Synthesis and HPLC enantioseparation of the cyclopropane analogue of valine (c3Val)

A new and efficient method is presented for the preparation of the N-Boc-protected cyclopropane analogue of valine, 1-(N-tert-butoxycarbonyl)amino-2,2-dimethylcyclopropanecarboxylic acid, both in racemic and enantiomerically pure forms. Cyclopropanation of the exocyclic double bond of 2-phenyl-4-isopropylidene-5(4H)-oxazolone with diazomethane followed by elaboration of the heterocyclic moiety provided multigram quantities of the racemic target compound. Subsequent HPLC resolution of a racemic precursor on a noncommercial chiral stationary phase has given access to enantiomerically pure products. Almost 1.5 g of the first-eluted enantiomer and 1.0 g of the second-eluted enantiomer have been isolated in optically pure form using a 150 x 20 mm ID column containing mixed 10-undecenoate/3,5-dimethylphenylcarbamate of cellulose covalently bonded to allylsilica gel with a mixture of hexanes/tert-butyl methyl ether/ethyl acetate as the mobile phase.     

HPLC and SFC enantioseparation of (±)‐Corey lactone diol: Impact of the amylose tris‐(3,5‐dimethylphenylcarbamate) coating amount on chiral preparation

As an important intermediate of prostaglandins and entecavir, optically pure Corey lactone diol (CLD) has great value in the pharmaceutical industry. In this work, the enantioseparation of (±)‐CLD was evaluated using high‐performance liquid (HPLC) and supercritical fluid chromatography (SFC). In HPLC, the separations of CLD enantiomers on polysaccharide‐based chiral stationary phases with both normal phase and polar organic phase were screened. And the conditions for the enantioseparation were optimized in HPLC and SFC, including the selection of mobile phase, temperature, back‐pressure, and other conditions. More important, it was found that the chiral resolutions were greatly enhanced by the increase of the coating amount of ADMPC (amylose tris‐(3,5‐dimethylphenylcarbamate)) under both HPLC and SFC conditions, which can lead to the increase of the productivity and the decrease of the solvent consumption. The preparations of optically pure CLD were evaluated on a semi‐preparative (2 × 25 cm) column packed with 30% ADMPC‐coated CSP under HPLC and SFC conditions. Preparative performances in terms of kkd are 1.536 kg racemate/kg CSP/day and 1.248 kg racemate/kg CSP/day in HPLC and SFC, respectively.     

Enantioselective and diastereoselective separation of synthetic pyrethroid insecticides on a novel chiral stationary phase by high-performance liquid chromatography

A novel chiral packing material for high-performance liquid chromatography (HPLC) was prepared by connecting (R)-1-phenyl-2-(4-methylphenyl) ethylamine (PTE) amide derivative of (S)-isoleucine to aminopropyl silica gel through 2-amino-3,5-dinitro-1-carboxamido-benzene unit. This chiral stationary phase was applied to the enantioselective and diastereoselective separation of five pyrethroid insecticides by HPLC under normal phase condition. To achieve satisfactory baseline separation an optimization of the variables of mobile phase composition was required. The two enantiomers of fenpropathrin and four stereoisomers of fenvalerate were baseline separated using hexane-1,2-dichloroethane-2-propanol as mobile phase. The results show that the enantioselectivity of CSP is better than Pirkle type 1-A column for these compounds. Only partial separations for the cypermethrin and cyfluthrin stereoisomers were observed. Seven peaks and eight peaks were observed for cypermethrin and cyfluthrin, respectively. The elution orders were assigned by using different stereoisomer-enriched products.     

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.     

Direct Enantioseparation of Nitrogen‐Heterocyclic Pesticides on Cellulose‐Based Chiral Column by High‐Performance Liquid Chromatography

The enantiomeric separation of eight pesticides including bitertanol ( 1 ), diclobutrazol ( 2 ), fenbuconazole ( 3 ), triticonazole ( 4 ), imazalil ( 5 ), triapenthenol ( 6 ), ancymidol ( 7 ), and carfentrazone‐ethyl ( 8 ) was achieved, using normal‐phase high‐performance liquid chromatography on two cellulosed‐based chiral columns. The effects of isopropanol composition from 2% to 30% in the mobile phase and column temperature from 5 to 40 °C were investigated. Satisfactory resolutions were obtained for bitertanol ( 1 ), triticonazole ( 4 ), imazalil ( 5 ) with the (+)‐enantiomer eluted first and fenbuconazole ( 3 ) with the (—)‐enantiomer eluted first on Lux Cellulose‐2 and Lux Cellulose‐3. (+)‐Enantiomers of diclobutrazol ( 2 ) and triapenthenol ( 6 ) were first eluted on Lux Cellulose‐2. (—)‐Carfentrazone‐ethyl ( 8 ) were eluted first on Lux Cellulose‐2 and Lux Cellulose‐3 with incomplete separation. Reversed elution orders were obtained for ancymidol (7). (+)‐Ancymidol was first eluted on Lux Cellulose‐2 while on Lux Cellulose‐3 (—)‐ancymidol was first eluted. The results of the elution order at different column temperatures suggested that column temperature did not affect the optical signals of the enantiomers. These results will be helpful to prepare and analyze individual enantiomers of chiral pesticides. Chirality 27:32–38, 2015. © 2014 Wiley Periodicals, Inc.     

Separation of Nadolol Stereoisomers Using Chiralpak IA Chiral Stationary Phase

Chiralpak IA adsorbent is used for both analytical and preparative chromatographic separation of nadolol stereoisomers. The results include a complete screening of the mobile phase composition for both the baseline resolution of all four nadolol stereoisomers (analytical separation) and the simulated moving bed (SMB) pseudo‐binary separation of the most retained stereoisomer. The experimental results show that analytical baseline resolution of nadolol stereoisomers can be achieved using alcohol/hydrocarbon and alcohol/acetonitrile solvent mixtures. The 10%ethanol/90%acetonitrile mixture is presented as the one that presents baseline resolution with lower retention. For the preparative pseudo‐binary separation, pure ethanol, pure methanol, alcohol/acetonitrile, and alcohol/tetrahydrofuran mixtures proved to allow good separation results. The 100%methanol/0.1%diethylamine solvent composition was selected to perform the experimental SMB separation. Using a 10 g/L total feed concentration, the more retained stereoisomer was recovered at the extract outlet stream with 99.5% purity, obtaining a system productivity of 1.98 gL^?1 h^?1 and requiring a solvent consumption of 3.13 L/g of product. Comparing these results with the ones recently presented by Ribeiro et al. (2013), this work shows that the Chiralpak IA chiral adsorbent is an interesting alternative to Chiralpak AD for the separation of nadolol stereoisomers at both analytical and preparative scales. Chirality 28:399–408, 2016. © 2016 Wiley Periodicals, Inc.     

High‐performance liquid chromatography separation of enantiomers of mandelic acid and its analogs on a chiral stationary phase

The enantiomers of mandelic acid and its analogs have been chromatographically separated on a chiral stationary phase (CSP) derived from 4‐(3,5‐dinitrobenzamido) tetrahydrophenanthrene. The rationale of separations of these compounds is discussed with respect to the method development for determining enantiomeric purity and possibility of obtaining enantiomerically pure materials by high‐pressure liquid chromatography. The relationship of analyte structure to the extent of enantiomeric separation has been examined and separation factors (α) are presented for various groups of structurally related compounds. Chiral recognition models have been suggested to account for the observed separations. These models provide mechanistic insights into the chiral recognition process. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Access to Enantiomerically Pure cis‐ and trans‐β‐Phenylproline by High‐Performance Liquid Chromatography Resolution

The preparation of all four stereoisomers of the proline analog that bears a phenyl group attached to the β carbon either cis or trans to the carboxylic acid (cis‐ and trans‐β‐phenylproline, respectively) has been addressed. The methodology developed allows access to multigram quantities of the target amino acids in enantiomerically pure form and suitably protected for use in peptide synthesis. Racemic precursors of cis‐β‐phenylproline and trans‐β‐phenylproline were prepared from easily available starting materials and subjected to high‐performance liquid chromatography enantioseparation. Semipreparative columns (250 × 20 mm) containing chiral stationary phases based on amylose (Chiralpak IA) (Daicel‐Chiral Technologies Europe, Illkirch, France) or cellulose (Chiralpak IC) were used respectively for the resolution of the cis‐ and trans‐β‐phenylproline precursors. Chirality, 24:1082‐1091, 2012. © 2012 Wiley Periodicals, Inc.     

Efficient synthesis of novel glutamate homologues and investigation of their affinity and selectivity profile at ionotropic glutamate receptors

A convenient synthesis of four new enantiomerically pure acidic amino acids is reported and their affinity at ionotropic glutamate receptors was determined. The new compounds are higher homologues of glutamic acid in which the molecular complexity has been increased by introducing an aromatic/heteroaromatic ring, that is a phenyl or a thiophene ring, that could give additional electronic interactions with the receptors. The results of the present investigation indicate that the insertion of an aromatic/heteroaromatic ring into the amino acid skeleton of glutamate higher homologues is well tolerated and this modification could be exploited to generate a new class of NMDA antagonists.     

Resolution of terfenadine enantiomers by beta-cyclodextrin chiral stationary phase high-performance liquid chromatography.

Enantiomers of terfenadine were resolved by high-performance liquid chromatography (HPLC) using a chiral stationary phase (CSP) column packed with beta-cyclodextrin (beta-CD) covalently bound to silica. Separation was achieved in both the reverse phase and normal phase modes. Resolution of enantiomers was confirmed by ultraviolet-visible absorption, circular dichroism, and mass spectral analysis.     

Separation of nicotine and nornicotine enantiomers via normal phase HPLC on derivatized cellulose chiral stationary phases

This paper describes the enantiorecognition of (±)nicotine and (±)nornicotine by high-performance liquid chromatography using two derivatized cellulose chiral stationary phases (CSPs) operated in the normal phase mode. It was found that different substituents linked to the cellulose backbone significantly influence the chiral selectivity of the derivatized CSP. The results showed that, in general, the tris(4-methylbenzoyl) cellulose CSP (Chiralcel OJ) surpasses tris(3,5-dimethylphenyl carbamoyl) cellulose CSP (Chiralcel OD). On the former column, the resolution (±)nicotine and (±)nornicotine enantiomers depended largely on mobile phase compositions. For the separation of the nicotine enantiomers, the addition of trifluoroacetic acid to a 95:5 hexane/alcohol mobile phase greatly improved the enantioresolution, probably due to enhanced hydrogen bonding interactions between the protonated analytes and the CSP. For (±)nornicotine separation, a reduction in the concentration of alcohol in the mobile phase was more effective than the addition of trifluoroacetic acid. Possible solute-mobile phase-stationary phase interactions are discussed to explain how different additives in the mobile phase and different substituents on the cellulose glucose units of the CSPs affect the separation of both pairs of enantiomers. Chirality 10:364–369, 1998. Published 1998 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

A validated enantiospecific method for determination and purity assay of clopridogrel

A new and accurate HPLC method using β‐cyclodextrin chemically bonded to spherical silica particles as chiral stationary phase (CSP) was developed and validated for determination of S‐clopidogrel and its impurities R‐enantiomer and S‐acid as a hydrolytic product. The effects of acetonitrile and methanol content in the mobile phase and temperature on the resolution and retention of enantiomers were investigated. A satisfactory resolution of S‐clopidogrel active form and its impurities was achieved on ChiraDex® column (5 μm, 4 × 250 mm) at a flow rate of 1.0 ml/min and 17°C using acetonitrile, methanol and 0.01 M potassium dihydrogen phosphate solution (15:5:80 v/v/v) as mobile phase. The detection wavelength was set at 220 nm. The method was validated in terms of accuracy, precision, linearity, and robustness. The limit of detection for R‐enantiomer and S‐acid were 0.75 and 0.09 μg/ml, respectively, injection volume being 20 μl. Finally, the molecular modeling of the inclusion complexes between the analytes and β‐cyclodextrin was performed to investigate the mechanism of the enantiorecognition and to study the quantitative structure–retention relationships. Chirality, 2009. © 2008 Wiley‐Liss, Inc.     

Practical access to the proline analogs (S,S,S)- and (R,R,R)-2-methyloctahydroindole-2-carboxylic acids by HPLC enantioseparation

An efficient methodology for the preparation of the α‐tetrasubstituted proline analog (S,S,S)‐2‐methyloctahydroindole‐2‐carboxylic acid, (S,S,S)‐(αMe)Oic, and its enantiomer, (R,R,R)‐(αMe)Oic, has been developed. Starting from easily available substrates and through simple transformations, a racemic precursor has been synthesized in excellent yield and further subjected to HPLC resolution using a cellulose‐derived chiral stationary phase. Specifically, a semipreparative (250 mm × 20 mm ID) Chiralpak® IC column has allowed the efficient resolution of more than 4 g of racemate using a mixture of n‐hexane/tert‐butyl methyl ether/2‐propanol as the eluent. Multigram quantities of the target amino acids have been isolated in enantiomerically pure form and suitably protected for incorporation into peptides. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

Direct Enantioseparation of Nitrogen‐heterocyclic Pesticides on Amylose‐tris‐(5‐chloro‐2‐methylphenylcarbamate) by Reversed‐Phase High‐Performance Liquid Chromatography

In this study, 11 nitrogen‐heterocyclic pesticides were stereoselectively separated on amylose‐tris‐(5‐chloro‐2‐methylphenylcarbamate) chiral stationary phase, using reversed‐phase high‐performance liquid chromatography with diode array detector and optical rotation detector at 426 nm. The effects of mobile phase composition and column temperature (5–40 °C) on separation were investigated. When acetonitrile and water were used as mobile phase, satisfactory separations were obtained on amylose‐tris‐(5‐chloro‐2‐methylphenylcarbamate) for four pesticides with elution orders of (+)/(?)‐simeconazole (1) , (?)/(+)‐nuarimol (3) , (?)/(+)‐carfentrazone‐ethyl (4) , and (?)/(+)/(?)/(+)‐bromuconazole (9) and part separations for three with elution orders of (?)/(+)‐famoxadone (6) , (+)/(?)‐fenbuconazole (10) , and (?)/(+)‐triapenthenol (11) . Only two chromatographic peaks on diode array detector were obtained for diclobutrazol (2) , cyproconazole (5) , etaconazole (7) , and metconazole (8) , although they should have four stereoisomers in theory because of presences of two chiral centers in molecules. The stereoisomeric optical signals of all pesticides did not reverse with temperature changes but would reverse with different solvent types for some pesticides. These results will be useful to prepare and analyze individual enantiomers of chiral pesticides. Chirality 24:1031–1036, 2012. © 2012 Wiley Periodicals, Inc.     

Effect of Chromatographic Conditions on Enantioseparation of Bovine Serum Albumin Chiral Stationary Phase in HPLC and Thermodynamic Studies

Twelve chiral compounds were enantiomerically resolved on bovine serum albumin chiral stationary phase (BSA‐CSP) by high‐performance liquid chromatography (HPLC) in reversed‐phase modes. Chromatographic conditions such as mobile phase pH, the percentage of organic modifier, and concentration of analyte were optimized for separation of enantiomers. For N‐(2, 4‐dinitrophenyl)‐serine (DNP‐ser), the retention factors (k) greatly increase from 0.81 to 6.23 as the pH decreasing from 7.21 to 5.14, and the resolution factor (R_s) exhibited a similar increasing trend (from 0 to 1.34). More interestingly, the retention factors for N‐(2, 4‐dinitrophenyl)‐proline (DNP‐pro) decrease along with increasing 1‐propanol in mobile phase (3%, 5%, 7% and 9% by volume), whereas the resolution factor shows an upward trend (from 0.96 to 2.04). Moreover, chiral recognition mechanisms for chiral analytes were further investigated through thermodynamic methods. Chirality 25:487–492, 2013. © 2013 Wiley Periodicals, Inc.     

Chiral analysis of butaclamol enantiomers in human plasma by HPLC using a macrocyclic antibiotic (vancomycin) chiral stationary phase and solid phase extraction

An enantioseparation of the antipsychotic drug butaclamol in human plasma by high-performance liquid chromatography (HPLC) with solid phase extraction is presented. The separation was achieved on the vancomycin macrocyclic antibiotic chiral stationary phase (CSP) Chirobiotic V with a polar ionic mobile phase (PIM) consisting of methanol : glacial acetic acid : triethylamine (100:0.2:0.05, v/v/v) at a flow rate of 0.5 ml/min. The detection wavelength was 262 nm. Bond Elut C18 solid phase extraction cartridges were used in the sample preparation of butaclamol samples from plasma. The method was validated over the range of 100-3,000 ng/ml for each enantiomer concentration (R(2) > 0.999). Recoveries for (+)- and (-)-butaclamol were in the range of 94-104% at the 300-2,500 ng/ml level. The method proved to be precise (within-run precision ranged from 1.1-2.6% and between-run precision ranged from 1.9-3.2%) and accurate (within-run accuracies ranged from 1.5-5.8% and between-run accuracies ranged from 2.7-7.7%). The limit of quantitation (LOQ) and limit of detection (LOD) for each enantiomer in human plasma were 100 ng/ml and 50 ng/ml, respectively.     

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.     

(Z)-1,1-dichloro-2-(4-benzyloxyphenyl)-2,3-bis(4-methoxyphenyl)cyclopropane: The synthesis and enantiomeric separation of an antitumor agent

(Z)-1,1-Dichloro-2-(4-benzyloxyphenyl)-2,3-bis(4-methoxyphenyl)cyclopropane ( 5 ), a potential antitumor agent designed to treat breast cancer, was prepared in three steps. A stereospecific palladium-catalyzed cross coupling reaction which provided the intermediate (Z)-triaryl alkene 4 was a crucial step in the synthesis. Makosza phase transfer reaction on 4 gave the enantiomeric (Z)-dichlorocyclopropane derivatives 5 which were resolved by semipreparative HPLC on a chiral stationary phase consisting of amylose tris-3,5-dimethylphenyl carbamate coated on silica gel. © 1994 Wiley-Liss, Inc.     

The direct chiral separations of fungicide enantiomers on amylopectin based chiral stationary phase by HPLC

Amylopectin-tris(phenylcarbamate) was synthesized and coated to aminopropylsilica to prepare chiral stationary phase. The chiral separations of fungicide enantiomers were performed by the CSP using high-performance liquid chromatography. Mobile phase was n-hexane and isopropanol, and flow rate was 1.0 ml/min. Detection wavelength was 230 nm. The influence of the percentage of isopropanol in the mobile phase on the separations was studied. Twelve chiral fungicides were tested and seven of them were found to show stereoselectivity on the CSP. The enantiomers of metalaxyl and benalaxyl got near baseline separations and myclobutanil, hexconazole, tebuconazole, uniconazole, and paclobutrazol enantiomers were completely separated. The decreasing percentage of isopropanol in the mobile phase resulted in better separation and longer analysis time. The enantiomers were identified by a circular dichroism (CD) detector and the CD spectra of the individual enantiomers were also studied by online scanning.     

Enantiomeric separation of prothioconazole and prothioconazole‐desthio on chiral stationary phases

Prothioconazole is a type of broad‐spectrum triazole thione fungicide developed by the Bayer Company. Prothioconazole‐desthio is the main metabolite of prothioconazole in the environment. In our study, enantiomeric separation of prothioconazole and prothioconazole‐desthio was performed on various chiral stationary phases (CSPs) by high‐performance liquid chromatography (HPLC). It was found that polysaccharide CSPs showed better ability than brushing CSPs in enantiomeric separation. The successful chiral separation of prothioconazole could be achieved on self‐made Chiralcel OD, commercialized Chiralcel OJ‐H and Lux Cellulose‐1. Chiralpak IA, Chiralpak IB, Chiralpak IC, Chiralcel OD, Chiralpak AY‐H, Chiralpak AZ‐H, and Lux Cellulose‐1 realized the baseline separation of prothioconazole‐desthio enantiomers. Simultaneous enantiomeric separation of prothioconazole and prothioconazole‐desthio was performed on Lux Cellulose‐1 using acetonitrile (ACN) and water as mobile phase. In most cases, low temperature favored the separation of two compounds. The influence of the mobile phase ratio or type was deeply discussed. We obtained larger Rs and longer analysis time with a smaller proportion of isopropanol (IPA) or ethanol and more water content at the same temperature. The ratio of ACN and water had influences on the outflow orders of prothioconazole‐desthio enantiomers. This work provides a new approach for chiral separation of prothioconazole and prothioconazole‐desthio with a discussion of chiral separation mechanism on different CSPs.