Triggering dissymmetry in achiral dye molecules by chiral solvents: Circular dichroism experiments and DFT calculations

The electronic circular dichroism spectra of achiral product “Lumogen F Red” (ROT‐300) in four different chiral solvents are recorded at different temperatures. DFT calculations allow to identify two enantiomeric conformers for ROT‐300. In vacuo they are equally populated; in chiral solvents one enantiomer prevails. Thermodynamic quantities involved in the chiral preference are derived. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

Enantioseparation of Racemic Flurbiprofen by Aqueous Two‐Phase Extraction With Binary Chiral Selectors of L‐dioctyl Tartrate and L‐tryptophan

A novel method for chiral separation of flurbiprofen enantiomers was developed using aqueous two‐phase extraction (ATPE) coupled with biphasic recognition chiral extraction (BRCE). An aqueous two‐phase system (ATPS) was used as an extracting solvent which was composed of ethanol (35.0% w/w) and ammonium sulfate (18.0% w/w). The chiral selectors in ATPS for BRCE consideration were L‐dioctyl tartrate and L‐tryptophan, which were screened from amino acids, β‐cyclodextrin derivatives, and L‐tartrate esters. Factors such as the amounts of L‐dioctyl tartrate and L‐tryptophan, pH, flurbiprofen concentration, and the operation temperature were investigated in terms of chiral separation of flurbiprofen enantiomers. The optimum conditions were as follows: L‐dioctyl tartrate, 80 mg; L‐tryptophan, 40 mg; pH, 4.0; flurbiprofen concentration, 0.10 mmol/L; and temperature, 25 °C. The maximum separation factor α for flurbiprofen enantiomers could reach 2.34. The mechanism of chiral separation of flurbiprofen enantiomers is discussed and studied. The results showed that synergistic extraction has been established by L‐dioctyl tartrate and L‐tryptophan, which enantioselectively recognized R‐ and S‐enantiomers in top and bottom phases, respectively. Compared to conventional liquid–liquid extraction, ATPE coupled with BRCE possessed higher separation efficiency and enantioselectivity without the use of any other organic solvents. The proposed method is a potential and powerful alternative to conventional extraction for separation of various enantiomers. Chirality 27:650–657, 2015. © 2015 Wiley Periodicals, Inc.     

Stereoselective drug release from Ketoprofen and Ricobendazole matrix tablets

Crystalline characteristics of racemic, pure R and S enantiomers and physical mixtures of Ketoprofen (KET) have been studied by DSC and X‐ray diffractometry. Aqueous solubilities were 182.6 ± 9.1 μg/ml for racemic KET, 259.6 ± 6.6 μg/ml for R‐KET, and 304.3 ± 2.7 μg/ml for S‐KET. Matrix tablets made with racemic and physical mixtures of KET show stereoselective drug release, which is faster for S‐KET than for R‐KET. This effect is more marked when the chiral excipient hydroxypropylmethylcellulose (HPMC) is used in place of the achiral Eudragit RL. Stereoselectivity of release is also affected by the amount of KET. Similar results were obtained when another chiral drug with low solubility, Ricobendazole (RBZ), is used. Depending on the excipient and drug dosage, more or less marked stereoselective drug release is obtained in RBZ matrix tablet formulations. Chirality 11:611–615, 1999. © 1999 Wiley‐Liss, Inc.     

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.     

Immobilization and chromatographic evaluation of novel regioselectively substituted amylose‐based chiral packing materials for HPLC

The regioselectively substituted amylose derivatives bearing a 4‐tert‐butylbenzoate or 4‐chlorobenzoate group at 2‐position, and 3,5‐dichlorophenylcarbamate and a small amount of 3‐(triethoxysilyl)propylcarbamate groups at 3‐ and 6‐positions were synthesized by a two‐step process based on the esterification of 2‐position of a glucose unit. The obtained derivatives were effectively immobilized onto macroporous silica gel by intermolecular polycondensation of triethoxysilyl groups. Their chiral recognition abilities were evaluated as chiral packing materials (CPMs) for high‐performance liquid chromatography. These CPMs showed high chiral recognition as well as the conventional coated‐type CPM, and can be used with the eluents‐containing chloroform and tetrahydrofuran. With the extended use of these eluents, improvement of chiral recognition and reversed elution orders were realized. For some racemates, the immobilized CPM exhibited ability comparable or better to the commercial immobilized amylose‐ or cellulose‐based columns, Chiralpak IA, IB, and IC. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

Enantioseparation of racecadotril using polysaccharide‐type chiral stationary phases in polar organic mode

Enantioseparation of the antidiarrheal drug, racecadotril, was investigated by liquid chromatography using polysaccharide‐type chiral stationary phases in polar organic mode. The enantiodiscrimininating properties of 4 different chiral columns (Chiralpak AD, Chiralcel OD, Chiralpak AS, Chiralcel OJ) with 5 different solvents (methanol, ethanol, 1‐propanol, 2‐propanol, and acetonitrile) at 5 different temperatures (5–40 °C) were investigated. Apart from Chiralpak AS column the other 3 columns showed significant enantioseparation capabilities. Among the tested mobile phases, alcohol type solvents were superior over acetonitrile, and significant differences in enantioselective performance of the selector were observed depending on the type of alcohol employed. Van't Hoff analysis was used for calculation of thermodynamic parameters which revealed that enantioseparation is mainly enthalpy controlled; however, enthropic control was also observed. Enantiopure standard was used to determine the enantiomer elution order, revealing chiral selector—and mobile‐phase dependent reversal of enantiomer elution order. Using the optimized method (Chiralcel OJ stationary phase, thermostated at 10 °C, 100% methanol, flow rate: 0.6 mL/min) baseline separation of racecadotril enantiomers (resolution = 3.00 ± 0.02) was achieved, with the R‐enantiomer eluting first. The method was validated according to the ICH guidelines, and its application was tested on capsule and granules containing the racemic mixture of the drug.     

Simultaneous determination of enantiomerization and hydrolysis kinetic parameters of chiral N‐alkylbenzothiadiazine derivatives

On‐column stopped flow multidimensional HPLC (sfMDHPLC) and dynamic high‐performance liquid chromatography were applied to investigate the influence of alkyl substituents at the sulfonamidic and amino moieties of benzothiadiazine 1,1‐dioxide derivatives on hydrolysis and enantiomerization rate constants. The data obtained indicate the presence of pyrrolo substituent at the 3,4 positions on benzothiadiazine rings inhibits the hydrolysis, whereas the enantiomerization occurs in acidic medium. Hydrolysis rates are quite similar for the two benzothiadiazines methyl substituted to nitrogen at 2‐ and 4‐positions. Conversely, enantiomerization rate of 4‐N‐methyl substituted is significantly higher than 2‐N‐methyl substituted. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Application of a new amidophosphite ligand to Rh‐catalyzed asymmetric hydrogenation of β‐dehydroamino acid derivatives in supercritical carbon dioxide: Activation effect of protic co‐solvents

New chiral amidophosphite ligand was synthesized and tested in the Rh‐catalyzed asymmetric hydrogenation of (Z)‐β‐(acylamino)acrylates in protic solvents and supercritical carbon dioxide (scCO₂) The catalytic performance is affected greatly by the acidity of the solvents. Better enantioselectivity (up to 88% ee) was achieved in scCO₂ containing 1,1,1,3,3,3‐hexafluoro‐2‐propanol, compared to neat protic solvents. Chirality, 2011. © 2011 Wiley‐Liss, Inc.     

Enantiomerization of chiral 2,3,3a,4-tetrahydro-1H-pyrrolo[2,1-c][1,2,4] benzothiadiazine 5,5-dioxide by stopped-flow multidimensional HPLC

An on-column stopped-flow multidimensional HPLC (sfMDHPLC) procedure using two chiral stationary phases (CSPs) and one achiral C18 column was developed for the determination of rate constants and free energy barriers of enantiomerization of (+/-)(R,S)-2,3,3a,4-tetrahydro-1H-pyrrolo[2,1-c][1,2,4]benzothiadiazine 5,5-dioxide. Moreover, a stopped-flow HPLC (sfHPLC) method previously developed was applied to the determination of kinetic parameters of enantiomerization of the above compound in the presence of a CSP. The individual enantiomers of the studied compound were isolated in parallel by preparative HPLC and the rate constants and free energy barriers of enantiomerization were determined in different solvents (off-column method). The data obtained by sfMDHPLC, sfHPLC and off-column methods were compared. The (S) enantiomer of the studied compound (S18986) was prepared by asymmetric synthesis and subsequently purified by preparative HPLC, followed by the determination of rate constants and free energy barriers of enantiomerization in different buffer solutions at pH 2-9.3.     

Absolute Configuration and Enantiodifferentiation of a Hemicryptophane Incorporating an Azaphosphatrane Moiety

The hemicryptophane racemate (±)‐ M-1 , P-1 was optically resolved by semipreparative HPLC on Chiralpak IC column. The absolute configuration of each isolated enantiomer was established from the analysis of their electronic circular dichroism spectra. Enantiodifferentiation of the chiral cationic cage (±)‐ M-1 , P-1 was evidenced in solution using Δ‐TRISPHAT as chiral solvating agent, and the diastereomeric associations were observed in ¹H and ³¹P NMR spectra. Chirality 24:1077–1081, 2012. © 2012 Wiley Periodicals, Inc.     

Bidirectional Chiral Inversion of Trantinterol Enantiomers After Separate Doses to Rats

The chiral inversion and pharmacokinetics of two enantiomers of trantinterol, a new β₂ agonist, were studied in rats dosed (+)‐ or (?)‐trantinterol separately. Plasma concentrations of (+)‐ and (?)‐trantinterol were measured by chiral stationary phase liquid chromatography tandem mass spectroscopy (LC‐MS/MS). The apparent inversion ratio was calculated as the ratio of AUC_0‐t of (?)‐trantinterol or (+)‐trantinterol inverted from their antipodes to the sum of the AUC_0‐t of (?)‐ and (+)‐trantinterol. Following single intravenous administration, both given enantiomers declined in similar plasma concentrations, suggesting that the two enantiomers have approximately the same disposition kinetics by the route of intravenous administration. However, after single oral administration, plasma concentrations of uninverted (?)‐trantinterol at many timepoints were significantly higher than those of uninverted (+)‐trantinterol, suggesting that the two enantiomers undergo apparently different absorption or metabolism after oral administration. Significant bidirectional chiral inversion occurred after intravenous and oral administration of (+)‐ or (?)‐trantinterol. After dosing with optically pure enantiomer, the concentration of the administered enantiomer predominated in vivo. The AUC_0‐36 of (+)‐trantinterol after intravenous and oral dosing of (?)‐trantinterol were 16.6 ± 5.2 and 33.3 ± 16%, respectively of those of total [(+) + (?)] trantinterol. The AUC_0‐36 of (?)‐trantinterol after intravenous and oral dosing of (+)‐trantinterol were 19.6 ± 8.8 and 37.9 ± 4.5%, respectively, of those of total [(?) + (+)] trantinterol. After intravenous administration of (+)‐ and (?)‐trantinterol the chiral inversion ratios of the two enantiomers were not significantly different and similar results were found for oral administration. The extent of chiral inversion after intravenous administration was apparently lower, indicating that the bidirectional chiral inversion was not only systemic but also presystemic. Chirality 25:934–938, 2013.© 2013 Wiley Periodicals, Inc.     

Chiral separation of catechin by capillary electrophoresis using mono‐, di‐, tri‐succinyl‐β‐cyclodextrin as chiral selectors

The chiral separation of (±)‐catechin was investigated by capillary electrophoresis using characterized succinyl‐β‐cyclodextrins (Suc‐β‐CDs) with one to three degree of substitution values. The effects of nature and concentration of Suc‐β‐CDs and running buffer pH on the migration time and resolution of (±)‐catechin are discussed. All three kinds of Suc‐β‐CDs show a clear baseline separation of (±)‐catechin in capillary electrophoresis. Mono‐Suc‐β‐CD effectively separated (±)‐catechin, and additional substituted CDs (di‐ and tri‐Suc‐β‐CD) were capable of chiral separation at a broad pH range. The optimum running conditions were found to be 100 mM borate buffer (pH 9.8) containing 5 mM mono‐Suc‐β‐CD with no methanol organic modifier. Chirality, 2009. © 2009 Wiley‐Liss, Inc.     

Solid-phase extraction on Styrosorb cartridges as a sample pretreatment method in the stereoselective analysis of propranolol in human serum

Sample pretreatment using solid-phase extraction (SPE) on cartridges filled with small-particle Styrosorb porous polystyrene-based sorbent has been used in the analysis of propranolol enantiomers in human serum by high-performance liquid chromatography (HPLC) with fluorescent detection. SPE on Sep-Pak C₁₈ cartridges was used as a reference pretreatment method. The propranolol content of the samples was determined by achiral normal-phase HPLC and the enantiomeric ratio of propranolol (S/R) was then determined by chiral HPLC on a column with silica-bonded cellulose-tris(3,5-dimethylphenyl carbamate). Recoveries of propranolol from serum using SPE on Styrosorb and C₁₈ phases were 97±5% and 96±5%, respectively. Detection and quantification limits for propranolol enantiomers were 4 and 7 ng/ml, respectively.     

Lipase‐catalyzed kinetic resolution of novel antitubercular benzoxazole derivatives

Novel benzoxazole derivatives were synthesized, and their antitubercular activity against sensitive and drug‐resistant Mycobacterium tuberculosis strains (M. tuberculosis H₃₇Rv, M. tuberculosis sp. 210, M. tuberculosis sp. 192, Mycobacterium scrofulaceum, Mycobacterium intracellulare, Mycobacterium fortuitum, Mycobacterium avium, and Mycobacterium kansasii) was evaluated. The chemical step included preparation of ketones, alcohols, and esters bearing benzoxazole moiety. All racemic mixtures of alcohols and esters were separated in Novozyme SP 435‐catalyzed transesterification and hydrolysis, respectively. The transesterification reactions were carried out in various organic solvents (tert‐butyl methyl ether, toluene, diethyl ether, and diisopropyl ether), and depending on the solvent, the enantioselectivity of the reactions ranged from 4 to >100. The enzymatic hydrolysis of esters was performed in 2 phase tert‐butyl methyl ether/phosphate buffer (pH = 7.2) system and provided also enantiomerically enriched products (ee 88‐99%). The antitubercular activity assay has shown that synthesized compounds exhibit an interesting antitubercular activity. Racemic mixtures of alcohols, (±)‐4‐(1,3‐benzoxazol‐2‐ylsulfanyl)butan‐2‐ol ((±)‐ 3a ), (±)‐4‐[(5‐bromo‐1,3‐benzoxazol‐2‐yl)sulfanyl]butan‐2‐ol ((±)‐ 3b ), and (±)‐4‐[(5,7‐dibromo‐1,3‐benzoxazol‐2‐yl)sulfanyl]butan‐2‐ol ((±)‐ 3c ), displayed as high activity against M. scrofulaceum, M. intracellulare, M. fortuitum, and M. kansasii as commercially available antituberculosis drug‐Isoniazid. Moreover, these compounds exhibited twice higher activity toward M. avium (MIC 12.5) compared with Isoniazid (MIC 50).     

Dynamic Behavior of Clobazam on High‐Performance Liquid Chromatography Chiral Stationary Phases

Clobazam, a 1,5‐benzodiazepin‐2,4‐dione, is a chiral molecule because its ground state conformation features a nonplanar seven‐membered ring lacking reflection symmetry elements. The two conformational enantiomers of clobazam interconvert at room temperature by a simple ring‐flipping process. Variable temperature HPLC on the Pirkle type (R)‐N‐(3,5‐dinitronenzoyl)phenylglycine and (R,R)‐Whelk‐O1 chiral stationary phases (CSPs) allowed us to separate for the first time the conformational enantiomers of clobazam and to observe peak coalescence‐decoalescence phenomena due to concomitant separation and interconversion processes occurring on the same time scale. Clobazam showed temperature dependent dynamic high‐performance liquid chromatography (HPLC) profiles with interconversion plateaus on the two CSPs indicative of on‐column enantiomer interconversion. (enantiomerization) in the column temperature range between T_col = 10°C and T_col = 30°C, whereas on‐column interconversion was absent at temperature close to or lower than T_col = 5°C. Computer simulation of exchange‐deformed HPLC profiles using a program based on the stochastic model yielded the apparent rate constants for the on‐column enantiomerization and the corresponding free energy activation barriers. At T_col = 20°C the averaged enantiomerization barriers, ΔG^?, for clobazam were found in the range 21.08–21.53 kcal mol^?1 on the two CSPs. The experimental dynamic chromatograms and the corresponding interconversion barriers reported in this article are consistent with the literature data measured by DNMR at higher temperatures and in different solvents. Chirality 28:17–21, 2016. © 2015 Wiley Periodicals, Inc.     

Stereoselective Toxicity and Metabolism of Lactofen in Primary Hepatocytes From Rat

The stereoselective metabolism of lactofen in primary rat hepatocytes was studied using a chiral high‐performance liquid chromatographic (HPLC) method. Rac‐lactofen and its two enantiomers, S‐(+)‐ and R‐(?)‐lactofen, as well as two of its major metabolites, acifluorfen, S‐(+)‐ and R‐(?)‐desethyl lactofen, were used as substrates,. The single and joint cytotoxicity of parent compounds and the metabolites were assessed by coincubation with rat hepatocytes as target cells. Cytotoxicity was determined by the methyl tetrazolium (MTT) assay. In hepatocyte incubations, S‐(+)‐lactofen was degraded more rapidly than R‐(?)‐lactofen, and a stereospecific formation of S‐(+)‐desethyl lactofen was detected. Metabolism of lactofen to desethyl lactofen was processed with the retention of configuration, and the achiral compound, acifluorfen, was the shared metabolite generated from both S‐(+)‐ and R‐(?)‐lactofen. There was no chiral conversion of lactofen or desethyl lactofen enantiomers during the incubation. For the cytotoxicity research, the calculated EC₅₀ values indicated that when being applied individually, the parent compound was less toxic than its metabolites, while the combination with metabolites enhanced its cytotoxic effects. The data presented here would be helpful for a more comprehensive assessment of the ecotoxicological and environmental risks of lactofen. Chirality 25:743–750, 2013. © 2013 Wiley Periodicals, Inc.     

Optical resolution and enantiomeric purity determination of the analgesic cizolirtine

The optical resolution of (±)‐cizolirtine was accomplished with excellent results (>99% ee) by means of crystallization with (+)‐ or (−)‐di‐p‐toluoyltartaric acid. The optical purity of the samples was controlled by three independent methods: ¹H NMR, capillary electrophoresis (CE) (using β‐cyclodextrins as chiral resolving agents), and HPLC (using a glycoproteic column). The use of a rapid analytical technique like ¹H NMR for estimating the relative amounts of each enantiomer, together with the high sensitivity of CE, afforded a convenient strategy for monitoring the entire process leading to enantiopure compounds. Chirality 11:63–69, 1999. © 1999 Wiley‐Liss, Inc.     

Synthesis of enantiopure planar chiral bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophanes

A new type of planar chiral (R_p)‐ and (S_p)‐4,7,12,15‐tetrasubstituted [2.2]paracyclophanes was prepared from racemic 4,7,12,15‐tetrabromo[2.2]paracyclophane as the starting substrate. Regioselective lithiation and transformations afforded racemic bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophane (4,15‐dibromo‐7,12‐dihydroxy[2.2]paracyclophane). Its optical resolution was performed by the diastereomer method using a chiral camphanoyl group as the chiral auxiliary. The diastereoisomers were readily isolated by simple silica gel column chromatography, and the successive hydrolysis afforded (R_p)‐ and (S_p)‐bis‐(para)‐pseudo‐meta‐type [2.2]paracyclophanes ((R_p)‐ and (S_p)‐4,15‐dibromo‐7,12‐dihydroxy[2.2]paracyclophanes). They can be used as pseudo‐meta‐substituted chiral building blocks.     

Nonaqueous Capillary Electrophoretic Enantioseparation of Water Insoluble Tröger's Base Derivatives Using β‐Cyclodextrin as Chiral Selector

Racemic mixtures of six Tröger's base derivatives were separated by chiral nonaqueous capillary electrophoresis. The separation protocol was optimized first for suitable solvents. Then the applicability of various salts dissolved in organic solvents and their mixtures was evaluated. As chiral selectors β‐cyclodextrin and heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin at various concentrations were used. The best enantioselectivity for the studied analytes was obtained utilizing formamide as organic nonaqueous solvent containing a mixture of sodium citrate and tris(hydroxymethyl)aminomethane acetate as electrolytes, and β‐cyclodextrin as chiral additive. The experimental results demonstrated the feasibility of nonaqueous capillary electrophoresis for enantioseparation of Tröger's base derivatives. This technique represents a suitable alternative to more commonly used capillary electrophoresis in aqueous environment. Chirality 25:810–813, 2013. © 2013 Wiley Periodicals, Inc.     

Benzoylcellulose derivatives as chiral stationary phases for open tubular column chromatography

The application of cellulose-based stationary phases for chiral separations has been extended to open tubular column chromatography. Efficient columns were obtained by coating the capillaries with mixtures of chiral cellulose materials and conventional achiral stationary phases for gas chromatography. In this study, various siloxane and polyethylene glycol polymers were used as achiral components and mixed with different substituted benzoylcellulose derivatives as chiral components. Systematic investigations were carried out to determine the optimal ratio for the components of the stationary phase. Depending on the chromatographic mode—gas chromatography (GC) or supercritical fluid chromatography (SFC)—the stationary phases were found to behave differently. The applicability of the technique was demonstrated by the resolution of various racemic compounds. © 1993 Wiley-Liss, Inc.