Home  About us  Contact
 

Enantiomeric Pairs (enantiomeric + pair)

Distribution by Scientific Domains
Chemistry66%

Selected Abstracts

The effect of co-surfactant-modified micelles on chiral separations in EKC

ELECTROPHORESIS, Issue 16 2009
Adeline B. Kojtari
Abstract The use of chiral pseudostationary phases in EKC provides high efficiencies and excellent resolution for enantiomeric separations. The chiral pseudostationary phases of interest in this study are alcohol-modified ("swollen") micelles, in which a co-surfactant (medium chain-length alcohol) is added with the surfactant. In this study, the chiral surfactant, dodecoxycarbonylvaline (DDCV), along with the co-surfactant, 2-hexanol, has been prepared as swollen micelle in order to investigate the chiral separation of enantiomeric pairs. Three sets of experiments were investigated in which swollen micelle systems contained: chiral surfactant and racemic co-surfactant; chiral surfactant and chiral co-surfactant; and phase ratio increases, in which both chiral surfactant and chiral co-surfactant were employed. In the first two sets of experiments, co-surfactant concentration was held constant and the surfactant concentration was increased. In the third set of experiments, both surfactant and chiral surfactant concentrations were increased proportionally. The chromatographic figures of merit for each enantiomeric pair were investigated and compared with various chiral aggregate systems. In swollen micelle compositions using constant racemic 2-hexanol concentration, when DDCV concentration increased, enantioselectivity and resolution increased; whereas, efficiency remained constant for most of the test compounds. Compositions using constant S -2-hexanol concentration reached a maximum in all chromatographic figures of merit when DDCV concentration was increased from 2 to 3%. An increase in both surfactant and co-surfactant concentrations led to noisy baselines and chiral aggregates that were generally unstable in solution. [source]

Optimization of capillary electrophoretic enantioseparation for basic drugs with native ,-CD as a chiral selector

ELECTROPHORESIS, Issue 12 2006
Nerissa L. Deńola
Abstract This study presents the advantages of the 20,µm inner diameter (id) capillary for the enantioseparation of ten basic drugs with native ,-CD as the chiral selector. The apparent binding constants of each enantiomeric pair were determined to calculate the optimum ,-CD concentration ([,-CD]opt) and the optimization was subsequently carried out. Comparison of the 20,µm id with 50,µm id were made in terms of the results obtained in the optimization and detection limits. Applying the optimum conditions for each compound, reproducible results (RSD from 0,3; n>5) were obtained for the 20,µm id capillary. Although the sensitivity is lower in the 20,µm id capillary, the LOD determined using this capillary is still found to be acceptable for the ten basic drugs studied. Enhanced resolution and faster analysis times were the main advantages observed with the use of this capillary in enantioseparation. [source]

Determination of amino acids in rat vitreous perfusates by capillary electrophoresis

ELECTROPHORESIS, Issue 17 2004
Kongthong Thongkhao-On
Abstract In vivo determinations of amino acids are important for improving our understanding of physiological states of biological tissue function and dysfunction. However, the chemically complex matrix of different biological fluids complicates the assay of this important class of molecules. We introduce a method for characterizing the amino acid composition of submicroliter volumes of vitreous humor perfusates. Low-flow push-pull perfusion sampling is compatible with collecting small volume samples in a complicated matrix that are potentially difficult to separate. An efficient, sensitive, and rapid analysis of amino acids from in vivo perfusates of the vitreous is presented with 3-(4-carboxybenzoyl)-2-quinoline-carboxaldehyde (CBQCA) derivatitation and capillary electrophoresis (CE) separation with laser-induced fluorescence detection (LIF). Derivatization with CBQCA for up to 2 h provided high sensitivity and low detection limits at the nM level. Seventeen amino acids including D -serine (D -Ser) and D -aspartate (D -Asp) were resolved in less than 10 min. Importantly, D -Ser is separated from its enantiomeric pair. Characterization of vitreal amino acids with this assay technique will be useful for understanding ocular diseases and physiological mechanisms in vision. [source]

Reactions of [Et4N][Tp*W(,3 -S)(,-S)2­(CuSCN)2] with Nitrogen Donor Ligands: Syntheses, Structures, and Third-Order Nonlinear Optical Properties

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 28 2009
Zhen-Hong Wei
Abstract Reactions of the preformed cluster [Et4N][Tp*W(,3 -S)(,-S)2(CuSCN)2] (1) with pyridine (py), 4,4,-bipyridine (4,4,-bipy), or 1,3-bis(4-pyridyl)propane (bpp) led to the formation of three neutral [Tp*W(,3 -S)(,-S)2Cu2]-based compounds [Tp*W(,3 -S)(,-S)2Cu2(SCN)(py)2] (2), [{Tp*W(,3 -S)(,-S)2Cu2(SCN)}2(4,4,-bipy)]·3.5H2O (3·3.5H2O), and [Tp*W(,3 -S)(,-S)2Cu2(SCN)(bpp)]2 (4), respectively. Compounds 2,4 were characterized by elemental analysis, IR spectra, UV/Vis spectra, 1H NMR, and X-ray analysis. There are two linkage isomers [Tp*W(,3 -S)(,-S)2Cu2(SCN)(py)2] and [Tp*W(,3 -S)(,-S)2Cu2(NCS)(py)2], each of which has its own enantiomeric pair in the crystal of 2. Compound 3 has a double butterfly-shaped structure in which two [Tp*W(,3 -S)(,-S)2Cu2(SCN)] fragments are linked with a single 4,4,-bipy bridge. For 4, the two butterfly-shaped [Tp*W(,3 -S)(,-S)2Cu2(SCN)] fragments are interconnected by a pair of bpp bridges. The third-order nonlinear optical (NLO) performances of 2,4 in DMF were also investigated by Z -scan techniques.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Helical chirality in hexamethylene triperoxide diamine

MAGNETIC RESONANCE IN CHEMISTRY, Issue 9 2006
Chunlei Guo
Abstract The primary explosive hexamethylenetriperoxide diamine has previously been found to exist in the solid state as a racemic mixture of helically chiral, threefold symmetric enantiomers; another enantiomeric pair of low-energy conformers has been predicted, but has never been observed. We show by solution 2D NMR at 14 T, in achiral solution and by addition of chiral shift reagents, that all four optically isomeric conformers coexist at slow equilibrium on the NMR timescale at room temperature, and can be observed. Calculations of the 1H and 13C NMR chemical shifts using gauge-including atomic orbital methods are in excellent agreement with experiment; thermochemical calculation of the free energies in solution are in somewhat worse agreement, but correctly predict the relative stability of the conformers. Analysis of the effects of chiral shift reagents on the NMR spectra suggests that discrimination between chiral isomers is primarily around the molecular equator, around which the enantiomeric gauche OO linkages are arrayed. Copyright © 2006 John Wiley & Sons, Ltd. [source]

The effect of co-surfactant-modified micelles on chiral separations in EKC

ELECTROPHORESIS, Issue 16 2009
Adeline B. Kojtari
Abstract The use of chiral pseudostationary phases in EKC provides high efficiencies and excellent resolution for enantiomeric separations. The chiral pseudostationary phases of interest in this study are alcohol-modified ("swollen") micelles, in which a co-surfactant (medium chain-length alcohol) is added with the surfactant. In this study, the chiral surfactant, dodecoxycarbonylvaline (DDCV), along with the co-surfactant, 2-hexanol, has been prepared as swollen micelle in order to investigate the chiral separation of enantiomeric pairs. Three sets of experiments were investigated in which swollen micelle systems contained: chiral surfactant and racemic co-surfactant; chiral surfactant and chiral co-surfactant; and phase ratio increases, in which both chiral surfactant and chiral co-surfactant were employed. In the first two sets of experiments, co-surfactant concentration was held constant and the surfactant concentration was increased. In the third set of experiments, both surfactant and chiral surfactant concentrations were increased proportionally. The chromatographic figures of merit for each enantiomeric pair were investigated and compared with various chiral aggregate systems. In swollen micelle compositions using constant racemic 2-hexanol concentration, when DDCV concentration increased, enantioselectivity and resolution increased; whereas, efficiency remained constant for most of the test compounds. Compositions using constant S -2-hexanol concentration reached a maximum in all chromatographic figures of merit when DDCV concentration was increased from 2 to 3%. An increase in both surfactant and co-surfactant concentrations led to noisy baselines and chiral aggregates that were generally unstable in solution. [source]

Polymeric alkenoxy amino acid surfactants: II.,Chiral separations of ,-blockers with multiple stereogenic centers

ELECTROPHORESIS, Issue 6 2004
Syed A. A. Rizvi
Abstract Two amino acid-based (leucine and isoleucine) alkenoxy micelle polymers were employed in this study for the separation of multichiral center-bearing ,-blockers, nadolol and labetalol. These polymers include polysodium N -undecenoxy carbonyl- L -leucinate (poly- L -SUCL) and polysodium N -undecenoxy carbonyl- L -isoleucinate (poly- L -SUCIL). Detailed synthesis and characterization were reported in our previous paper [26]. It was found that poly- L -SUCIL gives better chiral separation than poly- L -SUCL for both nadolol and labetalol isomers. The use of 50,100 mM poly- L -SUCIL as a single chiral selector provided separation of four and three isomers of labetalol and nadolol, respectively. Further optimization in separation of both enantiomeric pairs of nadolol and labetalol was achieved by evaluation of type and concentration of organic solvents, capillary temperature as well type and concentration of cyclodextrins. A synergistic approach, using a combination of poly- L -SUCIL and sulfated ,-CD (S-,-CD) was evaluated and it showed dramatic separation for enantiomeric pairs of nadolol. On the other hand for labetalol enantiomers, separation was slightly decreased or remain unaffected using the dual chiral selector system. Finally, simultaneous separation of both nadolol and labetalol enantiomers was achieved in a single run using 25 mM poly- L -SUCIL and 5% w/v of S-,-CD in less then 35 min highlighting the importance of high-throughput chiral analysis. [source]

Preparation of Optically Active ,-Amino[3]ferrocenophanes , Building Blocks for Chelate Ligands in Asymmetric Catalysis

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 23 2003
Patrick Liptau
Treatment of 1,1,-diacetylferrocene (4) with dimethylamine and TiCl4 yielded the unsaturated dimethylamino-substituted [3]ferrocenophane product 5. Its catalytic hydrogenation gave the corresponding saturated [3]ferrocenophane system 6 (trans/cis , 7:1). The rac -[3]ferrocenophane amine 6 was partially resolved (to ca. 80% ee) by means of L - or D - O,O, -dibenzoyltartrate salt formation. Treatment of 4 with the pure (R)- or (S)-methyl(1-phenylethyl)amine (8)/TiCl4 gave the corresponding optically active unsaturated [3]ferrocenophane amines (R)-(+)- 9 and (S)-(,)- 9, respectively. Their catalytic hydrogenation again proceeded trans -selectively, giving the corresponding saturated diastereomeric [3]ferrocenophane amines (1R,3R,5R)- 10a and (1S,3S,5R)- 10b [starting from (R)- 9], their enantiomers ent - 10a and ent - 10b were obtained from (S)- 9, but with a poor asymmetric induction (10a/10b < 2:1). Quaternization of 6 (CH3I) followed by amine exchange using (R)- or (S)-methyl(1-phenylethyl)amine (8), respectively, proceeded with overall retention. Subsequent chromatographic separation gave the pure diastereoisomers (1R,3R,5R)- 10a and (1S,3S,5R)- 10b [from (R)- 8, ent - 10a and ent - 10b from (S)- 8] in > 60% yield. Subsequently, the benzylic (1-phenylethyl) auxiliary was removed from the nitrogen atom by catalytic hydrogenolysis to yield the enantiomerically pure (> 98%) ([3]ferrocenophanyl)methylamines (1R,3R)- 11 and (1S,3S)- 11, respectively, which were converted into the corresponding dimethylamino-substituted [3]ferrocenophanes (1R,3R)- 6 and (1S,3S)- 6. Each enantiomer from the following enantiomeric pairs was isolated in its pure form and characterized by X-ray diffraction: (R)- 9/(S)- 9; (1R,3R,5R)- 10a/(1S,3S,5S)- 10a; (1R,3R,5S)- 10b/(1S,3S,5R)- 10b; (1R,3R)- 11/(1S,3S)- 11. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Novel results obtained by freezing berry pseudorotation of phosphoranes (10-P-5)

HETEROATOM CHEMISTRY, Issue 5 2002
Kin-ya Akiba
By freezing Berry pseudorotation of spirophosphoranes with recourse to the rigidity of the Martin bidentate ligand, we successfully prepared configurationally stable enantiomeric pairs of optically active phosphoranes, and could isolate "anti-apicophilic" C-apical O-equatorial (O-cis) phosphoranes. The effect of ,*PO orbital of the O-cis phosphorane was investigated both experimentally and theoretically. O-cis phosphoranes were revealed to be much more electrophilic at the phosphorus atom than O-trans isomers by experimental studies. The acidity of the ,-proton of an O-cis benzylphosphorane was found to be higher than that of the corresponding O-trans isomer. By the reaction of the ,-carbanion of an O-cis benzylphosphorane with PhCHO, we succeeded in the first isolation and full structural characterization of a 12-P-6 phosphate bearing an oxaphosphetane ring, the intermediate in the Wittig type reaction using a 10-P-5 phosphorane. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:390,396, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10072 [source]

Structure elucidation, conformational analysis and thermal effects on membrane bilayers of an antimicrobial myricetin ether derivative

JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 3 2001
C. Demetzos
The membrane perturbing 3,7,4,,5,-tetramethyl ether of myricetin 1 was isolated from Cistus monspelien-sis L. Its structure was elucidated and its conformational properties were explored using a combination of 2D NMR spectroscopy and computational chemistry. The obtained results showed that compound 1 adopts four enantiomeric pairs of low energy conformers characterized: (a) by an aromatic ring B twisted through rotation about C2-C1, bond from the rigid isoflavone ring; (b) a 4,-O-CH3 bond oriented out of the plane with equal probability upwards or downwards the phenyl ring B, while all the other O-CH3 bonds are oriented in the plane of the aryl ring. Two of these enantiomeric pairs are lowest in energy. These possible bioactive con-formers are possibly stabilized by van Der Waals interactions. The 3,,5-diacetyl derivative 2 of compound 1 was synthesized and its structure elucidation was achieved based on the chemical shift assignment of the parent compound 1. The Differential Scanning Calorimetry (DSC) results revealed that the degree of the thermal effects exerted by the flavonoids at dipalmitoylphosphatidyl choline (DPPC) bilayers followed the order 1 > 2 > myricetin. Their antimicrobial activity against Gram positive bacteria followed the same order. [source]

Configurational assignments of the diastereomers of 3,3,-(1,2-ethanediyl)bis[2-(3-fluorophenyl)-5-methyl-4-thiazolidinone] derivative with four stereogenic centers

JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 2 2001
Maria Gabriella Vigorita
Diastereomers of antiinflammatory/analgesic and antihistaminic 3,3,[(1,2-ethanediyl)bis(2-aryl-4-thiazo-lidinone)] derivatives possessing two stereogenic centers (indicated as BIS 2*C) have been widely investigated in recent years. The 5,5,-dimethyl analogues (BIS 4*C), now reported, have been synthesized by reaction of (±) ,-mercaptopropionic acid and N,N' -di(3-fluorobenzyliden)ethylenediamine. Because the 2 and 2,carbons bear the same groups and similarly the 5 and 5, carbons, and the latter groups are different from the former, four enantiomeric pairs and two meso forms exist in this situation. These diastereomers were identified by the concerted use of nmr spectroscopy and hplc on chiral stationary phase. [source]

Study of the conformational profile of the norbornane analogues of phenylalanine

JOURNAL OF PEPTIDE SCIENCE, Issue 6 2002
Arnau Cordomí
Abstract The conformational profile of the eight stereoisomeric 2-amino-3-phenylnorbornane-2-carboxylic acids (2-amino-3-phenylbicyclo[2.2.1]heptane-2-carboxylic acids) has been assessed by computational methods. These molecules constitute a series of four enantiomeric pairs that can be considered as rigid analogues of either L - or D -phenylalanine. The conformational space of their N -acetyl methylamide derivatives has been explored within the molecular mechanics framework, using the parm94 set of parameters of the AMBER force field. Local minimum energy conformations have been further investigated at the ab initio level by means of the Hartree-Fock and second order Moller-Plesset perturbation energy calculations using a 6,31G(d) basis set. The results of the present work suggest that the bulky norbornane structure induces two kinds of conformational constraints on the residues. On one hand, those of a steric nature directly imposed by the bicycle on the peptide backbone and, on the other hand, those that limit the orientations attainable by the phenyl ring which, in turn, reduces further the flexibility of the peptide backbone. A comparative analysis of the conformational profile of the phenylnorbornane amino acids with that of the norbornane amino acids devoid of the ,-phenyl substituent suggests that the norbornane system hampers the residue to adopt extended conformations in favour of C7-like structures. However, the bicycle itself does not impart a clear preference for any of the two possible C7 minima. It is the aromatic side chain, which is forced to adopt an almost eclipsed orientation, that breaks this symmetry introducing a marked preference for a single region of the (,, ,) conformational space in each of the phenylalanine norbornane analogues investigated. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd. [source]

Hydrogen-bonding controls the solid-state and enantiomeric comformations of the amino alcohol ligand 2-[(2-hydroxyethyl)amino]cyclohexanol

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2010
Alvaro S. de Sousa
The crystal structure of the title compound, C8H17NO2, consists of (R,R) and (S,S) enantiomeric pairs packed in adjacent double layers which are characterized by centrosymmetric hydrogen-bonded dimers, generated via N,H...O and O,H...O interactions, respectively. Intermolecular interactions, related to acceptor and donor molecule chirality, link the achiral double layers into tubular columns, which consist of a staggered hydrophilic inner core surrounded by a hydrophobic cycloalkyl outer surface and extend in the [011] direction. [source]

{4,10-Bis[2-(2-oxidobenzyl­idene­amino-,2N,O)benz­yl]-1,7-dioxa-4,10-diaza­cyclo­dodecane-,4O1,N4,O3,N10}ytterbium(III) perchlorate acetonitrile solvate

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2006
Marina González-Lorenzo
In the crystal structure of the title compound, [Yb(C36H38N4O4)]ClO4·CH3CN, the ytterbium ion is eight-coordinated and deeply buried in the cavity of the dianionic Schiff base ligand. The coordination polyhedron may be described as a distorted square anti­prism that shows a twist angle of 29.5,(1)° between the two square planes. The receptor adopts a syn arrangement, with both pendent arms on the same side of the crown group, and there are two helicities (one associated with this layout of the pendent arms and the other with the conformation of the crown ring), which give rise to enantiomeric pairs of diastereoisomers, viz. ,(,,,,) and ,(,,,,). [source]

Percutaneous permeation of enantiomers and racemates of chiral drugs and prediction of their flux ratios using thermal data: A pharmaceutical perspective

CHIRALITY, Issue 5 2003
Mohsen I. Afouna
Abstract Albeit pharmacological, pharmacokinetic, and toxicological differences between enantiomeric pairs or between the pure enantiomers and racemate of chiral drugs are known to exist for decades, we are just beginning to realize that there are apparent differences between these species with respect to their percutaneous permeation as well. Such differences in permeation are likely to be enhanced when chiral drugs are formulated with chiral excipients, necessitating a careful assessment of the effect of formulation excipients on the permeation as well as the overall therapeutic outcomes. The in vitro transport data from the preclinical investigations, using laboratory animal models and/or in vitro cell culture systems, must be carefully validated in vivo as there are differences between these models and the human skin. Mathematical models such as MTMT that utilize the interdependence of certain physicochemical characteristics and percutaneous permeability have a predictive value in assessing the flux behavior of enantiomers and racemates. Chirality 15:456,465, 2003. © 2003 Wiley-Liss, Inc. [source]