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  • Organic Chemistry  (68,935)
  • Wiley-Blackwell  (68,935)
  • Nature Publishing Group
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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 195-209 
    ISSN: 0899-0042
    Keywords: molecular imprinting ; molecular recognition ; chirality ; chromatography ; catalysis ; biosensor ; immunoassay ; antibody mimic ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular imprinting is a technique for the fabrication of biomimetic polymeric recognition sites or “plastic antibodies/receptors” which is attracting rapidly increasing interest. By this technology, recognition matrices can be prepared which possess high substrate selectivity and specificity. In the development of this technology, several applications have been foreseen in which imprinted materials may be exchanged for natural recognition elements. Thus, molecularly imprinted polymers have been used as antibody/receptor binding mimics in immunoassay-type analyses, as enzyme mimics in catalytic applications and as recognition matrices in biosensors. The best developed application area for imprinted materials, though, has been as stationary phases for chromatography, in general, and chiral chromatography, in particular. This review seeks to highlight some of the more intriguing advantages of the technique as well as pointing out some of the difficulties encountered. The prospects for future development will also be considered. Chirality 10:195-209, 1998. © 1998 Wiley-Liss, Inc.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 210-216 
    ISSN: 0899-0042
    Keywords: enantiospecific assay ; rat ; dog ; human ; enantiomer disposition ; HIV protease inhibitor ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: PNU-103017, 4-Cyano-N-(3-(cyclopropyl(5,6,7,8,9,10-hexahydro-4-hydroxy-2-oxo-2H-cycloocta(b) pyran-3-yl)methyl)phenyl)-benzenesulfonamide, is a selective HIV aspartyl protease inhibitor under evaluation as a potential oral treatment of Acquired Immunodeficiency Diseases. PNU-103017 is a racemic mixture of two enantiomers, designated PNU-103264 (R-) and PNU-103265 (S-). Stereoselective pharmacokinetics of the two enantiomers of PNU-103017 were observed in the dog, rat, and human after single and multiple dose administration of the racemate and were apparently species-dependent. Mean enantiomeric ratios of plasma concentrations (R-/S-) at each time point were greater than 1 in the dog, ranging from 1.22 to 3.06, but less than 1 in the rat and in the human, ranging from 0.44 to 0.80 and 0.23 to 0.73, respectively. A trend towards increased or decreased (farther from 1:1, R-/S-) enantiomeric ratio of plasma concentrations with time after each administration was also observed. The enantiomeric ratio remained unchanged after multiple dose administration in the rat, dog, and human although enzyme induction and increased plasma clearance were observed for both enantiomers. Chirality 10:210-216, 1998. © 1998 Wiley-Liss, Inc.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 217-222 
    ISSN: 0899-0042
    Keywords: amylose ; 3,5-dimethylphenyl-carbamate ; polysaccharide phase ; tert-butyl 2-tert-butyl-4-methoxy-2,5-dihydro-1,3-imidazole-1-carboxylate; amino acid ester synthesis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparative separation of the enantiomers of the title compound, a versatile chiral building block for the synthesis of unnatural amino acid esters, by high performance liquid chromatography on a chiral stationary phase (CSP), is reported for the first time. The CSP consists of amylose-(3,5-dimethylphenyl-carbamate), which has been coated onto the surface of macroporous aminopropyl-functionalized silica gel. The effect of mobile phase composition and the amount of amylose derivative on the silica gel has been thoroughly investigated. Using 2-propanol as organic modifier in hexane as mobile phase, on a semi-preparative column (200 mm × 40 mm ID, containing 192 g of stationary phase) about 200 mg of the racemate was separated per injection. Running the equipment under automatic conditions with repetitive injection mode allowed for the separation of 30 g per day. Both enantiomers were obtained with enantiopurities 〉99.75:0.25. Chirality 10:217-222, 1998. © 1998 Wiley-Liss, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 223-228 
    ISSN: 0899-0042
    Keywords: thalidomide enantiomers ; in vitro kinetics ; blood distribution ; human serum albumin ; chiral inversion ; plasma protein binding ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aim of this investigation was to elucidate the distribution and reactions of the enantiomers of thalidomide at their main site of biotransformation in vivo, i.e., in human blood. Plasma protein binding, erythrocyte: plasma distribution, and the kinetics of chiral inversion and degradation in buffer, plasma, and solutions of human serum albumin (HSA) were studied by means of a stereospecific HPLC assay. The enantiomers of thalidomide were not extensively bound to blood or plasma components. The geometric mean plasma protein binding was 55% and 66%, respectively, for (+)-(R)- and (-)-(S)-thalidomide. The corresponding geometric mean blood:plasma concentration ratios were 0.86 and 0.95 (at a haematocrit of 0.37) and erythrocyte:plasma distributions were 0.58 and 0.87. The rates of inversion and hydrolysis of the enantiomers increased with pH over the range 7.0-7.5. HSA, and to a lesser extent human plasma, catalysed the chiral inversion, but not the degradation, of (+)-(R)- and (-)-(S)-thalidomide. The addition of capric acid or preincubation of HSA with acetylsalicylic acid or physostigmine impaired the catalysis to varying extents. Correction for distribution in blood enhances previously observed differences between the pharmacokinetics of the enantiomers in vivo. The findings also support the notion that chiral inversion in vivo takes place mainly in the circulation and in albumin-rich extravascular spaces while hydrolysis occurs more uniformly in the body. In addition, the chiral inversion and hydrolysis of thalidomide apparently occur by several different mechanisms. Chirality 10:223-228, 1998. © 1998 Wiley-Liss, Inc.
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  • 5
    Electronic Resource
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 229-237 
    ISSN: 0899-0042
    Keywords: deuterium labelling ; menthocitronellol ; citronellol ; enantioselective multidimensional gas chromatography-mass spectrometry (enantio-MDGC-MS) ; dynamic headspace analysis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Mechanistic aspects of the biogenesis of the chiral monoterpenoid rose oxide in Pelargonium graveolens L'Héritier are investigated using deuterium-labelled precursors. After administration of the precursors using the cut-stem method, the dynamic headspace extracts of the plants are analysed using multidimensional gas chromatography-mass spectrometry (enantio-MDGC-MS). It is unequivocally shown that this plant is able to convert citronellol and menthocitronellol into cis-/trans-rose oxide. Menthocitronellol is converted into rose oxide with a clearly detectable enantiodiscrimination. These facts may be explained with the presence of an oxidase, which is able to oxidize citronellol and menthocitronellol in allylic position. A photooxygenation mechanism including singlet oxygen as the oxidizing agent is rather unlikely. Chirality 10:229-237, 1998. © 1998 Wiley-Liss, Inc.
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  • 6
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    Chirality 10 (1998), S. 358-361 
    ISSN: 0899-0042
    Keywords: cyclic imides ; barbiturates ; piperidine-2,6-diones ; mephenytoin ; chiral recognition ; enantioselectivity ; vancomycin chiral stationary phase ; normal-phase mode ; reversed-phase mode ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several cyclic imidic compounds (barbiturates, piperidine-2,6-diones, and mephenytoin) are enantiomerically resolved via high-performance liquid chromatography (HPLC) on a macrocyclic antibiotic covalently bonded to a silica gel support. The Chirobiotic V chiral stationary phase (CSP) column contains the antibiotic vancomycin as the chiral selector. The results of the analysis show that the substituents at the chiral carbon position of the racemic drugs affect chiral resolution. In addition, ring size may also play a role when considering the formation of analyte-CSP inclusion complexes. Contrary to the piperidine-2,6-diones, the chromatographic parameters for the barbiturates are much the same under normal- or reversed-phase conditions. The details of these results are discussed. Chirality 10:358-361, 1998. © 1998 Wiley-Liss, Inc.
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  • 7
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 364-369 
    ISSN: 0899-0042
    Keywords: (±)nicotine ; (±)nornicotine ; chiral separation ; enantiomers ; normal phase HPLC ; mobile phase additive ; cellulose-based chiral stationary phase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.This article is a US Government work and, as such, is in the public domain in the United States of America.
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  • 8
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 371-372 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 9
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 373-374 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 10
    Electronic Resource
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 382-395 
    ISSN: 0899-0042
    Keywords: selector/selectand associates ; hydrogen bonding ; chiral separation ; chiral phases ; enantioselectivity ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The progress made in the development of chiral stationary phases based on hydrogen-bonding selector/selectand associates is reviewed here. The structure of the different selectand/selector systems was established through X-ray diffraction and other spectroscopic techniques. The structure of the energetically more stable diastereomeric-associate was then correlated to the chromatographic results, namely to the elution order and the enantioselectivity of each of the systems. Chirality 10:382-395, 1998. © 1998 Wiley-Liss, Inc.
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  • 11
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 375-381 
    ISSN: 0899-0042
    Keywords: retention mechanisms ; separation of enantiomers ; chiral stationary phases ; equilibrium isotherms ; bonding ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The retention mechanisms and the separation of enantiomers on the classes of chiral stationary phases which are made by bonding isolated groups on the surface of an adsorbent are discussed. It is shown that retention on these phases originates from mixed mechanisms and how the individual contributions of these two mechanisms can be separated, by determining and modeling the equilibrium isotherms. A contribution originating from interactions of the isomers with the nonselective sites (type-I) and another one due to interactions with the enantioselective sites (type-II) can be determined and their importance studied as a function of several parameters, e.g., temperature or pH. This approach is illustrated with results obtained with different pairs of enantiomers on bovine serum albumin, 4-methylcellulose tribenzoate, or cellobiohydrolase immobilized on silica. Chirality 10:375-381, 1998. © 1998 Wiley-Liss, Inc.
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  • 12
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    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 396-404 
    ISSN: 0899-0042
    Keywords: chirality ; helicity ; GC-stationary phase ; conformations ; 1H-NMR-studies ; molecular mechanics calculations ; enantiomer separation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The manner of hydrogen-bonding to peptide selectors in enantiomer separation is examined with the help of a structural model. This model relies on a C3-symmetric trispeptide selector, which is stabilized by a network of distinct intramolecular hydrogen bonds. A combination of experimental and theoretical tools enables us to identify the lowest-energy conformation of the trispeptide selector and the sites of selector-substrate interactions. Experimental tools include temperature dependent 1H-NMR studies, 1D-NOE-measurements, and titration experiments, with the theoretical tools being EFF and CFF91 molecular mechanics calculations. The structural information deduced from these investigations is shown to bear on the enantioseparation of the corresponding chiral stationary phase towards derivatized amino acids. These observations, taken together, help to rationalize the mode of enantiomer-separation by amide phases as involving predominantly C7-hydrogen bonding sites. Chirality 10:396-404, 1998. © 1998 Wiley-Liss, Inc.
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  • 13
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    Chirality 10 (1998), S. 1-2 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 14
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    Chirality 10 (1998), S. 813-820 
    ISSN: 0899-0042
    Keywords: quinuclidine derivatives ; chromatographic separation ; borane complexes ; fractional crystallization ; resolution ; X-ray crystallography ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The four stereoisomers of the antimuscarinic 3-(2,3-dihydrobenzofuran-2-yl)quinuclidine have been prepared by a method involving chromatographic separation of the racemic diastereoisomers as borane complexes. The relative and absolute configurations of the stereoisomers were determined by X-ray crystallographic methods. The crystal structure of (2′R,3R)-3-(2,3-dihydrobenzofuran-2-yl)quinuclidine · HCl · H2O contains two independent molecules with different conformations of both the quinuclidine moiety and the dihydrofuran ring. Chirality 10:813-820, 1998. © 1998 Wiley-Liss, Inc.
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  • 15
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    Chirality 10 (1998), S. 808-812 
    ISSN: 0899-0042
    Keywords: calcium antagonist ; stereoselective analytical method ; isradipine enantiomers ; Chiral OJ ; HPLC ; GC ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: rac-Isradipine is a dihydropyridine type calcium antagonist. Its calcium entry blocking effect is due primarily to the (+)-(S)-enantiomer. This study describes a sensitive enantioselective method for the determination of isradipine in human serum. Following alkaline extraction into hexane, the enantiomers of isradipine are separated quantitatively by high-performance liquid chromatography on a Chiralcel OJ column at 39°C. The collected fractions were evaporated and assayed using capillary gas chromatography on a HP 50+ column with nitrogen selective detection. Using 2.0 ml of serum, 0.7 nmol/1 (0.26 ng/ml) of each enantiomer could be determined with acceptable precision. The method has successfully been used to measure (+)-(S)- and (-)-(R)-isradipine concentrations in samples from volunteers after intravenous and oral administration of isradipine. Chirality 10:808-812, 1998. © 1998 Wiley-Liss, Inc.
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  • 16
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    Chirality 10 (1998), S. 821-830 
    ISSN: 0899-0042
    Keywords: α-hydroxy acids ; chiral stationary phases ; enantiomer resolution ; copper ternary complexes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Direct separation of several α-hydroxy acid racemic mixtures was performed by the aid of ligand exchange chromatography using L-hydroxyproline chemically bound to silica stationary phase and aqueous solutions of copper (II) sulphate as a mobile phase. The elution order of the D- and L-enantiomers of α-hydroxy acids is interpreted in terms of a modified Davankov's rule. Several aspects of the Davankov's model of selectand-Cu(II)-selector ternary complexes are discussed based on the theoretical calculations within the quantum mechanical semiempirical and density functional theories. Chirality 10:821-830, 1998. © 1998 Wiley-Liss, Inc.
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  • 17
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    Chirality 10 (1998), S. 3-7 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 18
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    Chirality 10 (1998), S. 8-13 
    ISSN: 0899-0042
    Keywords: stereochemistry ; ion channel proteins ; nAChR ; K+ channels ; subunit arrangement ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Studies of ion channel proteins present many special challenges, including the potential for novel stereochemical phenomena. Here we describe several examples in which modern studies of ion channels have involved or made use of stereochemistry. Chirality 10:8-13, 1998. © 1998 Wiley-Liss, Inc.
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  • 19
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    Chirality 10 (1998), S. 24-27 
    ISSN: 0899-0042
    Keywords: homochirality ; origins of chirality ; spontaneous resolution ; molecular evolution ; racemic state ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An evaluation of some common misconceptions of the racemic state in combination with an articulation of principles for the spontaneous generation and amplification of chirality leads to the conclusion that homochirality in nature is a stereochemical imperative. Chirality 10:24-27, 1998. © 1998 Wiley-Liss, Inc.
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  • 20
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    Chirality 10 (1998), S. 14-23 
    ISSN: 0899-0042
    Keywords: biological signalling ; ristocetin A ; cooperativity ; dimerization ; asymmetry ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-regulation of biological signalling receptors via homodimerization is discussed in relation to the symmetry changes occurring when these receptors bind their target ligand. The idea of positive and negative cooperativity between dimerization and ligand binding, mediated by changes in the symmetry of the system, as a source of signalling control is considered; and an analogy made with the homodimerization of a glycopeptide antibiotic, ristocetin A, which displays negative cooperativity. Finally, the regulation of the bacterial aspartate receptor and the human growth hormone receptor is discussed as a function of ligand-induced asymmetry. Chirality 10:14-23, 1998. © 1998 Wiley-Liss, Inc.
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  • 21
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    Chirality 10 (1998), S. 35-40 
    ISSN: 0899-0042
    Keywords: de novo design ; self-assembly ; metalloproteins ; diastereoselection ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stereochemical consequences of the metal-ion assisted self-assembly of parallel three-helix peptide bundles are investigated. Chiral induction in the self-assembly of systems containing extensive protein secondary structure is compared with the racemic synthesis of short metallopeptides. Isolation and characterization of the individual stereoisomers of an exchange-inert metalloprotein provide structural insights into analogous exchange-labile systems. Chirality 10:35-40, 1998. © 1998 Wiley-Liss,Inc.
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  • 22
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    Chirality 10 (1998), S. 28-34 
    ISSN: 0899-0042
    Keywords: collagen ; nuclear magnetic resonance (NMR) ; nuclear Overhauser effect (NOE) ; stereospecific assignment ; triple-helix ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An improved model-based method for the stereospecific assignment of prochiral centers in collagen-like triple-helical molecules is introduced. Using the concepts of reporter atoms and of ensemble NOEs, the proposed methodology extracts the stereochemical information contained in the chiral elements of triple-helices and transfers it to prochiral centers with nondegenerate proton resonances. The improved approach has been successfully validated using -(Gly-Pro-Hyp)n- triple-helices for which the stereospecific assignment was previously obtained with established techniques. We have applied our stereochemical characterization to novel peptoid containing triple-helices for which existing methods of stereospecific assignment can not be used for all the prochiral centers. In our approach, several different NOE measurements are employed to make a given stereospecific assignment. The multiple NOE comparisons allow internal cross checks, which reduce the chance of erroneous assignments caused by experimental artifacts including spin diffusion and bias from anisotropic rotational motions. In addition, the multiple NOE comparisons are useful in overcoming problems associated with resonance overlap often encountered in the 1H-NMR spectra of collagen-like molecules. Our stereochemical analysis is anticipated to improve the precision and accuracy of the characterization of collagen-like triple-helices through a better correlation of structures with their 1H-NMR spectra. Chirality 10:28-34, 1998. © 1998 Wiley-Liss, Inc.
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  • 23
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    Chirality 10 (1998), S. 46-52 
    ISSN: 0899-0042
    Keywords: dendrimers ; circular dichroism ; accidental degeneracy ; cryptochirality ; solketal ; dendritic wedges ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and characterization of both enantiomers of 2-(benzyloxy)-1- [3,5-bis[[3,5-bis(benzyloxy)]benzyloxy]benzyloxy]-3-[[3,5-bis(benzyloxy)]benzyloxy] propane (S-7 and R-7) are described. The chirality is based on the linkage of three constitutionally different, but chemically similar, dendritic wedges with a chiral glycerol derived core. Both enantiomers are synthesized from the same starting material: S-(+)-Solketal. Despite their enantiomeric purity, S-7 and R-7 lack any optical activity and may be regarded as the first macromolecular analogues of the well-known organic molecules with “accidental degeneracy” or “cryptochirality.” Chirality 10:46-52, 1998. © 1998 Wiley-Liss,Inc.
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  • 24
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    Chirality 10 (1998), S. 53-59 
    ISSN: 0899-0042
    Keywords: macromolecular chirality ; chiral dendrimers ; cryptochirality ; fuzzy chirality ; starburst polymers ; cascade polymers ; chiral catalysts ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparation, chiroptical properties and potential application of chiral dendritic polymers are reviewed. Mislow's analysis of the manifestation of chirality, cryptochirality and fuzzy chirality is useful in studying these unique chiral macromolecules. Chirality 10:53-59, 1998. © 1998 Wiley-Liss,Inc.
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  • 25
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    Chirality 10 (1998), S. 41-45 
    ISSN: 0899-0042
    Keywords: polymers ; polyisocyanates ; helical conformation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A class of polymers synthesized at DuPont in the late 1950's, the polyisocyanates, are the simplest analogs of the Nylons and have proven valuable as experimental models for theories of wormlike macromolecules. The macromolecular dimensional properties associated with all wormlike polymers arise from a strongly preferred local conformation of the chain and in the polyisocyanates this conformation is helical with an interesting additional property in that the mirror helical senses are of equal probability. Recent experiments have shown that discrimination between the helical senses can be accomplished with surprisingly small chiral influences indicating high cooperativity which arises from a conformational state in which long sections of one helical sense are separated from the other sense by infrequent helical reversals. This can be seen to be analogous to theoretical ideas about one-dimensional paramagnetic materials in which the spin states and the domain boundaries are analogs to the helical sense and the helical reversals in the polyisocyanates. The mathematical formalisms of the one-dimensional magnetic materials precisely describe the chiral properties of the polyisocyanates. Chirality 10:41-45, 1998. © 1998 Wiley-Liss,Inc.
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  • 26
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    Chirality 10 (1998), S. 190-193 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 27
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    Chirality 10 (1998), S. 180-189 
    ISSN: 0899-0042
    Keywords: hypercoordinate silicon in naphthylsilanes ; X-ray structures ; NMR spectra ; reinterpretation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hexacoordinate silicon has been claimed to occur in bis(8-dimethylamino-naphth-1-yl)silanes on the basis of X-ray data and NMR spectra. Very much the same effects are shown by a bis(8-dimethylamino-naphth-1-yl)carbinol, pointing to essentially the same bonding in the Si and C compounds. Faced with the choice that either the carbon in the carbinol is hexacoordinate, too, or that the silicon in the silanes is, in fact, tetracovalent, we prefer the latter conclusion. Chirality 10:180-189, 1998. © 1998 Wiley-Liss, Inc.
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  • 28
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    Chirality 10 (1998), S. 587-591 
    ISSN: 0899-0042
    Keywords: snuff ; chewing tobacco ; Turkish tobacco ; Burley tobacco ; Virginia tobacco ; transdermal patches ; gum ; nasal spray ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantiomeric composition of nicotine in 18 smokeless tobaccos, 3 strains of tobacco leaf, 8 pharmaceutical products, and 4 commercial reagents was determined. The relative amount of the minor enantiomeric component, (R)-(+)-nicotine, ranged from ∼0.1% to ∼1.2% of the total nicotine in all samples. In some cases it appears that (R)-(+)-nicotine may be considered one of the five most common alkaloids in tobacco products. The highest level of (R)-(+)-nicotine was found in a commercial transdermal patch. The extraction and purification processes used in obtaining commercial (S)-(-)-nicotine supplies from tobacco do not appear to decrease the amount of (R)-(+)-nicotine present. Chirality 10:587-591, 1998. © 1998 Wiley-Liss, Inc.
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  • 29
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    Chirality 10 (1998), S. 600-618 
    ISSN: 0899-0042
    Keywords: steroidal bile acids ; inclusion compounds ; crystal structures ; facial molecules ; amphiphilic molecules ; amphiphilic sheets ; hydrogen-bonding networks ; head-to-tail ; head-to-head ; tail-to-tail ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chiral compounds, N-methyl-, N-ethyl-, and N-n-propylcholamides, form crystalline inclusion compounds with water or small organic substances. The compounds were analyzed by X-ray diffraction methods. It was found that the crystals have bilayered structures accumulated by chiral molecular sheets. The chiral molecules associate in a unique head-to-head/tail-to-tail and right-to-left motif to give chiral and amphiphilic sheets. Such sheets stack by adhesions between the hydrophilic sides and between the lipophilic sides. The alkyl groups of the amides prompt the formation of a hydrogen-bonding network between the tails instead of a cyclic one between the head and tail. The guest molecules are accommodated into small cavities between the steroidal side chains. Chirality 10:600-618, 1998. © 1998 Wiley-Liss, Inc.
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  • 30
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    Chirality 10 (1998), S. 619-626 
    ISSN: 0899-0042
    Keywords: nucleophilic aromatic substitution ; optical resolution ; asymmetric synthesis ; diastereoselective reaction ; Grignard reaction ; atrolactic acid derivatives ; biaryl coupling reaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel 1-aryl-9H-fluoren-9-ols 1 were conveniently synthesized by using the ester-mediated nucleophilic aromatic substitution on 2,6-dimethoxybenzoate 2 by aryl Grignard reagents as the key step. Racemic 1-phenylfluorenol 1a was converted to the diastereomeric esters 8 of (S)-2′-methoxy-1,1′-binaphthyl-2-carboxylic acid, which were readily separable by silica-gel column chromatography. Reduction of the optically pure diastereomer (+)-8 with LiAlH4 accompanied an appreciable racemization to give (+)-1a of 89% ee, which provides the first isolation of an optically active fluorenol of defined enantiomeric purity. Intrinsic chiral induction abilities of the 9-fluorenols 1 were examined in the atrolactic acid synthesis from phenylglyoxylates 9 and methylmagnesium iodide with diastereoselectivity of up to 85% de and the binaphthyl coupling of 1-methoxy-2-naphthoates 11 with 2-methoxy-1-naphthylmagnesium bromide with up to 73% de. Chirality 10:619-626, 1998. © 1998 Wiley-Liss, Inc.
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  • 31
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    Chirality 10 (1998), S. 699-704 
    ISSN: 0899-0042
    Keywords: ormeloxifene ; chiral separation ; sulfated cyclodextrin ; enantiomers ; capillary electrophoresis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (-)-Ormeloxifene, a drug candidate under development, was separated from (+)-ormeloxifene using capillary electrophoresis (CE) with sulfated β-cyclodextrin as chiral buffer additive. With conventional long-end injection the method showed high efficiency, since the theoretical plate number for (-)-ormeloxifene was over 1 million per m and the enantiomeric resolution was more than 100. However, the relatively long separation time of ∼22 min was a limiting factor. In order to reduce separation time, short-end injection experiments were carried out. By using the instrumental limits for capillary dimensions and field strength, the separation time was reduced to 〈40 sec. A further and significant reduction was achieved by applying extended short-end injection, which is a novel injection technique presented in this paper. With the extended short-end injection procedure, a plug of run buffer is injected after the sample has been injected, thus moving the sample closer to the detector and resulting in very short effective capillary lengths. Using the extended short-end injection technique, the separation was performed on 1.8 cm capillary (effective length) and the enantiomers were separated within 10 sec, which is a reduction of the original separation time by a factor of ∼155. Chirality 10:699-704, 1998. © 1998 Wiley-Liss, Inc.
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  • 32
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    Chirality 10 (1998), S. 705-710 
    ISSN: 0899-0042
    Keywords: preferential enrichment ; enantiomeric enrichment ; mixed crystal ; solid solution ; racemic compound crystals ; X-ray crystallography ; chiral ammonium sulfonate ; reversal of chirality ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray crystal structure of (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]dimethylammonium p-nitrobenzenesulfonate [(±)-NNMe2], which shows the novel enantiomeric resolution phenomenon Preferential Enrichment, has been compared with that of (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy) phenylcarbamoyl]ethyl]dimethylammonium p-toluenesulfonate [(±)-NTMe2], which does not show the phenomenon. The stable crystalline form of (±)-NNMe2 is a racemic compound, while that of (±)-NTMe2 is a mixed (disordered) crystal composed of the two enantiomers. The intermolecular hydrogen bonding mode in the crystal of (±)-NNMe2 was very different from that of (±)-NTMe2. Chirality 10:705-710, 1998. © 1998 Wiley-Liss, Inc.
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  • 33
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    Chirality 10 (1998), S. 95-99 
    ISSN: 0899-0042
    Keywords: β-amino alcohols ; enantiopure ; chiral ligands ; regioselective ; stereoselective ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both enantiomers of the important chiral β-amino alcohols erythro-2-amino-1,2-diphenylethanol were obtained via the separation of diastereoisomeric N-chlorinated derivatives of racemic trans-stilbene oxide and (S)-α-methylbenzylamine. The incorporation of chlorine in these precursors facilitated both the separation of diastereoisomers and the removal of the α-methylbenzyl chiral adjuvant. Each enantiopure β-amino alcohol was converted (Mitsunobu conditions) into the corresponding trans-2,3-diphenylaziridine in good overall yield. Chirality 10:95-99, 1998. © 1998 Wiley-Liss, Inc.
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  • 34
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    Chirality 10 (1998), S. 100-105 
    ISSN: 0899-0042
    Keywords: chiral separation ; chiral stationary phases ; hydroxyphosphonates ; chiroptical study ; enantiomeric excess ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantiomers of 17 α-hydroxybenzylphosphonate diethylesters containing para, or ortho substituents or other aromatic rings (1-naphthyl, 2-naphthyl, and 2-thienyl) have been successfully separated by HPLC on a Whelk-O 1 chiral stationary phase which is superior to other CSPs. The effect of the substituents, particularly halogens, on the enantioselectivity was investigated and related to a chiral recognition model. The absolute configurations of 4-methyl and 2-methyl substituted α-hydroxybenzylphosphonates were obtained by measurement of the circular dichroism spectra of the isolated enantiomers. Chirality 10:100-105, 1998. © 1998 Wiley-Liss,Inc.
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  • 35
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    Chirality 10 (1998), S. 106-119 
    ISSN: 0899-0042
    Keywords: porphyrin ; chiral ; oxidation ; mechanism ; enantioselective ; asymmetric catalysis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantioselective catalytic epoxidation of olefins is an important problem from both practical and mechanistic points of view. The origins of chiral induction by asymmetric porphyrin and salen complexes were investigated by FT-NMR T1 relaxation techniques. A new chiral vaulted porphyrin (1) that carries (S)-binaphthyl-L-alanine straps across both faces of the porphyrin macrocycle was synthesized and characterized. (R)-styrene oxide was obtained in 〉90% ee in the initial stages of styrene epoxidation with F5PhIO catalyzed by 1-Fe(III)Cl. The transition state for olefin epoxidation with high-valent metal-oxo species was modeled by coordinating epoxides to paramagnetic copper complexes of the corresponding ligands. The epoxide enantiomer that better fit the chiral cavity of the catalyst, as revealed by T1 relaxation measurements, was also the major product of catalytic olefin epoxidation. These results are consistent with the “lock-and-key” mechanism of asymmetric catalysis by metalloporphyrins. The copper complex of a chiral salen ligand showed no differentiation in terms of T1 relaxation rates between the enantiomers of cis-β-methylstyrene oxide in contrast to the high enantioselectivity observed for catalytic epoxidation. Chirality 10:106-119, 1998. © 1998 Wiley-Liss,Inc.
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  • 36
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    Chirality 10 (1998), S. 120-124 
    ISSN: 0899-0042
    Keywords: libraries ; asymmetric reduction ; oxazaborolidine ; chiral hplc ; chiral benz- hydrols ; borane ; combinatorial chemistry ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The advantages and disadvantages of the one-pot multi-substrate screening in asymmetric catalysis are discussed. The specific case of asymmetric reduction of ketones has been studied and is examplified by borane reduction catalyzed by an oxazaborolidine derived from (S)-diphenylproline. Chirality 10:120-124, 1998. © 1998 Wiley-Liss,Inc.
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  • 37
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    Chirality 10 (1998), S. 430-433 
    ISSN: 0899-0042
    Keywords: Whelk-O 1 ; chromatography ; HPLC ; enantiodifferentiation ; heterocycles ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In concert with a larger study of the processes by which chiral stationary phase CSP 1 differentiates between enantiomers, we have investigated the chromatographic separation of the enantiomers of a series of aryl-substituted heterocycles of systematically varied structure. A mechanistic picture of how these and similar resolutions occur is emerging. The mechanistic hypothesis described herein is of predictive value. Chirality 10:430-433, 1998. © 1998 Wiley-Liss, Inc.
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  • 38
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    Chirality 10 (1998), S. 415-424 
    ISSN: 0899-0042
    Keywords: chiral separation ; two- and three-dimensional crystals ; grazing incidence X-ray diffraction ; atomic force microscopy ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Spontaneous separation of enantiomers in two- and three-dimensional crystals is driven by the same thermodynamic and kinetic factors. However, amphiphilic crystalline monolayers at an interface cannot possess a center of inversion, the most common symmetry element in bulk crystals. This fact should, in principle, lead to better chances for spontaneous separation in the Langmuir or Langmuir-Blodgett monomolecular films. On the other hand, the monolayers of most amphiphiles studied to date incorporate long aliphatic chains that have an intrinsic tendency to pack in a herring-bone motif requiring glide plane symmetry, thus creating a bias towards racemate formation. Moreover, 2-D crystals supposedly have a much higher degree of molecular and therefore enantiomeric disorder compared to bulk crystals. All these factors necessitate a careful choice of molecules to guarantee enantiomeric separation in two dimensions. Unambiguous detection of spontaneous resolution in 2-D appears to require atomic resolution of molecular packing arrangement, which can in principle be obtained by grazing incidence X-ray diffraction or atomic force microscopy, whereas in bulk solids spontaneous resolution can be easily detected by various macroscopic methods. This short review provides analogies between spontaneous separation in 3-D and recent examples in 2-D, showing that spontaneous separation generally depends upon subtle differences in molecular structure. Chirality 10:415-424, 1998. © 1998 Wiley-Liss, Inc.
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  • 39
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    Chirality 10 (1998), S. 425-429 
    ISSN: 0899-0042
    Keywords: atropisomeric polychlorinated biphenyls (PCBs) ; Chirasil-Dex ; rotational barrier ; stopped-flow multidimensional gas chromatographic technique ; on-line enantiomerization kinetics ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rotational barriers ΔG† (T) of the four atropisomeric polychlorinated biphenyls (PCBs) 2,2′,3,5′,6-pentachlorobiphenyl (PCB 95), 2,2′3,3′,4,6′-hexachlorobiphenyl (PCB 132), 2,2′,3,3′,6,6′-hexachlorobiphenyl (PCB 136), and 2,2′,3,4′,5′,6-hexachlorobiphenyl (PCB 149) were determined via on-line enantiomerization kinetics by a new stopped-flow multidimensional gas chromatographic technique (stopped-flow MDGC) employing Chirasil-Dex as chiral stationary phase for enantiomer separation. The calculated rotational barriers ΔG† (T) of the trichloro-ortho-substituted atropisomers are 184 ± 2 kJ/mol for PCB 95, 189 ± 4 kJ/mol for PCB 132, and 184 ± 1 kJ/mol for PCB 149 at 300°C. The rotational barrier ΔG† (T) of tetrachloro-ortho-substituted PCB 136 is at least (or higher than) 210 kJ/mol at 320°C. Chirality 10:425-429, 1998. © 1998 Wiley-Liss, Inc.
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  • 40
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    Chirality 10 (1998), S. 434-483 
    ISSN: 0899-0042
    Keywords: ristocetin A ; macrocyclic antibiotic ; enantiomeric separations ; underivatized amino acids ; chiral stationary phase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The macrocyclic glycopeptide, ristocetin A, was covalently bonded to a silica gel support and evaluated as a liquid chromatographic (LC) chiral stationary phase (CSP). Over 230 racemates were resolved in either the reversed-phase mode, the normal-phase mode, or the polar-organic mode. The retention behavior and selectivity of this CSP were examined in each mode. Optimization of separations on this column is discussed. The ristocetin A CSP appeared to be complimentary to other glycopeptide CSPs (i.e., vancomycin and teicoplanin). Column stability was excellent. The CSP was not irreversibly altered when going from one mobile phase mode to another. Chirality 10:434-483, 1998. © 1998 Wiley-Liss, Inc.
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  • 41
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    Chirality 10 (1998), S. 499-504 
    ISSN: 0899-0042
    Keywords: enantioselective capillary gas chromatography ; cyclodextrin derivatives ; stereochemistry of terpenes ; monoterpenes ; essential oils ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Capillary gas chromatography employing the unique selectivities of specifically substituted cyclodextrins is highly suited for stereochemical investigations of terpenoid compounds. The analysis of many essential oils have shown that monoterpene derivatives regularly occur as enantiomeric mixtures. In the case of sesquiterpene hydrocarbons, liverworts (Hepaticae) and other lower organisms usually biosynthesize compounds of opposite stereochemistry as compared to higher plants and enantiomeric mixtures occur only occasionally. The investigation of diterpene hydrocarbons has so far shown no indication of the presence of both enantiomers in the same plant. Chirality 10:499-504, 1998. © 1998 Wiley-Liss, Inc.
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  • 42
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    Chirality 10 (1998), S. 484-491 
    ISSN: 0899-0042
    Keywords: salbutamol ; chiral separation ; validated assay ; fluorescence detection ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A sensitive enantioselective high-performance chromatographic (HPLC) method was developed and validated to determine low levels of (-)-R and (+)-S-albuterol in plasma. Baseline resolution was achieved by using a teicoplanin-based chiral stationary phase with a polar organic mobile phase consisting of methanol/acetonitrile/glacial acetic acid/diethylamine, 40:60:0.3:0.2, (v/v/v/v) and a flow-rate of 1.0 ml/min. Enantioselectivity (α) equaled 1.18 and resolution (Rs) equaled 1.8. By using fluorescence detection maximized at 230 and 310 nm for excitation and emission, respectively, concentrations of each enantiomer could be measured down to 125 pg/ml from a 1-ml plasma sample. Initially, the method was applied to plasma samples from a small single-dose inhalation study of racemic albuterol in a human volunteer and, later, to in vivo samples from a canine inhalation study of the single enantiomer, (-)-R-albuterol. Results from the canine study showed that no chiral inversion of (-)-R-albuterol occurs in the dog. Chirality 10:484-491, 1998. © 1998 Wiley-Liss, Inc.
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  • 43
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    Chirality 10 (1998), S. 492-498 
    ISSN: 0899-0042
    Keywords: racemate ; enantiomer ; HPLC ; chiral stationary phase ; benzoylcellulose ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The advantages that can be gained from derivatization of various racemic aliphatic and aromatic alcohols prior to enantiomeric chromatographic separation have been systematically investigated for a series of benzoate derivatives. Three cellulose-based CSPs available in the pure polymeric form - tribenzoyl cellulose (TBC), meta-methylbenzoyl cellulose (MMBC), and para-methylbenzoyl cellulose (PMBC) - were selected and several benzoate derivatives varying in the nature and the position of the substituent on the benzoyl group were prepared and analysed. TBC clearly gives the broadest application range, and among the different benzoate esters the best selectivity was generally obtained with either the 4-methoxybenzoate or the 4-methylbenzoate derivatives. Based on these results, some empirical rules could be formulated for optimizing the enantiomeric separation of racemic alcohols, which make up one of the most important classes of chemical substances used as drugs and biocides, or as building blocks for their synthesis. An application of this approach to the preparative separation of the enantiomers of a drug intermediate is also shown. Chirality 10:492-498, 1998. © 1998 Wiley-Liss, Inc.
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  • 44
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    Chirality 10 (1998), S. 513-518 
    ISSN: 0899-0042
    Keywords: CBH I ; cellulase ; cation ; sodium ; potassium ; enantioselectivity and temperature ; ionic strength ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An increase in both retention and enantioselectivity for some β-blocking agents was observed when exchanging potassium to sodium ion in the buffer used as mobile phase. A large effect of ionic strength on retention was observed, while the enantioselectivity was constant. Chirality 10:513-518, 1998. © 1998 Wiley-Liss, Inc.
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  • 45
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    Chirality 10 (1998), S. 507-512 
    ISSN: 0899-0042
    Keywords: chiroptical method ; drug analysis ; β-lactam antibiotics ; CD spectroscopy ; human fluids ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A study of the applicability of circular dichroism (CD) for the determination of drug levels in human serum is described and a new method for the quantitative determination of optically active absorbing drugs having Cotton effects at wavelengths above 250 nm in human serum and/or plasma is proposed. The principal advantages of this method are speed, economy, and simplicity, no derivatization or chromatographic separation steps being needed. The validity of the CD determination was confirmed by analysis of variance, β-lactam antibiotics being chosen as model drugs. In addition, the validation studies performed confirm the accuracy and precision of the proposed method. For β-lactam antibiotics lacking Cotton effects above 250 nm, an alternative method based on the extraction of the drug from serum is considered. Chirality 10:507-512, 1998. © 1998 Wiley-Liss, Inc.
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  • 46
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    Chirality 10 (1998), S. 519-521 
    ISSN: 0899-0042
    Keywords: optical activity of enantiomers ; weak interactions ; stability of optical activity ; racemization ; tunneling effect ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Assuming the active molecule as a two-level system, we calculate the racemization, due to the tunneling effect, taking into account the effects of the weak interactions and of an external potential. We show that the weak interactions would block the tunneling racemization of enantiomers in compressed gases and liquids. Chirality 10:519-521, 1998. © 1998 Wiley-Liss, Inc.
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  • 47
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    Chirality 10 (1998), S. 522-527 
    ISSN: 0899-0042
    Keywords: chiral HPLC ; quantitative substituent effect ; recognition mechanism ; fluorene derivative-chiral separation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chromatographic parameters for 12 structurally related compounds in the 4a-methyl-2,3,4,4a-tetrahydro-1H-fluorene and 4a-methyl-1,2,3,4,4a,9a-hexahydro-fluoren-9-one series are reported on CTA-I and Chiralcel OJ chiral stationary phases. Arrangement of the k' values according to configurationally related enantiomer series (Class I and Class II) and not according to the actual order of elution, allows the treatment of the data by linear correlation with structure and substituent effect. A detailed analysis of the capacity factor variation with respect to the structural changes shows clearly that the framework and substitution effects do not result in the same response on the two cellulose ester chiral stationary phases. More interestingly, it emerges that chiral discimination may be attributed to certain areas of the molecule, these areas being different in the interaction within CTA-I and Chiralcel OJ. Furthermore, our analysis points out the relevance of attempting to develop quantitative relationships for configurationally related series of enantiomers (in our case Class I and Class II), the main effort being devoted to the understanding of the capacity factor variation in each class rather than of the α values, which are derived entities. Chirality 10:522-527, 1998. © 1998 Wiley-Liss, Inc.
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  • 48
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    Chirality 10 (1998), S. 535-539 
    ISSN: 0899-0042
    Keywords: asymmetric hydrogenation ; non-coded amino acids ; enantioselectivity ; dipeptides ; diastereoselectivity ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantiomers of Propranolol, Pindolol, and Carazolol, well-known β-blockers, have been used to prepare cationic aminophosphine phosphinite rhodium complexes. Propraphos-Rh and Pindophos-Rh are very efficient catalysts in the asymmetric hydrogenation of N-Boc-protected unusual dehydroamino acid derivatives. Carazolol-Rh is less suitable in both activity and enantioselectivity. Under the same conditions, N-Boc-protected dehydrodipeptides are hydrogenated with diastereoselectivities between 70 and 90% de. Chirality 10:535-539, 1998. © 1998 Wiley-Liss, Inc.
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  • 49
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    Chirality 10 (1998), S. 528-534 
    ISSN: 0899-0042
    Keywords: chiral inversion ; ibuprofen ; ketoprofen ; flurbiprofen ; indoprofen ; suprofen ; fenoprofen ; metabolism of 2-arylpropionic acids ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The fungus Cordyceps militaris has been previously shown to be capable of inverting the chirality of 2-phenylpropionic acid from its (R)-enantiomer to its (S)-antipode. The structure of this compound is similar to the 2-arylpropionic acid non-steroidal anti-inflammatory drugs, which have also been reported to undergo a similar chiral inversion process in mammals and man. We report here an investigation into the substrate specificity of the enzyme system present in C. militaris using pure enantiomers and racemates of ibuprofen and ketoprofen and racemates of indoprofen, suprofen, flurbiprofen, and fenoprofen and the structurally related compounds 2-phenylbutyric acid and 2-phenoxypropionic acid as substrates, using optimised incubation conditions developed for the inversion of 2-phenylpropionic acid. The results demonstrated that C. militaris is capable of inverting the chirality of all the compounds investigated, which suggests that the active sites of the enzymes are very flexible with regard to the molecular dimensions of the substrate molecule and the spatial occupation of the groups surrounding the chiral centre. Metabolism of all the substrates was observed but the rate of metabolism varied extensively depending on the substrate. Achiral HPLC analysis was used to detect any potential metabolites and the results suggested that the site of the metabolism appeared to be at the aliphatic side groups only, with the aromatic ring being left intact in all cases. These results suggest that C. militaris could be a valuable tool in the investigation of the prospective metabolic fates of new 2-arylpropionic acids during their development. Chirality 10:528-534, 1998. © 1998 Wiley-Liss, Inc.
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    Chirality 10 (1998), S. 555-555 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    Chirality 10 (1998), S. 573-577 
    ISSN: 0899-0042
    Keywords: hexahelicene ; sulphoxides ; carbohydrate carbamate ; chiral preparative ; silica ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preparative enantiomeric resolutions of hexahelicen-7-yl acetic acid methyl ester and two sulphoxides were performed on cellulose tris(3,5-dimethylphenylcarbamate) coated onto aminopropylated 5-μm silica with 120-Å diameter pore. High enantiomeric purity was obtained for both enantiomers. The enantioselectivity of the amylose tris(3,5-dimethylphenylcarbamate), cellulose and amylose tris(phenylcarbamate) phases for the hexahelicen-7-yl acetic acid derivative were also investigated. Chirality 10:573-577, 1998. © 1998 Wiley-Liss, Inc.
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    Chirality 10 (1998), S. 592-599 
    ISSN: 0899-0042
    Keywords: chiral separation ; chiral selector ; separation of enantiomers ; liquid chromatography ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A chiral stationary phase (CSP 1) prepared starting from (R)-4-hydroxyphenylglycine and then grafting (R)-N-butanoyl-4-allyloxyphenylglycine N-propyl amide to silica gel was found to be very effective in separating the enantiomers of N-(3,5-dinitrobenzoyl)-α-amino amides. From the chromatographic behaviors of the resolution of N-propyl amides, N,N-diethyl amides and ethyl esters of N-(3,5-dinitrobenzoyl)-α-amino acids and the resolution of various N-(substituted benzoyl)leucine N-propyl amides, the hydrogen bonding and the π-π donor-acceptor sites of the analyte for the interaction with the CSP have been proposed. Similarly, the hydrogen bonding donor and acceptor sites of the CSP for the interaction with the analyte have been proposed from the comparison of the chromatographic behaviors of the resolution of various N-(3,5-dinitrobenzoyl)-α-amino N-propyl amides on modified CSPs (CSP 7 containing trifluoroacetyl group instead of the butanoyl group of CSP 1 and CSP 8 containing N,N-diethyl group instead of the N-propyl group of CSP 1) with those on CSP 1. By correlating the interaction sites of the CSP and their complementary interaction sites of the analyte, a chiral recognition mechanism which utilizes the two hydrogen bonding interactions and the π-π donor-acceptor interaction between (R)-CSP 1 and more retained analytes, (S)-N-(3,5-dinitrobenzoyl)-α-amino amides, has been proposed. Chirality 10:592-599, 1998. © 1998 Wiley-Liss, Inc.
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    Chirality 10 (1998), S. 667-675 
    ISSN: 0899-0042
    Keywords: polysilane ; circular dichroism ; exciton couplet ; helix ; fluorescence ; poor solvent ; good solvent ; conformational property ; helix reversal ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An optically active, rigid rodlike helical polysilane with 6,9,12-trioxatetradecyl and (S)-2-methylbutyl substituents (1) was newly obtained as a very high molecular weight polymer of several million. Due to the presence of trietheral substituent, 1 was readily soluble in a polar solvent such as ethanol and a mixture of ethanol and water, but was insoluble in pure water. Polysilane 1 in pure ethanol at room temperature exhibited an intense and narrow ultraviolet (UV) and circular dichroism (CD) absorptions at 323 nm, associated with an almost mirror imaged fluorescence (FL) at 328 nm, that are characteristic of rigid rodlike, single-screw-sense helical polysilanes reported previously. When solution temperature was changed from 60°C to -104°C, a global shape of 1 expanded associated with an increase of segment length, whereas a screw pitch tended to be wound tightly. On the other hand, as a solvent polarity became poor, a global shape of 1 shrunk associated with an decrease of segment length and formed a chiral motif with an M-helicity between two helical segments with a kink. At a ratio of 50% of ethanol/water of 50:50 (v/v), 1 became insoluble and formed aggregates. Chirality 10:667-675, 1998. © 1998 Wiley-Liss, Inc.
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    Chirality 10 (1998), S. 711-716 
    ISSN: 0899-0042
    Keywords: stereospecific polymerization ; chiral α-substituted acrylate ; stereoregularity ; chiroptical property ; isotactic polymer ; regular arrangement ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel chiral monomer, benzyl α-[(1-phenylethoxy)methyl]acrylate was synthesized, and the stereoregularity and chiroptical property of the polymers obtained by radical and anionic polymerizations were investigated. Anionic polymerization in toluene provided a polymer with high isotacticity regardless of optical purity of the monomer and difference of counter cations, Li+ and Mg2+, while radical polymerization appears to yield an atactic polymer. The isotactic copolymers anionically obtained from a mixture of (R)- and (S)-monomers showed peak splitting probably due to the triad arrangement of (R) and (S) groups of side chains in the resonance of 13C-NMR spectra. The isotactic polymer prepared from enantiomerically pure (R)-monomer ([α] 25 365 = +157°) with n-BuLi in toluene exhibited higher specific rotation ([α] 25 365 = +262°) than that of the radical polymer ([α] 25 365 = +162°). This may be explained by regular arrangement of optically active groups along the polymer chain of the isotactic polymer. Chirality 10:711-716, 1998. © 1998 Wiley-Liss, Inc.
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Concerning the Products of the Reaction of Methyl Bromide and Ethyl Bromide with Potassium Hydroxide in Aqueous Methanolic Solutions and the Progress of this SN2-ReactionInvestigations of the reaction of methyl bromide and ethyl bromide with potassium hydroxide in methanolic and aqueous methanolic solutions show that the main products of these reactions are dimethyl ether and ethylmethyl ether. The reaction rates measured in methanolic or aqueous methanolic solutions are the same whether potassium hydroxide or potassium methoxide are used. These results are caused by an equilibrium between hydroxide and methoxide ions with which we could establish the equilibrium constant near 0.6. This means that a solution of sodium hydroxide c = 0.1 moll-1 in methanol contains roughly 99.8% of methoxide ions. The reaction rates in methanolic as well as in aqueous methanolic solutions are strict second order. The reaction rate measured at several temperatures permitted the calculation of EA≠, ΔH≠, ΔS≠ and ΔG≠. Furthermore the kinetic investigations show that the nucleophilicity of methoxide ions is lower compared to hydroxide ions. The calculation of the Swain-Scott-parameter n results in a nucleophilicity scale in order to methoxide, hydroxide, ethoxide ions. The kinetic investigations of the reaction of ethyl bromide with methoxide and hydroxide ions in methanolic solutions demonstrate that at high temperatures the rate constant of methoxide ions is higher than that of hydroxide ions. The opposite case can be observed at lower temperatures. At the temperature of 20 °C the rate constants of both reactions are equal. This is to do with the isokinetic effect which one is rarely able to observe at room temperatures.
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 59
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition of N-chlorophthalimide (1b) to alkenes 3 via phthalimidyl radicals introduces a chlorine atom and an imidyl moiety to vicinal C-atoms of a carbon chain. The yields depend on the substituents of the alkene 3. The regioselectivity can be explained by steric and electronic effects; differences in the behavior of the addition of N-chlorophthalimide (1b) and N-bromophthalimide (1a) can be explained by a reversible attack of the phthalimidyl radical to the double bond.
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    ISSN: 0941-1216
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several new push-pull azobenzene dyes were synthesized by diazonium salt coupling of electron-poor anilines with N,N-dialkylanilines. Their dipole moments and first-order hyperpolarizabilities were evaluated respectively from the concentration dependence of the dielectric constant of solutions in apolar solvents and by the electric-field-induced-second-harmonic-generation (EFISH) technique. Investigation of the influence of both nature and steric hindrance of the electron-withdrawing group allowed to highlight a significant deviation from linearity in the relationship between dipole moments and first-order hyperpolarizabilities. This effect was particularly noticeable when multiple electron withdrawing groups pointing in different directions were present on the same phenyl ring.
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