice of chiral amines used to make amides with heterocyclic acid chlorides had a dramatic effect on success. Analysis of the relationship between amines used for synthesis and the diastereomeric separation showed that amides made from chiral 1,2-amino alcohols gave superior separation to amides from chiral morpholines. The presence of a hydrogen bond donor on the amide side chain seems to be required for a better diastereomeric separation.