Absolute configuration of chiral terpenes in marking pheromones of bumblebees and cuckoo bumblebees

The absolute configurations of citronellol, 2,3-dihydrofarnesol, and 2,3-dihydrofarnesal in male marking pheromones of seven species of bumblebees and cuckoo bumblebees were determined by enantioselective gas chromatography on a capillary column coated with 60% heptakis(2,3-di-O-acetyl-6-O-TBDMS)-beta-cyclodextrin in polysiloxane PS 268. Pure (-)-S-enantiomers of all three terpenes were found in the labial glands of all investigated specimens of the following species: Bombus (Bombus) terrestris, B. (Bombus) lucorum, B. (Pyrobombus) pratorum, B. (Pyrobombus) pyrenaeus, B. (Pyrobombus) jonellus, B. (Pyrobombus) impatiens, and the cuckoo bumblebee B. (Ashtonipsithyrus) bohemicus. Within species, specimens were collected at different localities and in different years. Except for 2,3-dihydrofarnesol in B. terrestris, this is the first report on the absolute configuration of terpenes in marking pheromones of bumblebees.     

A mixed stationary phase containing two versatile cyclodextrin-based selectors for the simultaneous gas chromatographic enantioseparation of racemic alkanes and racemic alpha-amino acid derivatives

In an effort to simultaneously enantioseparate racemic unfunctionalized alkanes and racemic alpha-amino acid derivatives by gas chromatography (GC) in forthcoming experiments related to the search for extraterrestrial homochirality, the two versatile modified cyclodextrin (CD) selectors octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin were dissolved in a polysiloxane and the mixed binary chiral selector system was coated onto a 50m x 0.25 mm i.d. fused silica capillary column. Whereas the former CD selector enantioseparates racemic unfunctionalized alkanes the latter CD selector preferentially resolves N-(O,S)-trifluoroacetyl-alpha-amino acid alkyl esters. With both CD selectors employed as mixed binary chiral selector system present in one chiral stationary phase (CSP), the simultaneous gas chromatographic enantioseparation of racemic alkanes and of racemic derivatized alpha-amino acids is achieved in a single temperature-programmed run. Also for other classes of racemic compounds, the scope of enantioseparation could be extended as compared to the conventional use of the single CD selectors in GC.     

Enantiomer separation of 1,4-sulfanylalcohols by conventional and low-temperature gas chromatography

Since 1990, the family of organosulfur molecules has assumed increasing importance in pheromone, flavour, and fragrance chemistry. Depending on the constitution of the functional groups, various volatile sulfur-containing compounds are chiral. However, hitherto it has been impossible to study the chirality of 1,4-sulfanylalcohols, since no adequate enantioselective analytical technique has been available. Here we report on the enantiomer separation of ten volatile 1,4-sulfanylalcohol homologues by applying an heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-methyl)-beta-cyclodextrin phase, involving in some cases the use of a low-temperature gas chromatographic (ltGC) technique. The results are expected to open research potential on the asymmetry of volatile organosulfur molecules, particularly in the fields of pheromone, flavour, and fragrance research.     

Lipase-catalyzed irreversible transesterification of 1-(2-furyl)ethanol using isopropenyl acetate

Asymmetric acetylation of racemic 1-(2-furyl)ethanol with the innocuous acyl donor isopropenyl acetate catalyzed by lipases in organic media afforded the chiral alcohol and acetate in high enantiomeric excess (up to 99%). The effect of molecular sieves as well as organic solvents on the kinetic resolution were studied. An effective separation of the enantiomers of both substrate and product was performed using gas chromatography on the chiral stationary phase heptakis-(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin.     

Chiral separation of 2,3-allenoic acid by capillary zone electrophoresis using cyclodextrin derivatives

In this paper, five of six samples of 2,3-allenoic acid enantiomers were separated by capillary zone electrophoresis (CZE) using hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) as chiral selectors. Using HP-beta-CD for chiral separation, three of the six enantiomers were separated. Five experimental conditions including HP-beta-CD concentration, pH, buffer concentration, temperature, and running voltage were investigated for their influence on separation and migration using enantiomers of 2-methyl-4-phenyl-2,3-butadienoic acid (A) and 2-(n-propyl)-4-phenyl-2,3-butadienoic acid (B) as samples. Good separation results were observed when [HP-beta-CD] = 3-12 mmol/L and pH = 7-9 for samples A and B. The temperature range of 15-25 degrees C can be selected for convenience. According to the chiral separation results, HP-beta-CD and HP-gamma-CD should be valuable selectors to separate 2,3-allenoic acids and HP-gamma-CD was suggested to separate the 2,3-allenoic acid samples with a group at 4-position bulkier than phenyl.     

Enantiomers of C(5)-chiral 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives: Analytical and semipreparative HPLC separation, chiroptical properties, absolute configuration, and inhibitory activity against monoamine oxidase

The HPLC enantiomer separation of a novel series of C(5)-chiral 1-acetyl-3-(4-hydroxy- and 2,4-dihydroxyphenyl)-5-phenyl-4,5-dihydro-(1H)-pyrazole derivatives, with inhibitory activity against monoamine oxidases (MAO) type A and B, was accomplished using polysaccharide-based chiral stationary phases (CSPs: Chiralpak AD, Chiralcel OD, and Chiralcel OJ). Pure alcohols, such as ethanol and 2-propanol, and typical normal-phase binary mixtures, such as n-hexane and alcohol modifier, were used as mobile phases. Single enantiomers of several analytes examined were isolated on a semipreparative scale, and their chiroptical properties were measured. The assignment of the absolute configuration was established for one compound by single-crystal X-ray diffraction method and for the other three by CD spectroscopy. The inhibitory activity against MAO of racemic samples and single enantiomers were evaluated in vitro.     

Liquid chromatographic retention behavior and enantiomeric separation of three chiral center beta-blocker drug (nadolol) using heptakis (6-azido-6-deoxy-2, 3-di-O-phenylcarbamolyted) beta-cyclodextrin bonded chiral stationary phase

Nadolol, a beta-blocker used in the management of hypertension and angina pectoris, has three chiral centers and is currently marketed as an equal mixture of its four stereoisomers. Enantiomeric separation of nadolol by high-performance liquid chromatography was studied on a column packed with novel heptakis (6-azido-6-deoxy-2, 3-di-O-phenylcarbamolyted) beta-cyclodextrin bonded chiral stationary phase. The retention behavior and resolution of nadolol enantiomers were investigated and discussed with respect to the mobile phase composition and flow rate, pH, ionic strength, and temperature. The optimal separation condition was found; the mobile phase contained 80% buffer solution (1% triethylamine acetate, pH 5.5) and 20% methanol with 0.3 ml/min mobile phase flow rate at a temperature of 20 degrees C. At the optimal conditions, resolution of three stereoisomers of nadolol was obtained with a complete separation of the most active enantiomer, (RSR)-nadolol. Thermodynamic properties including enthalpy and entropy change of binding to the CSP for the enantiomeric separation were also determined.     

Chiral discrimination with regioselectively substituted cellulose esters as chiral stationary phases

Four kinds of cellulose derivatives, including two regioselectively substituted cellulose esters (6-O-acetyl-2,3-di-O-benzoyl cellulose and 2,3-di-O-acetyl-6-O-benzoyl cellulose), were synthesized so that the effects of their functional group distribution on their chiral discrimination ability could be examined. The degree of substitution by functional groups appeared to have a critical effect on the separation in most cases, but the type of the functional group at the C-6 position also significantly influenced chiral discrimination when a series of neutral arylalcohol derivatives were used as racemates. Copyright 2000 Wiley-Liss, Inc.     

Enantiomers of a nonylphenol isomer: absolute configurations and estrogenic potencies

Enantiomers of 4-(1,1,2-trimethylhexyl)phenol, a chiral isomer of the endocrine disrupting chemical nonylphenol, have been resolved and isolated by preparative chiral HPLC. The absolute configurations of the enantiomers were then determined by an X-ray crystallographic study of the (-)-camphanoyl derivative of the first eluted enantiomer NP(35)E1. The first enantiomer (NP(35)E1) and the second enantiomer (NP(35)E2) eluted were found to have the S and R absolute configurations, respectively. The estrogenic potencies of the S and R enantiomers were tested by the E-screen assay. A slight difference was observed in the relative proliferative effect between the S enantiomer and R enantiomer in the E-screen assay.     

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.     

Optical resolution and absolute configuration of a nonsteroidal antiinflammatory drug, 2-(10,11-dihydro-10-oxodibenzo- [b,f]thiepin-2-yl)propionic acid

The title compound (+/-)-1 (CN-100) was efficiently resolved into a pair of enantiomers by fractional crystallization of the diastereomeric salts of (-)- and (+)-phenylethylamine. The purity of the enantiomers was determined using the chiral cellulose column (CHIRALCEL OJ) which was allowed direct separation of the enantiomers. A separation factor (alpha) of 1.73 was obtained. X-Ray crystallographic analysis of the (+)-isomer [salt of (-)-1-(4-bromophenyl)ethylamine] showed that this enantiomer has S-configuration. Biological studies have shown that only the (+)-isomer has antiinflammatory activity. Racemizaiton of (-)-isomer was carried out by heating its propionic acid solution in the presence of mineral acid, such as HBr.     

Solid phase synthesis and biological evaluation of enantiomerically pure wasp toxin analogues PhTX-343 and PhTX-12

PhTX-343 and PhTX-12, analogues of the natural polyamine wasp toxin PhTX-433, were synthesised in 40-60% yields as pure enantiomers using solid phase synthesis techniques. Capillary electrophoresis procedures were developed for chiral separation and determination of enantiomeric purity (ee) of the enantiomers of PhTX-343 and PhTX-12. The methods were optimised with respect to chiral selector, buffer pH, and temperature around the capillary. Thus, rac-PhTX-343 was resolved using a separation buffer containing 30 mM heptakis-(2, 6-di-O-methyl)-beta-cyclodextrin in 50 mM 6-aminocarproic acid (pH 4. 0) at 15 degrees C. rac-PhTX-12 was not resolvable in this system, but could be resolved using a separation buffer containing 10% w/v of dextrin 10, a linear maltodextrin, in 50 mM 6-aminocaproic acid (pH 4.0) at 15 degrees C. Using these methods, the optical purity of the synthetic enantiomers was determined to be ee > 99%. The enantiomers were also characterised by chiroptical methods. The antagonist potency of the enantiomers was tested on nicotinic acetylcholine receptors (human muscle-type nAChR) expressed in TE671 cells, ionotropic glutamate receptors in Xenopus laevis oocytes (expressing recombinant GluR1flop receptors), and locust muscle ionotropic glutamate receptors sensitive to quisqualate (qGluR). The potencies of each pair of enantiomers were similar (eudismic ratio close to 1).     

Comparative Enantioseparation of Ketoprofen with Trimethylated α‐, β‐, and γ‐Cyclodextrins in Capillary Electrophoresis and Study of Related Selector–Selectand Interactions Using Nuclear Magnetic Resonance Spectroscopy

The enantiomers of ketoprofen were separated by capillary electrophoresis using the (2,3,6‐tri‐O‐methyl)‐derivatives of α‐, β‐, and γ‐cyclodextrin (CyD) as chiral selectors. The affinity pattern of the ketoprofen enantiomers toward these CyDs changed depending on their cavity size. Thus, with hexakis (2,3,6‐tri‐O‐methyl)‐α‐CyD and heptakis (2,3,6‐tri‐O‐methyl)‐β‐CyD, the R enantiomer of the drug migrated first, whereas the enantiomer migration order was reversed in the presence of octakis(2,3,6‐tri‐O‐methyl)‐γ‐CyD. The change in the migration order was rationalized on the basis of changes in the structure of the complexes between the ketoprofen enantiomers and the chiral selectors as derived from nuclear magnetic resonance spectroscopy experiments. Chirality, 25:79–88, 2013.© 2012 Wiley Periodicals, Inc.     

Nonaqueous synthesis of a selectively modified, highly anionic sulfopropyl ether derivative of cyclomaltoheptaose (beta-cyclodextrin) in the presence of 18-crown-6

A highly anionic cyclomaltooligosaccharide (cyclodextrin, CD) derivative containing sulfopropyl functional groups on the primary face of the CD was synthesized. Heptakis(2,3-di-O-methyl)cyclomaltoheptaose [heptakis(2,3-di-O-methyl)-beta-cyclodextrin] was reacted with 1,3-propane sultone and potassium hydride (KH) in anhydrous tetrahydrofuran in the presence of 18-crown-6 to yield highly substituted potassium heptakis(2,3-di-O-methyl-6-O-sulfopropyl)cyclomaltoheptaose [heptakis(KSPDM)-beta-CD] with an average degree of substitution (DSCE) of 6.9 as determined by inverse detection capillary electrophoresis (CE). The principal species in the product is the fully substituted heptakis(KSPDM)-beta-CD. Complete NMR assignments of the hydrogen and carbon atoms are made using a combination of gCOSY and gHSQC. In the absence of 18-crown-6, the reaction generates a mixture of multiply charged derivatives with average DSCE of 4.1. The possible roles of the crown ether in the reaction are discussed. The ROESY NMR spectrum of the inclusion complex that forms between heptakis(KSPDM)-beta-CD and 2-naphthoic acid in D2O reveals that 2-naphthoic acid inserts with the carboxyl group toward the derivatized primary rim of the cyclodextrin.     

Simultaneous enantioselective analysis of chiral urinary metabolites in patients with Zellweger syndrome

Enantio-MDGC-MS analysis with heptakis-(2,3-di-O-methyl-6-O-tert.-butyl-dimethylsilyl)-beta-cyclodextrin as the chiral main column is a powerful tool for the separation of chiral compounds. This paper reports on the simultaneous stereodifferentiation of 2-hydroxyisocaproic acid (HICA), 3-methyladipic acid (3-MA), 2-hydroxyglutaric acid (2-HG), 3-(4-hydroxyphenyl)-lactic acid (HPLA), 2-hydroxysebacic acid (2-HS) and 3-hydroxysebacic acid (3-HS) in a single chromatographic run. These chiral urinary metabolites are useful in the diagnosis of peroxisomal diseases such as Zellweger syndrome (ZS). In this investigation, urine samples from nine patients with ZS were analysed in order to reveal the enantiomeric ratio of these chiral metabolites. The stereodifferentiation of the analysed chiral compounds may provide important information on their biochemical origin.     

Simultaneous determination of the enantiomers of esmolol and its acid metabolite in human plasma by reversed phase liquid chromatography with solid-phase extraction

A stereoselective RP-high performance liquid chromatography (HPLC) assay to determine simultaneously the enantiomers of esmolol and its acid metabolite in human plasma was developed. The method involved a solid-phase extraction and a reversed-phase chromatographic separation with UV detection (lambda = 224 nm) after chiral derivatization. 2,3,4,6-tetra-O-acetyl-beta-d-glucopyranosyl isothiocyanate (GITC) was employed as a pre-column chiral derivatization reagent. The assay was linear from 0.09 to 8.0 microg/ml for each enantiomer of esmolol and 0.07-8.0 microg/ml for each enantiomer of the acid metabolite. The absolute recoveries for all enantiomers were >73%. The intra- and inter-day variations were <15%. The validated method was applied to quantify the enantiomers of esmolol and its metabolite in human plasma for hydrolysis studies.     

Use of cyclodextrins in capillary zone electrophoresis for the separation of optical isomers: Resolution of racemic tryptophan derivatives

In this study capillary zone electrophoresis has been used for the separation of racemic tryptophan derivatives in their enantiomers. The effect of cyclodextrins with different shape, added to the background electrolyte, on the migration time of 10 compounds, including methyl tryptophan, hydroxy tryptophan, and tryptophan ester derivatives, has been studied. Furthermore, the effect of cyclodextrins with different shape and that of the composition of the background electrolyte on the enantiomer resolution are discussed. Among different cyclodextrins used α-cyclodextrin and heptakis(2,6-di-O-methyl)-β-cyclodextrin were found to possess the best complexing capacity and thus the resolution power toward analysed compounds.     

Combining synthetic and analytical strategies for preparative HPLC enantioseparation of monastrol racemic mixture

Large-scale resolution of racemic monastrol has been carried out by normal-phase mode HPLC on an amylose-based chiral stationary phase. Because monastrol solubility, in media of proper compositions for normal-mode HPLC separations (in terms of retention factors and selectivity), was significantly low and impractical for preparative scopes, racemic monastrol was transformed into the corresponding O-tert-butyldimethylsilyl derivative. The tert-butyldimethylsilyl group was chosen as a suitable derivatizing agent because it induced approximatively a six-times higher solubility and allowed for an almost quantitative recovery of pure monastrol from the derivatization-deprotection sequence. The competitive isotherms of the O-tert-butyldimethylsilyl compounds, measured through frontal analysis, were fitted to competitive Langmuir and four-parameter bi-Langmuir models. The equilibrium dispersive model of chromatography was used for modeling the nonlinear separation of the racemate and to optimize the experimental conditions for collection of highly concentrated fractions of pure (R,S)-O-tert-butyldimethylsilyl compounds, from which significant amounts of the corresponding enantiomers of monastrol (about 100 mg of each enantiomer with 30 runs on an analytical-scale column) were obtained by quantitative back-derivatization.     

Development and validation of a chiral liquid chromatographic method, based on Chiralpak to quantify enantiomers of (+/-)-DRF 2725 in rat plasma: lack of inversion of ragaglitazar (S(-)-DRF 2725) to its antipode in plasma

A selective, accurate and reproducible high-performance liquid chromatographic (HPLC) method for the separation of individual enantiomers of DRF 2725 [R(+)-DRF 2725 and S(-)-DRF 2725 or ragaglitazar] was obtained on a chiral HPLC column (Chiralpak). During method optimization, the separation of enantiomers of DRF 2725 was investigated to determine whether mobile phase composition, flow-rate and column temperature could be varied to yield the base line separation of the enantiomers. Following liquid-liquid extraction, separation of enantiomers of DRF 2725 and internal standard (I.S., desmethyl diazepam) was achieved using an amylose based chiral column (Chiralpak AD) with the mobile phase, n-hexane-propanol-ethanol-trifluoro acetic acid (TFA) in the ratio of 89.5:4:6:0.5 (v/v). Baseline separation of DRF 2725 enantiomers and I.S., free from endogenous interferences, was achieved in less than 25 min. The eluate was monitored using an UV detector set at 240 nm. Ratio of peak area of each enantiomer to I.S. was used for quantification of plasma samples. Nominal retention times of R(+)-DRF 2725, S(-)-DRF 2725 and I.S. were 15.8, 17.7 and 22.4 min, respectively. The standard curves for DRF 2725 enantiomers were linear (R(2) > 0.999) in the concentration range 0.3-50 microg/ml for each enantiomer. Absolute recovery, when compared to neat standards, was 70-85% for DRF 2725 enantiomers and 96% for I.S. from rat plasma. The lower limit of quantification (LLOQ) for each enantiomers of DRF 2725 was 0.3 microg/ml. The inter-day precisions were in the range of 1.71-4.60% and 3.77-5.91% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. The intra-day precisions were in the range of 1.06-11.5% and 0.58-12.7% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. Accuracy in the measurement of quality control (QC) samples was in the range 83.4-113% and 83.3-113% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. Both enantiomers and I.S. were stable in the battery of stability studies viz., bench-top (up to 6 h), auto-sampler (up to 12 h) and freeze/thaw cycles (n = 3). Stability of DRF 2725 enantiomers was established for 15 days at -20 degrees C. The application of the assay to a pharmacokinetic study of ragaglitazar [S(-)-DRF 2725] in rats is described. It was unequivocally demonstrated that ragaglitazar does not undergo chiral inversion to its antipode in vivo in rat plasma.     

2-(2,3-Dihydro-1H-indol-3-yl)ethanol: synthesis, separation of enantiomers, and assignment of absolute stereochemistry by X-ray structure analysis

The first direct resolution of racemic 2-(2,3-dihydro-lH-indol-3-yl)ethanol-prepared by catalytic hydrogenation of 2-(lH-indol-3-yl)ethanol-has been accomplished by chiral simulated moving bed (SMB) chromatography. The single enantiomers were isolated as their dihydrogen phosphate salts. Single-crystal X-ray analyses were successful, revealing that the (+)-enantiomer of 2-(2,3-dihydro-lH-indol-3-yl)ethanol has the (S) configuration. Chirality 16:126-130, 2004.