Direct resolution of some phenylglycines by liquid chromatography on a chiral crown ether phase

The optical resolution of a series of 12 amino acids of the phenylglycine family was studied using a chiral crown ether column. The effects of pH, temperature, and mobile phase additive were investigated. In three examples it was confirmed that the R enantiomer eluted prior to the S enantiomer. Most phenylglycines were resolved with large separation factors; those with two functional groups substituents on opposing sides of the phenyl ring, however, were not well separated.     

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.     

Chiral separation of underivatized amino acids in supercritical fluid chromatography with chiral crown ether derived column

The supercritical fluid chromatographic separation of underivatized amino acids was explored using immobilized chiral crown ether column CROWNPAK CR-I (+) and mass spectrometric detection. The type of modifier, acidic additives, and the role of water were investigated. Enantioseparation was achieved for all 18 amino acids investigated with short retention times (less than 3 minutes) and average resolution of greater than 5.0. Analysis of enantiomerically pure standards demonstrated the D enantiomer eluted first for all amino acids using a CROWNPAK CR-I (+) column.     

Enantiomeric separation of five amino acids by high-performance liquid chromatography on a chiral crown ether column

A chiral liquid chromatographic method was validated to analyze the D-and L-enantiomers of five amino acids contained in a commercial solution: aspartic acid, leucine, lysine, phenylalanine, and valine. These 10 compounds were separated on a chiral crown ether column with a mobile phase composed of water adjusted to pH 1.5 with perchloric acid, with ultraviolet detection at 220 nm. The method was applied to the commercial amino acid solution before and after sterilization by 5 kGy irradiation; no stereoconversion was observed following sterilization. Chirality 9:150–152, 1997. © 1997 Wiley-Liss, Inc.     

Liquid chromatographic separation of the enantiomers of fluoroquinolone antibacterials on a chiral stationary phase based on a chiral crown ether

A chiral stationary phase (CSP) recently developed by bonding (diphenyl-substituted 1,1'-binaphthyl) crown ether to silica gel for the liquid chromatographic separation of enantiomers was applied to the resolution of investigational fluoroquinolone antibacterial agents including gemifloxacin (formerly LB20304a). All fluoroquinolone compounds used in this study were resolved quite well on the CSP. Especially, the resolution of gemifloxacin and its analogs on the CSP was excellent and even greater than that on the commercial Crownpak CR(+). The resolution was found to be dependent on the type and the content of organic, acidic, and inorganic modifiers added to the mobile phase and on the column temperature.     

Absolute Configuration and Predominant Conformations of a Chiral Crown Ether‐Based Colorimetric Sensor: A Vibrational Circular Dichroism Spectroscopy and DFT Study of Chiral Recognition

In the present work, we report a comprehensive vibrational circular dichroism (VCD) spectroscopic study of a chiral crown ether which features an axial chiral 3.3'‐diphenyl‐1,1'‐binaphthyl group as chiral moiety. By comparing the experimental and calculated VCD spectra, we show that the presumably very flexible crown ether preferably adopts only one ring conformation. Conformational flexibility is observed in the 2,4‐dinitrophenyl‐diazophenol group, which was previously introduced for colorimetric detection of primary amines and amino alcohols (Cho et al., Chirality 2011;23:349–353). The VCD spectra of the host–guest complexes with phenyl glycinol (PG) and phenyl alaninol have been studied as well. Based on the spectra calculated, it is shown that the diastereomeric complexes in general can be differentiated using VCD spectroscopy. Furthermore, the experimental VCD spectra of the complexes of the host molecule with PG support the above finding. Chirality 25:294–300, 2013. © 2013 Wiley Periodicals, Inc.     

Comparison of Chiral Separations of Aminophosphonic Acids and Their Aminocarboxylic Acid Analogs Using a Crown Ether Column

Crown ethers are capable of complexing with primary amines and have been utilized in chromatography to separate amino acid racemates. This application has been extended to resolve (1‐amino‐1‐phenylmethyl)phosphonic acid and (1‐aminoethyl)phosphonic acid racemates, along with their aminocarboxylic acid analogs (2‐phenylglycine and alanine, respectively), via a ChiroSil RCA crown ether based chiral stationary phase. Effects of the organic modifier, temperature, and acid type and concentration on retention and selectivity were also investigated. Trends in retention and selectivity varied between aminophosponic acids and their aminocarboxylic analogs. Computer modeling and ¹H NMR analyses were performed to potentially gain a better understanding of interactions of the aforementioned molecules with the ChiroSil RCA chiral stationary phase. Theoretical predictions of the most stable conformations for (R)‐ and (S)‐enantiomers were compared to elution order; it was found that the elution order agreed with molecular modeling such that the longest retention correlated with the predicted most stable complex between the enantiomer and crown ether. ¹H NMR demonstrated interactions of aminophosphonic and aminocarboxylic racemates with (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid in solution and was utilized to determine enantiomeric excess of (1‐amino‐1‐phenylmethyl)phosphonic acid after its enantioenrichment via crystallization through diastereomeric salt formation with the crown ether followed by filtration. Chirality 25:369–378, 2013. © 2013 Wiley Periodicals, Inc.     

Synthesis and dopaminergic activity of heterocyclic analogues of 5,6-dihydroxy-2-aminotetralins

The heterocyclic analogues of 5,6-dihydroxy-2-aminotetralins (6) were synthesized and their in vitro dopaminergic activity was compared to that of (-)-DP-5,6-ADTN and the novel potent agonist Z12571. The results show that changing the cathecol ring for a heterocycle decreases the D1-like activity of the target molecules 6. However, the D2-like activity of tetrahydroquinoline (6j) was comparable to that of (-)-DP-5,6-ADTN.     

Liquid chromatographic enantiomer resolution of N-hydrazide derivatives of 2-aryloxypropionic acids on a crown ether derived chiral stationary phase

The liquid chromatographic separation of the enantiomers of several N-hydrazide derivatives of 2-aryloxypropionic acids was performed on a crown ether type chiral stationary phase derived from (18-crown-6)-2,3,11,12-tetracarboxylic acid. The behavior of chromatographic parameters by the change of mobile phases and additives for the resolution of these analytes was investigated. The enantiomers of all analytes were base-line resolved with a mobile phase of 100% methanol containing 20 mM H2SO4. These results are the first reported for enantiomer resolution of chiral acids of 2-aryloxypropionic acids as their N-hydrazide derivatives.     

A colorimetric chiral sensor based on chiral crown ether for the recognition of the two enantiomers of primary amino alcohols and amines

A new colorimetric chiral sensor material consisting of three different functional sites such as chromophore (2,4-dinitrophenylazophenol dye), binding site (crown ether), and chiral barrier (3,3'-diphenyl-1,1'-binaphthyl group) was prepared and applied to the recognition of the two enantiomers of primary amino alcohols and amines. Among five primary amino alcohols and two primary amines tested, the two enantiomers of phenylalaninol show the highest difference in the absorption maximum wavelength (Δλ(max)=43.5 nm) and in the association constants (K(S)/K(R)=2.51) upon complexation with the colorimetric chiral sensor material and, consequently, the two enantiomers of phenylalaninol were clearly distinguished from each other by the color difference.     

Development of HPLC Chiral Stationary Phases Based on (+)‐(18‐Crown‐6)‐2,3,11,12‐tetracarboxylic Acid and Their Applications

Crown ether‐based chiral stationary phases (CSPs) have been known to be useful for the resolution of racemic primary amino compounds. In particular, CSPs based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid have been reported to be useful for the resolution of secondary amino compounds as well as primary amino compounds. In this article, the process of developing various CSPs based on (+)‐(18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid to improve the chiral recognition efficiency and/or the stability of the CSPs and their applications to the resolution of various primary and nonprimary amino compounds are reviewed. Chirality 27:576–588, 2015. © 2015 Wiley Periodicals, Inc.     

Utilization of (18‐Crown‐6)‐2,3,11,12‐tetracarboxylic Acid as a Chiral NMR Solvating Agent for Diamines and β‐Amino Acids

The compound (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid was evaluated as a chiral nuclear magnetic resonance (NMR) solvating agent for a series of diamines and bicyclic β‐amino acids. The amine must be protonated for strong association with the crown ether. An advantage of (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid over many other crown ethers is that it undergoes a neutralization reaction with neutral amines to form the protonated species needed for binding. Twelve primary diamines in neutral and protonated forms were evaluated. Diamines with aryl and aliphatic groups were examined. Some are atropisomers with equivalent amine groups. Others have two nonequivalent amine groups. Association equilibria for these systems are complex, given the potential formation of 2:1, 1:1, and 1:2 crown‐amine complexes and given the various charged species in solution for mixtures of the crown ether with the neutral amine. The crown ether produced enantiomeric differentiation in the ¹H NMR spectrum of one or more resonances for every diamine substrate. Also, a series of five bicyclic β‐amino acids were examined and (18‐crown‐6)‐2,3,11,12‐tetracarboxylic acid caused enantiomeric differentiation in the ¹H NMR spectrum of three or more resonances of each compound. Chirality 27:708–715, 2015. © 2015 Wiley Periodicals, Inc.     

Enantiomeric separation of Novel Psychoactive Substances by capillary electrophoresis using (+)‐18‐crown‐6‐tetracarboxylic acid as chiral selector

In the recent years, hundreds of Novel Psychoactive Substances (NPS) have entered both the European and the global drug market. These drugs, which are mainly used for recreational matters, have caused serious social problems. Every year, the spectrum of these misused drugs is enlarged by new derivatives, which are produced by modifications of basic structures of already well‐known substances. Additionally, a lot of them possess a stereogenic center which leads to 2 enantiomeric forms. The fact that the pharmacological effects and potencies of the enantiomers of these chiral NPS may differ can be assumed from a broad spectrum of active pharmaceutical ingredients. For this reason, analytical method development regarding enantiomeric separation for these classes of substances is of great pharmaceutical and medical interest. The aim of this work was to create an easy‐to‐prepare chiral capillary electrophoresis method for the enantioseparation of NPS which contains a primary amino group by means of (+)‐18‐crown‐6‐tetracarboxylic acid as chiral selector. Novel Psychoactive Substances were purchased at various Internet stores or represent samples seized by Austrian police. The effects of selector concentration, the electrolyte composition, and the addition of organic modifiers to the background electrolyte on enantioseparation were investigated. Under optimized conditions, the use of 20‐mM (+)‐18‐crown‐6‐tetracarboxylic acid, 10‐mM Tris, and 30‐mM citric acid buffer at pH 2.10 turned out to be effective. Fifteen of 24 tested NPS were resolved in their enantiomers within 15 minutes. It was found that all NPS were traded as racemic mixtures.     

Chiral phase partitioning by enantioselective complexation: Characterisation by the semi‐empirical application of regular solution theory

The thermodynamics underlying enantioselective complexation and partitioning behaviour are poorly understood. This paper presents a model that decouples the effects of enantioselective complexation and subsequent diastereomer partitioning. Regular solution theory is applied in a semi‐empirical manner to describe the diastereomer partitioning process, which is reported to be governed by hydrophobic interactions. The model was shown to give a good fit to experimental partitioning for the enantioselective extraction of phenylalanine isomers by two chiral extractants; a modified amino acid [copper (II) N‐decyl‐(L)‐hydroxyproline] and a chiral crown ether [(S)‐bis(phenylnaphtho)‐20‐crown‐6]. A variety of aliphatic and aromatic solvents were tested. The predicted and observed experimental enantioselectivities were found to vary exponentially with the difference in the solubility parameters of the aqueous and organic phases and with those of the two diastereomeric complexes formed. This model provides the basis for a better understanding of enantioselective partitioning effects. Chirality 11:241–248, 1999. © 1999 Wiley‐Liss, Inc.     

Determination of the enantiomeric purity of dihydroxy- and dimethoxy-2-aminotetralins by high-performance capillary electrophoresis with cyclodextrins as chiral selector

A chiral separation of 5,6- and 6,7-dihydroxy-2-aminotetralin and of the corresponding dimethoxy analogues can be achieved by high-performance capillary electrophoresis using cyclodextrins as chiral selector. The selectivity of a number of native and derivatized cyclodextrins was screened and hydroxylalkyl-β-cyclodextrins turned out to be the most effective. Optimization of hydroxyethylcyclodextrin and hydroxypropylcyclodextrin concentration for each compound led to baseline separation (resolution factors from 1.7 to 2.3) resulting in a detection limit of 0.1% for the enantiomer present as impurity. Modifiers of the electroendosmotic flow, such as hydroxypropylcellulose and hexadecyltrimethylammoniun bromide, must be added to the background electrolyte to obtain such results. © 1995 Wiley-Liss, Inc.     

Microscale HPLC enables a new paradigm for commercialization of complex chiral stationary phases

The small column size (0.3 mm i.d. x 15 cm) used in microscale HPLC contains only a small fraction (<1%) of the chromatographic packing material of a typical analytical HPLC column. Consequently, chromatographic stationary phases that are prohibitively expensive in conventional HPLC, owing either to synthetic complexity or costly starting materials, may become commercially viable in the microscale format. To illustrate this point, a previously described, synthetically complex, crown ether chiral stationary phase was prepared and evaluated in the microscale format, showing excellent separation of the enantiomers of underivatized amine analytes.     

Identification of enantioselective extractants for chiral separation of amines and aminoalcohols

The major obstacle for the introduction of fractional reactive extraction as a chiral separation method in the chemical and pharmaceutical industries is the lack of versatile enantioselective extractants. Therefore, a rational approach is developed to transfer the extensive knowledge of chiral selectors reported in the literature on chiral recognition and other chiral separation techniques to extraction. Based on a similarity in separation mechanisms, it was expected that chiral selectors originating from a technique in which chiral recognition takes place in the liquid phase are most likely to function as enantioselective extractant. Using this approach, a selection of promising extractants was made from the literature and experimentally evaluated for the enantioseparation of aminoalcohols and amines. As a result, four enantioselective extractant systems, namely, dibutyl-L-tartrate with boric acid, N-(2-hydroxydodecyl)-L-hydroxyproline Cu(II) complex, N-dodecyl-L-hydroxyproline Cu(II) complex, and azophenolic crown ether, have been identified. The azophenolic crown ether system performed the best and demonstrated an enantioselectivity between 1.3-5.0 for five out of six test compounds. Identification of the enantioselective extractant systems was highly facilitated by the developed rational transfer approach that, although partially qualitative, appeared capable of reducing more than 50 encountered candidates to only three promising systems for further experimental evaluation. Therefore, it is expected that this approach can be successfully applied to identify enantioselective extractants for other classes of enantiomers as well.     

Enantiomeric Discrimination of Isoxazoline Fused β‐amino Acid Derivatives using (18‐Crown‐6)‐2,3,11,12‐tetracarboxylic Acid as a Chiral NMR Solvating Agent

(18‐Crown‐6)‐2,3,11,12‐tetracarboxylic acid is a useful chiral NMR solvating agent for isoxazoline‐fused β‐amino acid derivatives. Isoxazoline substrates are analyzed as their hydrochloride salts in methanol‐d₄. The crown ether and substrate associate through the formation of three hydrogen bonds between the protonated amine and crown ether oxygen atoms. Enantiomeric discrimination is observed for two or more resonances of every substrate. At least one of these resonances is free of overlap with other resonances in the spectrum and has large enough enantiomeric discrimination to enable the determination of enantiomeric purity. 2D COSY methods can be used to identify additional resonances that exhibit enantiomeric discrimination in the NMR spectrum. Chirality, 25:48‐53, 2013.© 2012 Wiley Periodicals, Inc.     

Application of cyanuric chloride-based six new chiral derivatizing reagents having amino acids and amino acid amides as chiral auxiliaries for enantioresolution of proteinogenic amino acids by reversed-phase high-performance liquid chromatography

Six dichloro-s-triazine (DCT) reagents having l-Leu, d-Phg, l-Val, l-Met, l-Ala and l-Met-NH₂ as chiral auxiliaries in cyanuric chloride were introduced for enantioseparation of 13 proteinogenic amino acids. Four other DCTs and six monochloro-s-triazine (MCT) reagents having amino acid amides as chiral auxiliaries were also synthesized. These 16 chiral derivatizing reagents (CDRs) were used for synthesis of diastereomers of all the 13 analytes using microwave irradiation, which were resolved by reversed-phase high-performance liquid chromatography (RP-HPLC) using C18 column and gradient eluting mixture of aqueous TFA and acetonitrile with UV detection at 230 nm. It required only 60–90 s for derivatization using microwave irradiation. Better resolution and lower retention times were observed for the diastereomers prepared with CDRs having amino acids as chiral auxiliaries as compared to counterparts prepared with reagents having amino acid amides as chiral auxiliaries. As the best resolution of all the 13 analytes was observed for their diastereomers prepared using the DCT reagent having l-Leu as chiral auxiliary, this CDR was further employed for derivatization of Lys, Tyr, His and Arg followed by RP-HPLC analysis of resulting diastereomers. The results are discussed in light of acid and amide groups of chiral auxiliaries constituting CDRs, electronegativities of the atoms of achiral moieties constituting CDRs and hydrophobicities of side chains of amino acids constituting CDRs and analytes.     

New chiral fluoroanthryl derivatives: Resolution of the enantiomers by chromatography on cellulose esters and their evaluation as chiral solvating agents in NMR spectroscopy

2,2,2-Trifluoro-1-(9-anthryl)ethanol (TFAE) has now been widely used as a powerful chiral solvating agent for NMR spectroscopy. In connection with the development of a new general synthesis of halogenoalkylalkanols, starting from the corresponding ketone or aldehyde, we synthesized some halogenoalkyl-1-(9-anthryl)methanol derivatives liable to work as chiral solvating agents. The racemic anthryl derivatives were preparatively resolved into their corresponding enantiomers by chromatography on triacetyl cellulose (CTA I) or on meta-methylbenzoyl cellulose beads as chiral stationary phases. Their effectiveness as chiral solvating agents was measured as the magnitude of the splitting induced in the ¹H-NMR spectra of 1-phenylethylamine and of (1-phenylethyl)methyl ether in comparison with splitting caused by TFAE. While TFAE induced the largest splitting for 1-phenylethylamine, 2,2,3,3,3-pentafluoro-1-(9-anthryl)propanol 2 was more effective in the case of (1-phenylethyl)methyl ether, pointing out that depending on the substrate, other derivatives of the TFAE type can be very useful as chiral solvating agents.