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A chromatographic and computational study on the driving force operating in the exceptionally large enantioseparation of N-thiocarbamoyl-3-(4 -biphenyl)-5-phenyl-4,5-dihydro-(1H) pyrazole on a 4-methylbenzoate cellulose-based chiral stationary phase
No full textThis paper describes the results obtained in the HPLC enantioseparation of N-thiocarbamoy1-3-(4'-biphenyl)-5-phenyl-4,5-dihydro-(1H) pyrazole on a cellulose tris(4-methylbenzoate) chiral stationary phase (OJ-3 CSP) using normal-phase and polar organic conditions. The enantioseparation factor (alpha = 207) observed using the mixture n-hexane-2-propanol 70:30 as a mobile phase is among the highest values ever reported in enantioselective HPLC. The enantioseparation process was investigated by means of molecular modelling techniques. Chromatographic and theoretical investigations addressed the extreme affinity of the most CSP retained (S)-enantiomer to the intermolecular H bond network between the ligand thioamide group and the stationary phase O atoms. (C) 2013 Elsevier B.V. All rights reserved.This paper describes the results obtained in the HPLC enantioseparation of N-thiocarbamoyl-3-(4’-biphenyl)-5-phenyl-4,5-dihydro-(1H) pyrazole on a cellulose tris(4-methylbenzoate) chiral stationary phase (OJ-3 CSP) using normal-phase and polar organic conditions. The enantioseparation factor (alpha = 204) observed using the mixture n-hexane-2-propanol 70:30 as a mobile phase is among the highest values ever reported in enantioselective HPLC. The enantioseparation process was investigated by means of molecular modelling techniques. Chromatographic and theoretical investigations addressed the extreme affinity of the most CSP retained (S)-enantiomer to the intermolecular H bond network between the ligand thioamide group and the stationary phase O atoms
Influence of the nature of alkyl substituents on the high-performance liquid chromatography enantioseparation and retention of new atropisomeric 1,1’-bibenzimidazole derivatives on amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase
No full textSix new atropisomeric heteroarenes were synthesized by connecting two 2-alkylbenzimidazole fragments via N-N junction. They differ by the substituent nature (methyl, ethyl, propyl, butyl, pentyl and hexyl) of the aliphatic function. The novel atropisomeric compounds were used as chiral probes to study the chromatographic behavior of the amylose tris(3,5-dimethylphenyl carbamate) (Chiralpak AD-3) chiral stationary phase (CSP) under normal phase mode. The pivotal role of the length and flexibility of the 2,2'-alkyl groups on retention, enantioselectivity and enantiomer elution order was demonstrated by enantioselective HPLC analysis. Additional information on the chiral recognition mechanism was obtained from the evaluation of the correlated thermodynamic data
Recognition mechanism of aromatic derivatives resolved by argentation chromatography: The driving role played by substituent groups
No full textArgentation chromatography is widely used nowadays as a powerful tool to separate complex mixtures of analytes containing unsaturated and/or aromatic fragments. Here we present the results of chromatographic and computational studies on a silver-thiolate stationary phase, in which the silver metal is covalently bonded to mercaptopropyl silica particles. The exceptionally high selectivity displayed by this organometallic moiety prompted us to deeply investigate its molecular recognition properties. The interactions of the silver atom with a series of benzene derivatives was investigated to gain information on the mechanism by which the different ring substituents modulate retention factors and selectivity. The experimental trend was fully rationalized by means of quantum-mechanical Density Functional Theory (DFT) calculations, which allowed us to elucidate the chromatographic results in the light of unusual and unexpected substituent effects
3-(Phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole: a fascinating molecular framework to study the enantioseparation ability of the amylose (3,5-dimethylphenylcarbamate) chiral stationary phase. Part II. Solvophobic effects in enantiorecognition process
No full textThe enantiomers of five chiral compounds incorporating the 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole scaffold and differing only in the linear alkyl chain (varying in length from 1 to 5 carbon atoms) linked to the oxygen atom were directly separated on the amylose (3,5-dimethylphenylcarbamate) based Chiralpak AD-3 chiral stationary phase. The effects of the mobile phase composition, the structure of the analytes and temperature on the retention and enantioselectivity were investigated. It was found that the enantiomeric separations were in all cases enthalpy-driven and disfavored by entropic term. U-shape curves obtained by plotting the chromatographic data versus the alcoholic percentage in n-pentane-methanol and n-hexane-ethanol mobile phases highlighted that, at higher alcohol concentrations, solvophobic interactions were operative in the retention mechanism. The unusual trend of such curves was linked to the nature of alkyl chain of the pyrazolines and it was indicative of the solvophobic contribution to the achievement of a high degree of enantioseparation
3-(Phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole: a fascinating molecular framework to study the enantioseparation ability of the amylose (3,5-dimethylphenylcarbamate) chiral stationary phase. Part I. Structure-enantioselectivity relationships
Full text linkChiral stationary phases (CSPs) based on amylose (3,5-dimethylphenylcarbamate) (ADMPC) exhibit awide-range of enantioselectivity in high-performance liquid chromatography (HPLC) and supercriticalfluid chromatography (SFC). Although this class of CSPs has been extensively used, chiral discriminationsat receptorial level, which are useful to develop predictive molecular models, have been rarely reportedin the literature.Herein, we describe the results obtained in the enantioselective HPLC of a set of six C5-chiral 4,5-dihydro-(1H)-pyrazole derivatives on the ADMPC-based Chiralpak AD-3 CSP (CSP) under normal-phaseand polar organic conditions. Using pure methanol as a mobile phase the exceptional enantioseparationfactor value of 50 at 25◦C was found for one of the investigated analytes. To the best of our knowledge, theenantiomeric bias represents the most outstanding enantioseparation ever recorded on ADMPC-basedCSPs.Systematic variations in chemical groups in specific positions of the 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole molecular framework resulted in peculiar changes in retention andenantioselectivity. A careful analysis of the chromatographic data permitted to advance some hypothesesconcerning the role played by the individual chemical groups in determining the exceptional enantiosep-aration.In particular, under methanol-rich mode, the prenyl moiety of the second eluted enantiomer of thebetter resolved analyte was recognized as a critical structural element to establish direct and favorablesolvophobic interactions with apolar portions of selector
3-Methylcyclohexanone thiosemicarbazone: Determination of E/Z isomerization barrier by dynamic high-performance liquid chromatography, configuration assignment and theoretical study of the mechanisms involved by the spontaneous, acid and base catalyzed processes
No full textDirect HPLC diastereo- and enantioseparation of 3-methylcyclohexanone thiosemicarbazone (3-MCET) on a polysaccharide-based chiral stationary phase under normal-phase conditions. The optimized chromatographic system was employed in dynamic HPLC experiments (DHPLC), as well as detection technique in a batch wise approach to determine the rate constants and the corresponding free energy activation barriers of the spontaneous, base- and acid-promoted E/Z diastereomerization of 3-MCET. The stereochemical characterization of four stereoisomers of 3-MCET was fully accomplished by integrating the results obtained by chemical correlation method with those derived by theoretical calculations and experimental investigations of circular dichroism (CD). As a final goal, a deepened analysis of the perturbing effect exercised by the stationary phase on rate constant values measured through DHPLC determinations as a function of the chromatographic separation factor α of the interconverting species was successfully accomplished. This revealed quite small deviations from the equivalent kinetic values obtained by off-column batch wise procedure, and suggested a possible effective correction of rate constants measured by DHPLC approac
Unexpected different chemoselectivity in the aerobic oxidation of methylated planar catechin and bent epicatechin derivatives catalysed by the Trametes villosa laccase/1-hydroxybenzotriazole system
Full text linkUnreported methylated catechin and epicatechin derivatives 5 and 6 were synthesized by an oxa-Pictet-
Spengler reaction. Catechin 5 shows the B and C rings coplanar because of the formation of a trans
junction between the C ring and the newly generated six-term cycle D, in turn condensed to ring B. In
contrast, epicatechin 6 presents a bent geometry due to the establishment of a cis junction between the
C ring and the newly formed cycle D. The oxidation of compounds 5 and 6 in the presence of the
Trametes villosa laccase/1-hydroxybenzotriazole (HBT) system was investigated under aerobic conditions
in both a biphasic system and a reverse micelle. The unexpected different chemoselective oxidation at
the benzylic position of catechin and epicatechin derivatives 5 and 6 has been rationalized using a
molecular modelling approach. These results demonstrate that the Trametes villosa laccase/HBT system
represents a useful tool to functionalize the C-2 or C-4 position of phenolic compounds depending on
the structural features
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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