Enantioseparation and thermodynamic study of naphthalene derivatives,new melatoninergic agonists,on coated amylose [tris(S)‐1‐phenylethylcarbamate] stationary phase. Transposition to preparative scale

This work reports a high‐performance liquid chromatography normal‐phase methodology to elucidate enantiomers of naphthalene derivatives, evaluated as melatoninergic agonists. For this purpose four different polysaccharide based chiral stationary phases were evaluated, namely Chiralcel OD‐H (cellulose tris‐3,5‐dimethylphenylcarbamate), Chiralcel OJ (cellulose tris‐methylbenzoate), Chiralpak AD (amylose tris‐3,5‐dimethylphenylcarbamate) and Chiralpak AS (amylose tris‐(S)‐1‐phenylethylcarbamate) with different alcoholic modifiers on different amounts in n‐heptane. A temperature study was carried out, between 20 and 40 °C and the apparent thermodynamic parameters were calculated thanks to the Van't Hoff linearization. For all compounds (except 3), ΔΔH° and ΔΔS° exhibited positive values ranging from 791.2 to 9999.3 J/mol and from 3.9 to 37.8 J/K/mol respectively, indicating entropically driven separations. Optimized conditions led to goof resolution of 2.37 for compound 1 on Chiralpak AS, with heptane–2‐propanol 90:10 (v/v), at a temperature of 30 °C. Then they were transposed to the preparative scale for compound 1, generating 22 mg of each enantiomer with an 80% yield. The limits of detection and of quantification were determined to allow the calculation of the enantiomeric excess. They were found with very low values, equal to 0.32 and 1.05 µ m and 0.33 and 1.11 µ m, respectively, for peaks 1 and 2 of compound 1. Copyright © 2014 John Wiley & Sons, Ltd.     

Chromatographic behavior of small organic compounds in low‐temperature high‐performance liquid chromatography using liquid carbon dioxide as the mobile phase

Low‐temperature high‐performance liquid chromatography, in which a loop injector, column, and detection cell were refrigerated at –35ºC, using liquid carbon dioxide as the mobile phase was developed. Small organic compounds (polyaromatic hydrocarbons, alkylbenzenes, and quinones) were separated by low‐temperature high‐performance liquid chromatography at temperatures from –35 to –5ºC. The combination of liquid carbon dioxide mobile phase with an octadecyl‐silica (C₁₈) column provided reversed phase mode separation, and a bare silica‐gel column resulted in normal phase mode separation. In both the cases, nonlinear behavior at approximately –15ºC was found in the relationship between the temperature and the retention factors of the analytes (van't Hoff plots). In contrast to general trends in high‐performance liquid chromatography, the decrease in temperature enhanced the separation efficiency of both the columns.     

Enantioseparation of racemic paroxol on an amylose‐based chiral stationary phase by supercritical fluid chromatography

The separation of racemic paroxol, a key precursor of trans‐(?)‐paroxetine, on Chiralpak AD‐H, an amylose‐based chiral stationary phase, by supercritical fluid chromatography was studied. Pulse experiments were investigated using supercritical carbon dioxide modified with methanol (MeOH), ethanol and 2‐propanol at 35°C and 15 MPa. Retention and separation factors were determined under analytical conditions for different mobile phase compositions. Among the modifiers used, MeOH was shown to be the best additive, and 5% v/v of MeOH was the preferable concentration at which selectivity of 1.14 and resolution of 3.0 was obtained. In order to evaluate the potential with respect to preparative separations, the adsorption isotherms of individual enantiomers of paroxol were estimated using the elution by characteristic point method. Isotherm parameters were determined from the overloaded elution profiles that were collected at pressure ranging from 15 to 24 MPa. The isotherms obtained were further validated by comparing experimentally recorded elution profiles with the predictions based on the equilibrium‐dispersive model. The results are important to the process design and optimization of preparative supercritical fluid chromatography application.     

A comparative study of hydrophobicity of octadecyl and alkylamide bonded phases based on methylene selectivity

The conformational dynamics of home‐made monomeric and polymeric (octadecyl – C18 and alkylamide – AP) stationary phases have been investigated by analyzing the dependence of retention on the reciprocal temperature for different organic compounds. Theoretical considerations were based on the results of methylene selectivity determination. Two selected PAHs – naphthacene and benzo[a]anthracene, whose structures differ in the position of the ring – were chosen as solutes to study the thermodynamics of retention behavior. Standard enthalpies (ΔH ⁰) were obtained from the slope of the plots of the capacity factor logarithm (ln k) against the reciprocal temperature (1/T), i. e. van't Hoff plots, in a temperature range from 285 to 315 K. The conformation of the stationary phase (monomeric and polymeric) structure and the length of bonded ligands with their specific functional (N‐acylamide) groups had a significant effect on retention.     

A Consistent Determination of the Dimerization Constants of the Self‐Association of 2,2‐Dimethyl‐3‐ethyl‐3‐pentanol in Carbon Tetrachloride from its Infrared Spectral Data

Two equations of linear type (Eqs. 10 and 17 in the text) have been derived to analyze the IR data to determine the dimerization constant consistently. Equation 10 is to be used to fit the integrated absorbances of the monomer band to obtain the molar monomer absorptivity, ?_m, and dimerization constant, K; Eq. 17 is to be used to fit the integrated absorbances of the dimer bands to obtain the molar dimer absorptivity, ?_d, and dimerization constant, K. Thus the same dimerization constant can be independently determined either from the monomer band or from the dimer band. The discrepancy between the two determined values provides an assessment of the consistency of determination. The monomer‐dimer self‐association of 2,2‐dimethyl‐3‐ethyl‐3‐pentanol in the solvent of carbon tetrachloride was chosen to demonstrate the utility of these two equations.     

A model investigation of ab initio geometries for identity and nonidentity substitution with three‐center four‐ and three‐electron transition states

With the van't Hoff model derived from the dynamics of a regular tetrahedron in which the interstitial carbon changes its position from tetrahedral into a trigonal (bi)pyramidal configuration, the introduction of dimensionless ratio numbers (R) can serve to localize significant points on the principal reaction coordinate. These numbers are expressed as R(d) and R(n) based on transition state geometries and the number of electrons involved in the three‐center bonding, respectively. Using this concept, we obtain a model for the evaluation of different ab initio calculations based on identity and nonidentity substitution reactions for three‐center four‐ and three‐electron bonding transition states. Similar ratio numbers have been derived for proton exchange reactions. The reactions under investigation show clearly that in spite of the differences in chemical outcome the transition steps reduced to the first principles of chemical bonding are similar. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Retention behavior of resorcinarene‐based cavitands on C8 and C18 stationary phases

The understanding of the retention behavior of large molecules is an area of interest in liquid chromatography. Resorcinarene‐based cavitands are cavity‐shaped cyclic oligomers that can create host–guest interactions. We have investigated the chromatographic behavior of two types of cyclic tetramers as analytes in high‐performance liquid chromatography. The experiments were performed at four different temperatures (15, 25, 35, 45°C) on two types of reversed stationary phases (C₈ and C₁₈) from two different manufacturers. We have found a huge difference between the retention of resorcinarenes and cavitands. In some cases, the retention factor of cavitands was even a hundred times larger than the retention factor of resorcinarenes. The retention of methylated derivates was two to four times larger compared to that of demethylated compounds on every column. The opposite retention behavior of the resorcinarenes and cavitands on the two types of stationary phases showed well the difference of the selectivity of the XTerra and BDS Hypersil columns. The retention mechanism was studied by the thermodynamic parameters calculated from the van't Hoff equation.     

Molecular model studies based on ab initio calculations of nucleophilic and electrophilic addition‐substitution reactions focused on carbon–halogen compounds

We describe with molecular model studies based on the intrinsic parameters of van't Hoff's regular tetrahedron nucleophilic and electrophilic addition‐substitution reactions via a selected reaction coordinate for the displacement of carbon. Geometries of stable, intermediate transition complexes or transition states are compared with the corresponding ab initio values. Specific attention is given on the hypervalent and nonhypervalent character of carbon supported by ab initio calculations, our model consideration and experimental evidence. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A combined experimental,theoretical, and Van't Hoff model study for identity methyl,proton, hydrogen atom,and hydride exchange reactions. Correlation with three‐center four‐, three‐, and two‐electron systems

We have studied carbon transfer reactions following an S_N2 reaction profile. With ab initio calculations and experimental geometries concerning the nature of the various complexes indicated as stable, intermediate, and transition state we were able to show the additional value of van't Hoff's tetrahedral configuration by changing its geometry via a trigonal pyramid into a trigonal bipyramid. The ratio of the apical bond and corresponding tetrahedral bond distances is then nearly 1.333. The relevance of this approach has also been shown for identity proton‐(hydrogen atom‐, and hydride‐) in‐line reactions. The use of this geometrical transmission will be demonstrated for the hydrogen bonding distances in e.g., DNA duplexes and other biological (supra) molecular systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Validated Method for the Simultaneous Determination of Gemifloxacin and H2‐receptor Antagonists in Bulk,Pharmaceutical Formulations and Human Serum by RP‐HPLC; In‐vitro Applications

Simple, isocratic and rapid RP‐HPLC method has been developed for the simultaneous analysis of gemifloxacin and H₂‐receptor antagonists i.e. Cimetidine, Famotidine and Ranitidine, in bulk, pharmaceutical formulation and human serum. Separation was achieved on the RP‐Mediterranea column [C18 (250 × 4.6 mm, 5 μ)] at ambient temperature using mobile phase consisting of acetonitrile: methanol: water (20:28:52 v/v/v pH 2.8 adjusted by phosphoric acid). Flow rate was 1.0 mL/min with an average operating pressure of 180 kg/cm². Gatifloxacin (GATI) was used as an internal standard (IS). Quantitation was achieved with UV detection at 221, 256 and 267 nm, respectively. Linear calibration curves, at concentration ranges of 0.05‐37.5 μgmL^‐L with a correlation coefficient of ±0.9994. The detection and quantification limits were in the ranges of 0.023‐0.250 μgmL^‐L and 0.071‐0.756 μgmL^‐L, respectively. Friedman's and Student's t‐test were applied to correlate these results. Method was validated in terms of selectivity, linearity, precision, robustness, recovery, limits of detection and quantitation and is applicable to the routine analysis of GFX and H₂‐receptor antagonists, alone or in combination.     

High‐performance liquid chromatography thermodynamic study of new potential antiepileptic compounds on a cholesterol column using isocratic elution with methanol/water and acetonitrile/water eluent systems

Basic thermodynamic functions responsible for retention of new 1,2,4‐triazole derivatives exhibiting varied antiepileptic activity on cholesterol‐based stationary phase were determined. Evaluation of the Gibbs energy change, the change in enthalpy and the change in entropy was based on the van't Hoff relationship representing lnk versus 1/T . A detailed discussion of the van't Hoff equation, exploring the influence of the phase ratio on deviations from linearity in a van't Hoff plot is presented. We show chromatographic evidence to the question of how a varied mobile phase composition may cause different thermodynamic phase ratios. The analysis of data from a differential scanning calorimetry excluded any phase transitions of either the individual solutes or cholesterol stationary phase suspended in the mobile phase components within the studied temperature range.     

Conformational analysis. Part 40: a theoretical and NMR investigation of the conformations of cis‐ and trans‐cyclopentane‐1,3‐diol

The conformations of cis‐ ( 1 ) and trans‐cyclopentane‐1,3‐diol ( 2 ) have been studied by ab initio (Gaussian 98) and molecular mechanics (PCMODEL) calculations and by NMR spectroscopy. The calculations gave two low‐energy conformations for ( 1 ), 1A and 1B , both with axial hydroxyl groups. Two conformations with equatorial hydroxyl groups ( 1C and 1D ) were found but with much higher energy (ca 4.0 kcal mol^?1). Five low‐energy conformers were found for 2 . Four were envelope conformations and one a half‐chair. The complete analysis of the 400 MHz ¹H NMR spectra of 1 in a variety of solvents and 2 in chloroform was performed by extensive decoupling experiments, iterative computer analysis and spectral simulation. This gave all the H,H couplings in the molecule, including in 1 a long‐range ⁴J(H,H) coupling between H‐2cis and H‐4,5cis. The ³J(H,H) couplings were used to determine the conformer populations in these molecules. This was initially achieved using the Haasnoot, de Leeuw and Altona equation. to obtain the conformer couplings. It was found that this equation was not accurate for the C·CH₂·CH₂·C fragment in these molecules and the following equation was derived for this fragment from five‐ and six‐ membered cyclic compounds in fixed conformations: (1) The conformer populations were obtained by calculating the conformer couplings which were then compared with the observed couplings. Compound 1 in benzene solution is an approximately equal mixture of conformers 1A and 1B with small (<4%) amounts of 1C and 1D . In the polar solvents acetone and acetonitrile the populations of 1A and 1B are again equal, with 20% of 1C and <2% of 1D . In 2 the major conformers are 2B and 2D with small amounts of 2C , 2E and 2A . These novel findings are considered with previous data on cyclopentanol and cis‐ and trans‐cyclopentane‐1,2‐diol and it is shown that the axial hydroxyl substituent at the fold of the envelope appears to be a major factor in determining the conformational energies of these compounds. Copyright © 2003 John Wiley & Sons, Ltd.     

Indirect reversed‐phase high‐performance liquid chromatographic and direct thin‐layer chromatographic enantioresolution of (R,S)‐Cinacalcet

Enantioresolution of the calcimimetic drug (R,S)‐Cinacalcet was achieved using both indirect and direct approaches. Six chiral variants of Marfey's reagent having l ‐Ala‐NH₂, l ‐Phe‐NH₂, l ‐Val‐NH₂, l ‐Leu‐NH₂, l ‐Met‐NH₂ and d ‐Phg‐NH₂ as chiral auxiliaries were used as derivatizing reagents under microwave irradiation. Derivatization conditions were optimized. Reversed‐phase high‐performance liquid chromatography was successful using binary mixtures of aqueous trifluoroacetic acid and acetonitrile for separation of diastereomeric pairs with detection at 340 nm. Thin silica gel layers impregnated with optically pure l ‐histidine and l ‐arginine were used for direct resolution of enantiomers. The limit of detection was found to be 60 pmol in HPLC while in TLC it was found to be in the range of 0.26–0.28 µg for each enantiomers. Copyright © 2010 John Wiley & Sons, Ltd.     

Direct separation of the enantiomers of ramosetron on a chlorinated cellulose‐based chiral stationary phase in hydrophilic interaction liquid chromatography mode

Ramosetron is an enantiopure active pharmaceutical ingredient marketed in Japan since 1996 and later in a few Southeast Asian countries predominantly as an antiemetic for patients receiving chemotherapy. In this study, a simple and rapid high‐performance liquid chromoatography method for the separation of ramosetron and its related enantiomeric impurity by using hydrophilic interaction liquid chromatography mode is presented. Chiral resolution was performed on an analytical column (100 mm × 4.6 mm id) packed with 3 μm particles of cellulose‐based Chiralpak IC‐3 chiral stationary phase. Using a mobile phase containing acetonitrile–water–diethylamine (100:10:0.1, v/v/v) and setting the column temperature at 35°C, the resolution value was 7.35. At a flow rate of 1 mL/min, the enantioseparation was completed within 5 min. The proposed method was partially validated and it has proven to be sensitive with limit of detection and limit of quantitation of the (S)‐enantiomer impurity of 44.5 and 133.6 ng/mL.     

High‐performance liquid chromatographic and subcritical fluid chromatographic separation of α‐arylated ß‐carboline,N‐alkylated tetrahydroisoquinolines and their bioisosteres on polysaccharide‐based chiral stationary phases

New, pharmacologically interesting chiral amino compounds, namely, stereoisomers of α‐hydroxynaphthyl‐ß‐carboline, benz[d]azepine and benz[c]azepine analogs as well as N‐α‐hydroxynaphthylbenzyl‐substituted isoquinolines were enantioseparated by high‐performance liquid chromatographic and subcritical fluid chromatographic methods on polysaccharide‐based chiral stationary phases. Separation of the stereoisomers was optimized in both subcritical fluid chromatography and normal phase liquid chromatographic modes by investigating the effects of the composition of the bulk solvent, temperature, and the structures of the analytes and selectors. Both normal phase liquid chromatography and subcritical fluid chromatography exhibited satisfactory performance, albeit with somewhat different effectiveness in the separation of the stereoisomers studied. The optimized methods offer the possibility to apply preparative‐scale separations thereby enabling further pharmacological investigations of the enantiomers.     

Enantioselective separation of twelve pairs of enantiomers on polysaccharide‐based chiral stationary phases and thermodynamic analysis of separation mechanism

The enantioseparation of twelve pairs of structurally related 1‐aryl‐1‐indanone derivatives was studied in the normal‐phase mode using three different polysaccharide‐type chiral stationary phases, namely Chiralpak IB, Chiralpak IC, and Chiralpak ID. n‐Hexane/2‐propanol and n‐hexane/ethanol were employed as mobile phases. Among all the investigated chiral columns, Chiralpak IC exhibited the most universal and the best enantioseparation ability toward all the racemates, particularly with the mobile phase composed of n‐hexane/2‐propanol (90/10, v/v). Then the effects of column temperature on retention and enantioselectivity were examined in the range of 25–40°C. Satisfactory enantioseparation was obtained at ambient temperature. The natural logarithm of retention and separation factors (ln k and ln α) versus the reciprocal of absolute temperature (1/T) (Van't Hoff plots) were found to be linear for all racemates, indicating that the retention and separation mechanisms were independent of temperature in the range investigated. Then, the thermodynamic parameters (ΔΔH°, ΔΔS°, and ΔΔG°) were calculated from Van't Hoff plots. These values indicated that the solute transfer from the mobile to stationary phase was enthalpically favorable, and the process of enantioseparation was mainly enthalpy controlled. At last, the impact of small changes in molecular structures of the tested 1‐indanone derivatives on enantioseparation was also discussed.     

High‐performance liquid chromatographic separations of stereoisomers of chiral basic agrochemicals with polysaccharide‐based chiral columns and polar organic mobile phases

The separation of the stereoisomers of 23 chiral basic agrochemicals was studied on six different polysaccharide‐based chiral columns in high‐performance liquid chromatography with various polar organic mobile phases. Along with the successful separation of analyte stereoisomers, emphasis was placed on the effect of the chiral selector and mobile phase composition on the elution order of stereoisomers. The interesting phenomenon of reversal of enantiomer/stereoisomer elution order function of the polysaccharide backbone (cellulose or amylose), type of derivative (carbamate or benzoate), nature, and position of the substituent(s) in the phenylcarbamate moiety (methyl or chloro) and the nature of the mobile phase was observed. For several of the analytes containing two chiral centers all four stereoisomers were resolved with at least one chiral selector/mobile phase combination.     

Chiral separation and thermodynamic investigation of WCK 3023: A novel oxazolidinone antibacterial agent,application to pre‐clinical pharmacokinetic study

A chiral liquid chromatographic method was developed and validated for the quantification of R‐enantiomer impurity (RE) in WCK 3023 (S‐enantiomer), a new drug substance. The separation was achieved on Chiralpak IA (amylose‐based immobilized chiral stationary phase), using a mobile phase consisting of n‐hexane–ethanol–trifluoroacetic acid (70:30:0.2, v/v/v) at a flow rate of 1.0 mL/min. The method was extensively validated for the quantification of RE in WCK 3023 and proved to be robust. For RE the detector response was linear over the concentration range of 0.11–5 μg/mL. The limit of quantitation and limit of detection for RE were 0.11 and 0.04 μg/mL respectively. Average recovery of the RE was in the range of 98.11–99.55%. The developed method was specific, sensitive, precise and accurate for quantitative determination of RE in WCK 3023. The impact of thermodynamic parameters on the chiral separation was evaluated. The method was employed for controlling the enantiomeric impurity in the lots of WCK 3023 used for pre‐clinical studies. The method was successfully applied to evaluate the possible conversion of WCK 3023 to RE in rat serum samples during pre‐clinical pharmacokinetic studies.     

Benedict's solution/ vitamin C: An alternative catalytic protocol for the synthesis of regioselective‐1,4‐disubstituted‐1H‐1,2,3‐triazoles at room temperature

A novel and highly efficient method for the synthesis of 1,4‐disubstituted‐1H‐1,2,3‐triazoles by copper‐catalyzed azide‐alkyne cycloaddition has been developed. This economic and sustainable protocol uses a readily available Benedict's solution/Vitamin C catalyst system affording a wide range of 1,4‐disubstituted‐1H‐1,2,3‐triazoles under mild conditions.