Dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins as chiral stationary phases for high-performance liquid chromatography

New dichloro-, dimethyl-, and chloromethylphenylcarbamate derivatives of cyclodextrins (CDs) were prepared and their enantiomeric recognition abilities were evaluated as chiral stationary phases (CSPs) in normal phase high-performance liquid chromatography (HPLC). The effects of the type of cyclodextrins, the nature and position of the substituents on the phenyl ring, binding mode and spacer on the chiral recognition were studied in detail. No marked change of chiral recognition abilities was established by reversing the binding side of CDs (i.e., by the narrower [primary] opening of the cone-shaped CD to silica gel with the wider [secondary] opening sides). This result indirectly proves the previously drawn conclusion about the minor role of inclusion phenomena in chiral recognition in this case. Nevertheless, chiral recognition of these CSPs toward some compounds critically depends on the type of CDs used. All CD derivatives described in this study show rather low enantiomeric resolving abilities compared with corresponding polysaccharide (cellulose and amylose) derivatives, although very high enantioselectivity of separation was observed for a few compounds, such as racemic flavanone and cyclopropanedicarboxilic acid dianilide. © 1996 Wiley-Liss, Inc.     

Enantioselective host-guest complexation of Ru(II) trisdiimine complexes using neutral and anionic derivatized cyclodextrins

Enantioselective host-guest complexation between five racemic Ru(II) trisdiimine complexes and eight derivatized cyclodextrins (CDs) has been examined by NMR techniques. The appearance of non-equivalent complexation-induced shifts of between the Δ and Λ-enantionomers of the Ru(II) trisdiimine complexes and derivatized CDs is readily observed by NMR. In particular, sulfobutyl ether-β-cyclodextrin sodium salt (SBE-β-CD), R-naphtylethyl carbamate β-cyclodextrin (RN-β-CD), and S-naphtylethyl carbamate β-cyclodextrin (SN-β-CD) showed good enantiodiscrimination for all five Ru complexes examined, which indicates that aromatic and anionic derivatizing groups are beneficial for chiral recognition. The complexation stoichiometry between SBE-β-CD and [Ru(phen)₃]²⁺ was found to be 1:1 and binding constants reveal that Λ-[Ru(phen)₃]²⁺ binds more strongly to SBE-β-CD than the Δ-enantiomer. Correlations between this NMR method and separative techniques based on CDs as chiral discriminating agents (i.e., selectors) are discussed in detail.     

Racemization and hydrolysis of tropic acid alkaloids in the presence of cyclodextrins

Because of the constantly increasing demand for optically pure drugs it is of great importance to elucidate factors affecting stereochemistry, in order to provide a stable formulation with a high chiral quality of the desired isomer. Therefore, the effects of cyclodextrins (CyDs) and their alkylated and hydroxyalkylated derivatives on racemization and hydrolysis of (?)-(S)-hyoscyamine and (?)-(S)-scopolamine were examined kinetically and spectroscopically (NMR). Direct methods, based on a chiral and achiral chromatographic phase system, were used to determine their degradation products and enantiomer composition during stability tests. All different CyDs, except α-CyD, retarded racemization and hydrolysis. The inclusion of the drug substances in CyDs inhibits the attack of hydroxyl ions and/or water molecules and thus retards the racemization and hydrolysis. The racemization of the tropic acid alkaloids is dependent on the pH and temperature. NMR studies were used to evidence the formation of a soluble 1:1 complex in aqueous solution. © 1993 Wiley-Liss, Inc.     

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

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

NMR chiral discrimination of chalcogen containing secondary alcohols

Here, we report the general strategies by which NMR spectroscopy can be used to determine the enantiopurity and absolute configuration of chalcogen containing secondary alcohols, including the evaluation of the use of chiral solvating and chiral derivatizing agents. The BINOL/DMAP ternary complex demonstrated a simple and fast protocol for determining enantiopurity. The drug Naproxen afforded a stable, nonhygroscopic, and readily available chiral derivatizing agent (CDA) for NMR chiral discrimination of chalcogen containing secondary alcohols. The chiral recognition by CDA and chiral solvating agent (CSA) was assessed using ¹H, ⁷⁷Se‐{1H}, and ¹²⁵Te‐{1H} NMR spectroscopy. A simple model for the assignment of the absolute configuration from NMR data is presented.     

Emerging subject for chiral separation science: cluster boron compounds

The structural chirality is an inherent feature of fully synthetic boron cluster compounds that sometimes exhibit unique biochemical effects. HPLC studies with zwitter-ionic cluster boron compounds and electrophoretic studies with boron cluster anions reveal that the chiral separability of these species is remarkably dissimilar to that of organic species, if uncharged cyclodextrins are used as chiral selectors. Furthermore, marked differences were found between the analytical characteristics of the chiral separations of the boron cluster species and those of the organic species with uncharged cyclodextrins. The present-day experimental database indicates that the rules valid for the chiral separations of the organic species cannot be applied to the chiral separations of the boron cluster species without experimental verification. The current extent of research work devoted to the investigation of chirality and chiral separations of boron cluster species is negligibly small in comparison with that devoted to the investigation of chirality and chiral separations of organic species. This makes difficult a reliable explanation of both the particularities observed in chiral separations of boron cluster species with cyclodextrins as chiral selectors and the strange effects related to these separations at the moment.     

NMR studies of chiral discrimination relevant to the enantioseparation of N-acylarylalkylamines by an (R)-phenylglycinol-derived chiral selector

Recently, it was reported that the chiral recognition ability of (R)-N-3,5-dinitrobenzoyl phenylglycinol derivative was examined as a new HPLC chiral stationary phase (CSP 1) for the resolution of racemic N-acylnaphthylalkylamines. However, the mechanism of chiral discrimination on the CSP remained elusive until now. In this study, a spectroscopic investigation of the chiral discrimination mechanism of CSP 1 was undertaken using mixtures of (R)-N-3,5-dinitrobenzoyl phenylglycinol-derived chiral selector (2) and each of the enantiomers of N-acylnaphthylalkylamines (3) by NMR study. First, the differences in free energy changes (DeltaDeltaG) upon diastereomeric complexation in solution between the complex of each isomer with chiral selector 2 by NMR titration were calculated. The values were then compared with those estimated by chiral HPLC. The chemical shift changes of each proton on the chiral selector and analytes were also checked and it was found that the chemical shift changes decreased continuously as the acyl group on analytes increased in length. This observation was consistent with the HPLC data. From these experimental results, the interaction mechanism of chiral discrimination between the chiral selector and the analytes is more precisely explained.     

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.     

Macrocycle conformation and self-inclusion phenomena in octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E) by NMR spectroscopy and molecular dynamics

Octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E) is a lipophilic chiral selector successfully used for the enantioselective gas chromatographic separation of a multitude of racemic analytes. NMR data (13C chemical shifts, 3J(HH), rotating frame NOEs (ROEs)) and molecular dynamics (MD) simulations point out that the macrocycle is distorted with respect to the canonical truncated-cone shape of native cyclodextrins, although C(8) symmetry is retained on the NMR timescale. ROE data and MD trajectories provide evidence for self-inclusion of one 6-O-pentyl pendant chain within the cavity of Lipodex E. The interpretation of long-range and low-intensity ROEs is supported by the calculation of average internuclear distances by using the radial distribution function (RDF) calculated from MD trajectories. MD simulations are eventually used to compare the flexibility of the macrocycle of Lipodex E with that of native gammaCD.     

Preparation and enantioseparation characteristics of two chiral stationary phases based on mono(6(A)-azido-6(A)-deoxy)-perphenylcarbamoylated alpha- and gamma-cyclodextrin

Two chiral stationary phases, ph-alpha-CD and ph-gamma-CD, were prepared from mono(6(A)-azido-6(A)-deoxy)perphenylcarbamoylated alpha- and gamma-cyclodextrin immobilized onto silica gel via the Staudinger reaction. The chromatographic characteristics of these two chiral stationary phases were evaluated. The influence of different cyclodextrins (CDs) on the enantioselectivities was also investigated in this study. Compared to ph-gamma-CD, ph-alpha-CD exhibited quite good enantioselectivity toward the analytes with bulky molecular structures. It was found that the formation of inclusion complex might play a quite important role in the chiral recognition not only under reverse phases but also under normal phases.     

Role of capillary electrophoresis methods in the drug development process

In the past several years, capillary electrophoresis (CE) has generated considerable interest from pharmaceutical companies for control of both the chiral and achiral purity of bulk drugs and drug products. This paper evaluates the use of CE as: (1) a technique complementary to HPLC for the determination of peak homogeneity of a drug, (2) for determination of chiral purity, and (3) for determination of achiral purity. It would be greatly advantageous if CE could be used to determine both the chiral and achiral purity in a single assay. This investigation compares the results obtained for the separation of the enantiomers of duloxetine using several neutral cyclodextrins to those obtained using anionic cyclodextrins (sulfobutyl ether derivatives) as chiral selectors added to the separation buffer. In addition, it reports chiral separations obtained by using neutral cyclodextrins in a sulfonic acid-coated capillary column, which give a negatively charged capillary surface and electro-osmotic flow even in low pH buffers. The possible mechanism of separation is discussed. © 1996 Wiley-Liss, Inc.     

Chromatographic enantioseparation by cycloalkylcarbamate derivatives of cellulose and amylose

Cyclopentyl and (+/-)-exo-2-norbornylcarbamates of cellulose and amylose were prepared and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography (HPLC) were evaluated. Among these carbamates, cellulose tris(cyclopentylcarbamate) and amylose tris((+/-)-exo-2-norbornylcarbamate) showed particularly high chiral recognition, which is comparable to that of several well-known phenylcarbamate derivatives. The chiral recognition mechanism of cellulose tris(cyclohexylcarbamate), which was previously found to be an effective chiral stationary phase for HPLC, was investigated using NMR spectroscopy. The derivative dissolved in chloroform exhibited the chiral discrimination of several enantiomers in NMR as well as in HPLC. For example, the 1,1'-bi-2-naphthol enantiomers were distinctly discriminated in the (1)H, (13)C, and 2D-NOESY spectra.     

Chiral discrimination by NMR spectroscopy of ephedrine and N-methylephedrine induced by β-cyclodextrin,heptakis(2,3-di-O-acetyl)β-cyclodextrin,and heptakis (6-O-acetyl)β-cyclodextrin

NMR spectroxcopy has been used to compare the interaction of ephedrine and N-methylephedrine with β-cyclodextrin, heptakis(2,3-di-O-acetyl)β-cyclodextrin, heptakis(6-O-acetyl)β-cyclodextrin. The stoichiometry of the complexes formed between all three cyclodextrins and N-methylephedrine was found to be 1:1 by UV spectroscopy by means of the Job technique. NMR spectra of the single enantiomers of ephedrine and N-methylephedrine in the presence of all three cyclodextrins gave information about the parts of the ligands which interact differently with the host molecules and may be responsible for the chiral discrimination. To quantify the complex stabilities, binding constants were calculated from the changes in the chemical shifts of the ligand signals upon complexation. Analyses of the coupling constants of both species showed that no significant conformational change occurs upon complexation. ROESY spectra of these optical isomers with all three cyclodextrins provided detailed information about the geometry of the complexes. Different intermolecular cross-peaks between the individual isomers of ephedrine and N-Methylephedrine were found for native β-cyclodextrin and its 2,3-diacetylated derivative but not for 6-acetyl cyclodextrin. Analyses of the intramolecular cross-signals of the ligands confirmed that no significant conformational change occurs upon complexation. Chirality 9:211–219, 1997. © 1997 Wiley-Liss, Inc.     

Cyclomaltooligosaccharide (cyclodextrin)-assisted enantiomeric recognition of benzo[lmn][3,8]phenanthroline-derived amino acids

Formation of self-assembly molecular aggregates and cyclomaltooligosaccharide (cyclodextrin) molecular aggregates with benzo[lmn][3,8]phenanthroline-derived amino acids is presented. The nature of the molecular aggregates was studied by negative-ion electrospray-ionization mass spectrometry (ESIMS). The enantiomeric recognition was demonstrated by NMR enantiomeric discrimination of the amino acid derivatives in aqueous solutions of cyclodextrins.     

Carboxymethylated cyclodextrins and their complexes with Pr(III) and Yb(III) as water‐soluble chiral NMR solvating agents for cationic compounds

Cyclodextrins that are indiscriminately carboxymethylated at the 2‐, 3‐, and 6‐positions are used as chiral NMR solvating agents for cationic substrates with phenyl, naphthyl, pyridyl, indoline, and indole rings. Enantiodifferentiation with the α‐, β‐, and γ‐cyclodextrin derivatives is compared. The carboxymethylated derivatives are almost always more effective as chiral NMR solvating agents for cationic substrates than native cyclodextrins. The most effective carboxymethylated cyclodextrin varies for different substrates, and at times even different resonances of the substrate. Addition of paramagnetic praseodymium(III) or ytterbium(III) to mixtures of the carboxymethylated cyclodextrin and substrate often causes enhancements in enantiomeric discrimination and facilitates measurements of enantiomeric purity. The lanthanide ion bonds to the carboxymethyl groups and causes perturbations in the chemical shifts in the NMR spectra of substrate molecules in the cyclodextrin cavity. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Structural background of cyclodextrin-protein interactions

Cyclodextrins are cyclic oligosaccharides with the shape of a hollow truncated cone. Their exterior is hydrophilic and their cavity is hydrophobic, which gives cyclodextrins the ability to accommodate hydrophobic molecules/moieties in the cavity. This special molecular arrangement accounts for the variety of beneficial effects cyclodextrins have on proteins, which is widely used in pharmacological applications. We have studied the interaction between beta-cyclodextrin and four non-carbohydrate-binding model proteins: ubiquitin, chymotrypsin inhibitor 2 (CI2), S6 and insulin SerB9Asp by NMR spectroscopy at varying structural detail. We demonstrate that the interaction of beta-cyclodextrin and our model proteins takes place at specific sites on the protein surface, and that solvent accessibility of those sites is a necessary but not compelling condition for the occurrence of an interaction. If this behaviour can be generalized, it might explain the wide range of different effects of cyclodextrins on different proteins: aggregation suppression (if residues responsible for aggregation are highly solvent accessible), protection against degradation (if point of attack of a protease is sterically 'masked' by cyclodextrin), alteration of function (if residues involved in function are 'masked' by cyclodextrin). The exact effect of cyclodextrins on a given protein will always be related to the particular structure of this protein.     

Synthesis and Utilization of Trialkylammonium‐Substituted Cyclodextrins as Water‐Soluble Chiral NMR Solvating Agents for Anionic Compounds

Cationic trialkylammonium‐substituted α‐, β‐, and γ‐cyclodextrins containing trimethyl‐, triethyl‐, and tri‐n‐propylammonium substituent groups were synthesized and analyzed for utility as water‐soluble chiral nuclear magnetic resonance (NMR) solvating agents. Racemic and enantiomerically pure (3‐chloro‐2‐hydroxypropyl)trimethyl‐, triethyl‐, and tri‐n‐propyl ammonium chloride were synthesized from the corresponding trialkyl amine hydrochloride and either racemic or enantiomerically pure epichlorohydrin. The ammonium salts were then reacted with α‐, β‐, and γ‐cyclodextrins at basic pH to provide the corresponding randomly substituted cationic cyclodextrins. The ¹H NMR spectra of a range of anionic, aromatic compounds was recorded with the cationic cyclodextrins. Cyclodextrins with a single stereochemistry at the hydroxy group on the (2‐hydroxypropyl)trialkylammonium chloride substituent were often but not always more effective than the corresponding cyclodextrin in which the C‐2 position was racemic. In several cases, the larger triethyl or tri‐n‐propyl derivatives were more effective than the corresponding trimethyl derivative at causing enantiomeric differentiation. None of the cyclodextrin derivatives were consistently the most effective for all of the anionic compounds studied. Chirality 28:299–305, 2016. © 2016 Wiley Periodicals, Inc.     

Chiral discrimination of the analgesic cizolirtine by using cyclodextrins: A (1)H NMR study on the solution structures of their host-guest complexes.

The use of four cyclodextrins (three native and one beta-CD derivative) as NMR chiral solvating agents to resolve the enantiomers of (+/-)-cizolirtine, 1, and its chemical precursor (the carbinol, (+/-)-2), was investigated. The best enantiodiscrimination occurred when beta-cyclodextrin was used. ROESY experiments were performed to qualitatively ascertain the most probable host-guest structures in D(2)O solution, and the binding features found were explained in terms of spatial fitting of the guest molecules into the macrocyclic cavities. No geometrical differences were noted between the two diastereomeric complexes formed by a cyclodextrin and a racemic substrate, so the magnetic nonequivalence induced on guest protons by the enantioselective binding had to be explained as a result of subtle disparities in the orientation and/or the conformational state of the complexed enantiomers.     

Comprehensive strategy for chiral separations using sulfated cyclodextrins in capillary electrophoresis

This review focuses on the emerging role of sulfated cyclodextrins in the capillary electrophoretic (CE) separation of chiral analytes. Since being introduced as enantioselective agents for CE in 1995, these anionic additives have continued to demonstrate remarkable application universality. The broad spectrum of chiral compounds successfully separated using this approach includes acidic, basic, neutral, and zwitterionic species. This impressive array of analyte structures is derived from a growing diversity of compound classes including pharmaceuticals, plant extracts, biomarkers, herbicides, alkaloids, fungicides, and metal ions. Moreover, literature reports highlight the minimal optimization required to achieve a successful separation. Based on these findings, sulfated cyclodextrins appear to be well suited for the development of a more universal, comprehensive separation strategy for chiral compounds. This review explores this proposition by beginning with the structure and migration properties of sulfated cyclodextrins, using applications to highlight the separating power of this technique and ending with a pragmatic, comprehensive separation strategy.     

Stereoselective reduction with NADH model BNAH through chiral induction in cyclodextrins

Stereoselective reductive debromination-cyclopropanation of 2-bromo-1-phenylethylidene-malononitrile and 2-bromo-1-beta-naphthylethylidenemalononitrile by coenzyme NADH model BNAH through chiral induction in cyclodextrins is reported. The matching between substrates and cyclodextrins, the substituent effect, and the effect of cyclodextrin concentration on the optical yields have been investigated.