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  • 1995-1999  (2,691)
  • Organic Chemistry  (2,690)
  • Nuclear reactions
  • 101
    Electronic Resource
    Electronic Resource
    Synthesis, structure, and chiroptical properties of the first 4-Oxa[7]Paracyclophane (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 147-153 
    Details
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the chiral 9,12-dimethyl-4-oxa[7]paracyclophane 3 was achieved by the dithia route with subsequent sulfone pyrolysis. The conformational flexibility of the oxamethylene bridge with local Cs symmetry is evident from low temperature NMR experiments. Experimentally, an activation enthalpy of 11.4 kcal/mol is found for this process, which is in good agreement with the rotational barrier derived from semiempirical AM1 calculations (10.2 kcal/mol). Ab initio Hartree-Fock geometry optimizations have been performed for 3 and the corresponding hydrocarbon 9,12-dimethyl[7]paracyclophane 4. The distance of the O-atom to the center of the benzene ring in the structure of 3 is only 2.784 Å, i.e., significantly closer than the analogous C4-benzene ring distance in the slightly more strained 4 (3.112 Å). The enantiomeric separation of (±3) has been achieved by HPLC and the circular dichroism (CD) spectrum is reported. Ab initio all-electron DFT/SCI calculations of the CD spectrum are in good agreement with the experimental data and reveal the importance of p(O-atom)→ π* charge-transfer type excited states at relative low energies (6.6 eV, 188 nm) responsible for an intense negative CD band. The benzene type π → π* states are energetically lowered by 0.4-0.8 eV due to the boat-type deformation of the benzene moiety. By comparison of theoretical and experimental CD data the absolute configuration of 3 is assigned as (-)-(S). Chirality 10:147-153, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.23
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  • 102
    Electronic Resource
    Electronic Resource
    Stereochemistry of polycyclohexyl systems (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 159-168 
    Details
    ISSN: 0899-0042
    Keywords: symmetry ; conformation ; chirality ; stereochemistry ; NMR ; rotational barrier ; interconversion graph ; gear meshing ; molecular propellers ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The static and dynamic stereochemistries of tricyclohexylmethane, tetracyclohexylmethane, and tetracyclohexylethene are reviewed. Chirality 10:159-168, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.25
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  • 103
    Electronic Resource
    Electronic Resource
    Chiral recognition of phthalides and lactams (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 430-433 
    Details
    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.
    Additional Material: 4 Ill.
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  • 104
    Electronic Resource
    Electronic Resource
    Highly enantioselective HPLC separations using the covalently bonded macrocyclic antibiotic, ristocetin A, chiral stationary phase (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 434-483 
    Details
    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.
    Additional Material: 10 Ill.
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  • 105
    Electronic Resource
    Electronic Resource
    Determination of the enantiomers of albuterol in human and canine plasma by enantioselective high-performance liquid chromatography on a teicoplanin-based chiral stationary phase (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 484-491 
    Details
    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.
    Additional Material: 5 Ill.
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  • 106
    Electronic Resource
    Electronic Resource
    Achiral derivatization as a means of improving the chromatographic resolution of racemic alcohols on benzoylcellulose CSPs (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 492-498 
    Details
    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.
    Additional Material: 5 Ill.
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  • 107
    Electronic Resource
    Electronic Resource
    Enantioselective capillary gas chromatography in the investigation of stereochemical correlations of terpenoids (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 499-504 
    Details
    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.
    Additional Material: 7 Ill.
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  • 108
    Electronic Resource
    Electronic Resource
    Unexpected difference in enantioselective retention on cellulase (CBH I) silica stationary phase caused by exchange of potassium for sodium ion in the mobile phase (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 513-518 
    Details
    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.
    Additional Material: 5 Ill.
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  • 109
    Electronic Resource
    Electronic Resource
    Weak interactions and the tunneling racemization (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 519-521 
    Details
    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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  • 110
    Electronic Resource
    Electronic Resource
    Determination of drug levels in human serum by circular dichroism (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 507-512 
    Details
    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.
    Additional Material: 5 Ill.
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  • 111
    Electronic Resource
    Electronic Resource
    Chiral objects with a dendritic architecture (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 46-52 
    Details
    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.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.9
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  • 112
    Electronic Resource
    Electronic Resource
    Two-dimensional crystalline structures and photochemical behavior of cinnamate monolayers on water surfaces (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 60-65 
    Details
    ISSN: 0899-0042
    Keywords: grazing-incidence X-ray diffraction ; topochemical photodimerization ; 4-alkoxycinnamic acid ; 4-alkoxycinnamamide ; 2-D crystallites on water surfaces ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A direct relationship between the structures of two-dimensional (2-D) crystallites of 4-octadecyloxy-E-cinnamic acid and 4-octadecyloxy-E-cinnamamide amphiphiles at the air-water interface and their photochemical behavior, is presented. The detailed packing arrangements of the monolayers were determined, close to the atomic level, from the diffraction pattern measured, in situ on the water surface, by the grazing incidence X-ray diffraction (GID) technique using synchrotron radiation. The products of the photochemical reaction, performed by irradiating the monolayer films directly on the water surface, were collected and analyzed. While the acid crystallites yield a β-truxinic acid product together with the cis-isomer, the amide undergoes only trans-cis isomerisation, in full agreement with the 2-D crystalline packing arrangements of the two amphiphiles on the water surface. Chirality 10:60-65, 1998. © 1998 Wiley-Liss,Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.11
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  • 113
    Electronic Resource
    Electronic Resource
    Separation of enantiomers of drugs by capillary electrophoresis, part 7: Gamma-cyclodextrin as chiral solvating agent (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 548-554 
    Details
    ISSN: 0899-0042
    Keywords: capillary electrophoresis ; enantiomer separation ; chiral drugs ; γ-cyclodextrin ; gamma-cylcodextrin ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Following an extended chiral drug screening program by capillary zone electrophoresis (CZE), the enantioseparation of 86 racemic drugs was tested with γ-cyclodextrin as a chiral solvating agent. Unified conditions were applied to all experiments. In total, 18 drug racemates were separated, 13 entries thereof that had not been separated at the lower CSA concentration applied in an earlier stage of the project. A comparison of the data with the results obtained for α- and β-cyclodextrin points to the significance of partial penetration (“side-on binding”) of aryl groups into the cyclodextrin cavity. Chirality 10:548-554, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 6 Ill.
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  • 114
    Electronic Resource
    Electronic Resource
    Conformationally switched-on polyether ionophores (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 78-87 
    Details
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of two types of conformationally switched podand ionophores and their ionophoric properties are described. Both feature cyclohexane rings with polyether groups as trans 1,2 substituents. In the “switched off” forms of the ionophores, the two podand substituents are constrained to a diaxial orientation and cannot chelate a metal ion. In both cases, a chemical reaction, hydrolysis of a ketone acetal, is used to remove the constraint allowing the two podand substituents to achieve a diequatorial orientation. In this conformation, the two diequatorial podand substituents can chelate a potassium ion and the ionophoric properties are “switched on.” In one case, the chains can be held in the diaxial orientation by a large group, an acetyle group derivatized as the ethylene glycol acetal, in the 4 position. When the size of the group is lowered, the polyether groups become equatorial and can complex a potassium ion as evidenced by a conformational change measured by low temperature NMR spectroscopy. In the second example, an annulated ring holds the cyclohexane ring rigidly in the non-complexing conformation. When the restraint is removed, complexation can occur as evidenced by transport of potassium picrate through a liquid membrane (chloroform layer). In both cases, the ionophoric properties are “switched on” by hydrolysis of a ketone acetal. Chirality 10:78-87, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.13
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  • 115
    Electronic Resource
    Electronic Resource
    Asymmetric syntheses of 1-deoxy-6-epicastanospermine and rhamno-1-deoxynojirimycin (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 88-90 
    Details
    ISSN: 0899-0042
    Keywords: allylic oxidation ; dihydroxylation ; chiral lactam ; chiral indolizidines ; phenyl glycinol ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concise and highly diastereoselective synthesis of the titles azasugars was accomplished using nonracemic bicyclic lactams. Chirality 10:88-90, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chir.14
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  • 116
    Electronic Resource
    Electronic Resource
    A short, stereoselective synthesis of (3R,4R)-4-acetoxy-3-[(R)-1′((t-butyldimethylsilyl)oxy) ethyl]-2-azetidinone, key intermediate for the preparation of carbapenem antibiotics (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 91-94 
    Details
    ISSN: 0899-0042
    Keywords: β-lactams ; carbapenem antibiotics ; titanium enolates ; imines ; stereoselective synthesis ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A short stereoselective synthesis of the acetoxy azetidinone (1), an important precursor of several biologically active β-lactam antibiotics, has been accomplished in seven steps and 32.8% overall yield from the easily available and inexpensive (R) ethyl 3-hydroxybutanoate. Chirality 10:91-94, 1998. © 1997 Wiley-Liss, Inc.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/chir.15
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  • 117
    Electronic Resource
    Electronic Resource
    Separation of the enantiomers of α-hydroxybenzylphosphonate esters by enantioselective HPLC and determination of their absolute configuration by circular dichroism (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 100-105 
    Details
    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.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/chir.17
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  • 118
    Electronic Resource
    Electronic Resource
    Enantiomers of 2-methyl- and 2,4-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid: Preparation by resolution, determination of absolute configuration, and Curtius rearrangement (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 791-799 
    Details
    ISSN: 0899-0042
    Keywords: 1,4-benzoxazine-2-carboxylic acids ; X-ray crystal structure determination ; 1-phenylethylamine ; 2-amino-1,4-benzoxazin-3-ones ; racemization ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both enantiomers of 2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid 2 and 2,4-dimethyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid 3 were prepared via resolution of the corresponding racemic carboxylic acids with (R)- and (S)-1-phenylethylamine, respectively. Absolute configuration of (-)-(R)-2-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-2-carboxylic acid was determined by X-ray crystallography. Curtius rearrangement of acyl azides prepared from enantiomers of these heterocyclic carboxylic acids carried out in benzyl alcohol afforded enantiomers of the corresponding benzyl carbamates, which upon hydrogenolysis gave racemic 2-amino-2-methyl-3,4-dihydro-2H-1,4-benzoxazin-3-one 4 and 2-amino-2,4-dimethyl-3,4-dihydro-2H-1,4h-benzoxazin-3-one 5. Chirality 10:791-799, 1998. © 1998 Wiley-Liss, Inc.
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  • 119
    Electronic Resource
    Electronic Resource
    Enantioselective sulfation of β2-receptor agonists by the human intestine and the recombinant M-form phenolsulfotransferase (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 800-803 
    Details
    ISSN: 0899-0042
    Keywords: β2-agonist drugs ; sulfotransferase ; M-form phenolsulfotransferase ; sulfation ; isoproterenol ; metaproterenol ; terbutaline ; albuterol ; salbutamol ; salmeterol ; formoterol ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The β2-receptor agonist class of drugs is metabolized in humans almost exclusively by sulfate conjugation. The objective of this investigation was to determine the influence of chemical structure on the stereoselectivity of the sulfoconjugation of these chiral drugs. The pure enantiomers of six β2-agonists, including those clinically most widely used, were all effectively sulfated both by the cytosol of the human intestine and the recombinant human M-form phenolsulfotransferase (PST). Whereas the apparent Km values (Km,app) for the sulfation of the individual drug enantiomers by the intestinal cytosol varied widely, ranging from 4.8 μM for (S)-isoproterenol to 889 μM for (S)-albuterol, these Km,app values were highly correlated with those obtained with M-PST (correlation coefficient 0.994). In contrast, the M-PST Vmax,app values were similar for all drug enantiomers, ranging from 276 to 914 pmol min-1 mg-1 protein, implying that substrate binding to M-PST by far is the main determinant of the sulfation activity. For isoproterenol, the Km,app for M-PST was 6.1 times higher for the active (R)- than for the inactive (S)-enantiomer. For other β2-agonists, the stereoselectivity decreased towards unity as the Km,app increased. However, for albuterol, containing a hydroxymethyl substituent at the aromatic ring, the stereoselectivity was dramatically reversed, with 10 times higher Km,app for the inactive (S)- than for the active (R)-enantiomer. Chirality 10:800-803, 1998. © 1998 Wiley-Liss, Inc.
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  • 120
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    Electronic Resource
    Influence of the nature and substitution of chiral 2,3-epoxy alcohol derivatives on the enantiomeric elution order on chiralcel OD column (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 804-807 
    Details
    ISSN: 0899-0042
    Keywords: 2,3-epoxyalcohols ; enantiomeric elution order ; protective groups ; chain length influence ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of 2,3-epoxy alcohol derivatives (1-16), obtained either as racemates or through the Sharpless asymmetric epoxidation reaction, were studied on a Chiralcel OD column. Nearly all compounds exhibit good enantioselective resolution on this chiral support. The order of elution of enantiomers is reversed between nerol and geraniol compounds. For 2,3-epoxy alcohols bearing a remote alkoxy (or silyloxy) group, the order of the enantiomeric elution alternates with the number n (n = 1-3) of methylenic groups present between the epoxide ring and the terminal OR (R = p - BrBn or OSitBuPh2) functionality. In the case of trans 2,3-epoxy alcohols for the same number n, the order of elution is reversed when changing the terminal group -OSi to -OR. The latter group greatly improves the separation of the two enantiomers. Chirality 10:804-807, 1998. © 1998 Wiley-Liss, Inc.
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    Kurt Mislow honorary issue of Chirality (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 1-2 
    Details
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/chir.1
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    Stereoisomers of 3-(2,3-dihydrobenzofuran-2-yl)quinuclidine: Preparation and muscarinic receptor affinities (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 813-820 
    Details
    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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    The chiral code: From DNA primary structures to quaternary assemblies (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 405-414 
    Details
    ISSN: 0899-0042
    Keywords: chiral-discrimination ; homochirality ; stereospecificity ; self-assembly ; supercoiling ; cholesteric mesophase ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nucleic acids are characterized by a predominant right-handed helical configuration that derives from the chirality of the sugar moiety. Hitherto, only “local” effects of this helical asymmetry, exemplified by DNA interactions with small compounds, have been documented. The results described in this study indicate that an enhanced asymmetry is required for the manifestation of chiral effects in DNA self-assembly processes or for chiral discrimination upon interactions with peptides. Two cases in which the intrinsic DNA asymmetry is enhanced are reported: rod-like superhelical species derived from linear DNA molecules, and topologically constrained supercoiled DNA. In the first case, the superhelical grooves within the DNA rods allow for a stereospecific complexation with peptides, resulting in chiral discrimination. In the second case, it is shown that the properties of cholesteric assemblies derived from supercoiled DNA are strictly determined by the enhanced asymmetry associated with molecular supercoiling. The results allow for new reflections on the concept of molecular complementarity, and indicate that spontaneously obtained chiral DNA mesophases might have played a key role in determining terrestrial homochirality. Chirality 10:405-414, 1998. © 1998 Wiley-Liss, Inc.
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    Editors' note (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. iii 
    Details
    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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    Chiroptical properties and absolute configuration of pyrroloisoquinoline antidepressants (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 169-172 
    Details
    ISSN: 0899-0042
    Keywords: stereochemistry ; chirality ; circular dichroism ; isoquinoline ; enantiomers ; alkaloids ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Circular dichroism (CD) spectra were determined for the bioactive (+)-enantiomers of 1 · HCl, 3 · HCl, and 4 · HBr to characterize the chiroptical properties of these pyrroloisoquinoline antidepressants. The compounds showed a low-intensity negative CD band with much fine structure between 252 and 272 nm, a medium negative CD band with fine structure between 215 and 225 nm, and a high-intensity negative CD maximum between 198 and 203 nm. Except for amplitude variation, the three CD spectra were essentially superimposable in sign and position of the bands. The CD curves for the (-)-enantiomers of 1 · HCl and 4 · HBr were opposite in sign and comparable within 5% to the (+)-enantiomers. The results are consistent with the previously assigned (Maryanoff et al. J. Med. Chem. 30:1433-1454, 1987) absolute configurations of (6S, 10bR) for 1 and 3, and (6R, 10bR) for 4. Chirality 10:169-172, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/chir.26
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    Mislow's label paradox, chirality-preserving conformational changes, and related chirality measures (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 173-179 
    Details
    ISSN: 0899-0042
    Keywords: chirality at low and high resolution ; functional groups ; fuzzy density fragments ; continuum models ; fuzzy sets ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Mislow's Label Paradox and the chirality preserving or abandoning properties of deformation paths of polyhedral models are extended to simple representations of electron density continua of molecules. Chirality 10:173-179, 1998. © 1998 Wiley-Liss, Inc.
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    URL: http://dx.doi.org/10.1002/chir.27
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    Hexacoordinated carbon or tetracovalent silicon? (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 180-189 
    Details
    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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    URL: http://dx.doi.org/10.1002/chir.28
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    Sclesinger, H. and Winterfeldt, E. Enantioselective route to an azadirachtin substructure. Chirality 9:454-458, 1997. (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 190-193 
    Details
    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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    URL: http://dx.doi.org/10.1002/chir.29
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    Dendrimers and chirality (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 53-59 
    Details
    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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    URL: http://dx.doi.org/10.1002/chir.10
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    A chiral polymeric analogy to a one-dimensional paramagnetic materialFigure 2 is reprinted with permission from Green MM, Peterson NC, Sato T, Teramoto A, Cook R, Lifson S. A helical polymer with a cooperative response to chiral information. Science 1995;268:1860. Copyright 1995 American Association for the Advancement of Science. Readers may view, browse, and/or download this material for temporary copying purposes only, provided these uses are for noncommerical personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior written permission from AAAS. (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 41-45 
    Details
    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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    URL: http://dx.doi.org/10.1002/chir.8
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    Enthalpies of mixing of R- and S- enantiomers of propane-1,2-diol and methyllactate at 298.15 K (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 722-725 
    Details
    ISSN: 0899-0042
    Keywords: R- and S-isomer ; chiral compound ; enthalpy of mixing ; propane-1,2-diol ; methyllactate ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enthalpies of mixing of chiral isomers R- and S- of propane-1,2-diol and methyllactate, respectively, have been measured over the whole range of mole fraction at 298.15 K. All the enthalpies of mixing measured are very small. The enthalpic differences between the interactions of molecules of like-configuration and those of opposite configuration have been evaluated precisely. Mixing of R- and S-enantiomers of propane-1,2-diol gives slight enthalpic stabilisation over the whole range of mole fraction, however methyllactate has shown the opposite effect on mixing. Chirality 10:722-725, 1998. © 1998 Wiley-Liss, Inc.
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    Chloroperoxidase-induced asymmetric sulfoxidation of some conformationally restricted sulfides (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 246-252 
    Details
    ISSN: 0899-0042
    Keywords: sulfoxides ; chloroperoxidase ; asymmetric oxidation ; enantioselective ; episulfide ; gas chromatography ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Asymmetric sulfoxidation by means of a chloroperoxidase from Caldariomyces fumago and hydrogen peroxide as the oxygen source was studied for a series of sterically well-defined substrates. The stereochemistry of the sulfoxidation was the same for all substrates studied. While 2,3-dihydrobenzo[b]thiophene (1) is an excellent substrate (giving 99.5% yield and 99% e.e. of the (R)-sulfoxide), replacement of a methylene group by either a more sterically demanding group or a heteroatom caused a substantial decrease in reactivity or in reactivity as well as enantioselectivity. A further investigation of the lowered catalytic efficiency of chloroperoxidase with these substrates has been carried out in a series of competitive reactions. Thus, benzo[1,3]oxathiole (5) acted as a competitive inhibitor of the enzyme, whereas 1-thiochroman (2) and 1-thiochroman-4-one (3) were shown to be too sterically demanding to significantly compete for the active site. For the oxidation of 2, 3, and 5, it was found that in the low CPO concentration range the chemical yield after 60 min reaction time increased almost linearly with the amount of CPO used. The products from 2 and 3 could be obtained in over 80% yield with an e.e. exceeding 96%. Chloroperoxidase was also found to be an effective catalyst in the oxidation of labile episulfides, yielding the corresponding anti-sulfoxides quantitatively and giving 12% e.e. of (1R, 2R)-sulfoxide in the oxidation of propylene sulfide. Chirality 10:246-252, 1998. © 1998 Wiley-Liss, Inc.
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    Immobilized vancomycin as chiral stationary phase in packed capillary liquid chromatography (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 273-280 
    Details
    ISSN: 0899-0042
    Keywords: direct chiral separation ; mobile phase composition ; NSAIDs ; retention model ; vancomycin ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fused silica-packed capillary columns containing vancomycin immobilized by reductive amination on an aldehyde-silica were used to separate enantiomers of some non-steroidal anti-inflammatory drugs. Attempts have been made to qualitatively explain the influence of various mobile phase compositions on the enantioselective retention. The effects of mobile phase pH, buffer, and organic modifier concentrations were investigated as well as the influence of salts of hydrophobic ions added to the mobile phase to induce ion pair retention. Chirality 10:273-280, 1998. © 1998 Wiley-Liss, Inc.
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    Sympathomimetic enantiomers and asthma (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 262-272 
    Details
    ISSN: 0899-0042
    Keywords: airway ; beta2-agonist ; racemic ; eutomer ; distomer ; hyperreactivity ; bronchospasm ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Airways of asthma patients can become hyperresponsive to airway spasmogens following regular use of isoprenaline or β2-selective sympathomimetics. Hyperreactivity that results from acute exposure of animals to these drugs is pre-empted by vagal section (a procedure which does not influence spasmolytic efficacy of sympathomimetics), is not diminished by antagonism of β2-adrenoceptors and is not associated with loss of responsivity of β2-adrenoceptors in the airways. Since activation, modulation, or blockade of β2-adrenoceptors does not determine this form of hyperreactivity, the possibility that distomers may induce hyperreactivity must be considered. Ocular and vascular responses to distomers of sympathomimetics have long been recognised and, more recently, comparable observations have been made for the airways. Thus, reactivity of guinea-pig airways to spasmogens was increased following exposure to S-isoprenaline, S-salbutamol, or S-terbutaline and exposure to S-isoprenaline or S-salbutamol can intensify symptoms in asthmatics. Regular exposure to the racemate, especially during or following an allergic reaction, predisposes to expression of hyperreactivity, which is nullified, acutely, by the eutomer. These observations imply that biological effects of sympathomimetic distomers may contribute to morbidity and mortality in asthma patients. Chirality 10:262-272, 1998. © 1998 Wiley-Liss, Inc.
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    1997 Chirality Medal (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 281-281 
    Details
    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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    Circular dichroism of axially chiral methaqualone, 3-Aryl-2-mercapto- and 3-aryl-2-alkylthio-4(3H)-quinazolinones: conformational dependence of CD and assignment of absolute configuration (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 253-261 
    Details
    ISSN: 0899-0042
    Keywords: chiroptical properties ; Cotton effect ; atropisomerism ; quantum-mechanical calculation ; AM1 ; CNDO/S ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rotational strengths calculated on the basis of quantum-mechanically obtained minimum energy geometries were used to determine the absolute configurations of axially chiral 3-aryl-4(3H)-quinazolinones from the sign of the observed Cotton effects (CEs). For the spectral data, CNDO/S calculations were used; for the geometries, ab initio (RHF/6-31G) and semiempirical (AM1) theories were used. Oscillator and rotational strengths of all excited states down to 200 nm were compared to experimental absorption and circular dichroism (CD) data. It was found that the sign of the 1Lb Cotton effects obtained for the enantiomers of methaqualone and derivatives of 3-aryl-2-alkylthio-4(3H)-quinazolinones can be correlated unambiguously with the absolute configuration. Furthermore, the sign of the Cotton effect of the π-π* transition of the thiocarbonyl chromophore of 3-aryl-2-mercapto-4(3H)-quinazolinones is suitable for a successful stereochemical correlation. Chirality 10:253-261, 1998. © 1998 Wiley-Liss, Inc.
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    Correlation between time reversal symmetry and chirality in certain molecular processes (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 289-293 
    Details
    ISSN: 0899-0042
    Keywords: chirality ; time reversal symmetry ; asymmetric synthesis ; enantiomerism ; isomerism ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: If a molecule is identified not only by its static spatial constructions, but also by the motions at the sub-molecular level, application of time reversal symmetry operation to a certain molecule could lead to another distinguishable from the original in the sense of sub-molecular motions, a phenomenon now defined as time reversal isomerism. Assessment of the consideration of certain enantiomers as distinguishable time reversal isomers is suggested in order to evoke a comprehensive interpretation of a likely correlation between the two types of isomerisms. The conceptual basis of a connection between absolute asymmetric synthesis under the influence of external fields and the intrinsic time reversal symmetry violation at the molecular level is also established to encourage new experimental investigations on this theme. Chirality 10:289-293, 1998. © 1998 Wiley-Liss, Inc.
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    Carbamates of cellulose bonded on silica gel: Chiral discrimination ability as HPLC chiral stationary phases (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 283-288 
    Details
    ISSN: 0899-0042
    Keywords: chiral HPLC ; cellulose carbamates ; enantiomeric resolution ; warfarin ; flurbiprofen ; lorazepam ; oxazepam ; pindolol ; tertatolol ; nicardipine ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Four cellulose mixed 10-undecenoate/carbamate derivatives, simultaneously bearing 10-undecenoyl and variously substituted phenylaminocarbonyl groups, were chemically bonded on allylsilica gel. The study of the effect of these substitutions on the performance of the resulting chiral supports, and a comparison with the recently described 10-undecenoate/3,5-dimethylphenylcarbamate derivative, are presented. In this study heptane/2-propanol or heptane/chloroform mixtures were used as mobile phases. Chirality 10:283-288, 1998. © 1998 Wiley-Liss, Inc.
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    Synthesis of regioselectively substituted cellulose derivatives and applications in chiral chromatography (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 294-306 
    Details
    ISSN: 0899-0042
    Keywords: cellulose ; regioselective derivatization ; chiral stationary phases ; liquid chromatography ; enantioseparation ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Various cellulose-2,3-bis-arylcarbamate-6-O-arylesters and cellulose-2,3-bis-arylester-6-O-arylcarbamates, designed to test the possible combined effects of the known tris-arylcarbamate and tris-arylester classes, were synthesized with high regioselectivity at O-C(6), and their use as CSPs in liquid chromatography for enantiomeric separations was investigated. The separations obtained with the synthesized CSPs were compared to the separations achieved on a self-packed reference column, consisting of cellulose-tris-(3,5-dimethylphenyl-carbamate) as CSP standard. Among the synthesized, regioselectively substituted cellulose derivatives, 2,3-bis-O-(3,5-dimethylphenylcarbamate)-6-O-benzoate-cellulose and 2,3-bis-O-(benzoate)-6-O-(3,5-dichlorophenylcarbamate)-cellulose gave the best CSPs for the separation of the test racemates. CSPs from regioselectively substituted cellulose derivatives seem to exhibit higher selectivities than cellulose-tris-(3,5-dimethylphenylcarbamate) for certain classes of racemic compounds. Chirality 10:294-306, 1998. © 1998 Wiley-Liss, Inc.
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    Kinetics of racemization of (+)- and (-)-diethylpropion: Studies in aqueous solution, with and without the addition of cyclodextrins, in organic solvents and in human plasma (1998)
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 307-315 
    Details
    ISSN: 0899-0042
    Keywords: configurational stability ; pH ; temperature ; ionic strength ; phosphate buffer concentration ; plasma protein affinity ; native cyclodextrins ; cyclodextrin derivatives ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The configurational stability of (+)- and (-)-diethylpropion [(+)- and (-)-2-(diethyl)-1-phenyl-1-propanone or (+)- and (-)-DEP] was investigated systematically from chemical, pharmaceutical, and pharmacological aspects. The enantiomeric ratio was monitored directly with a recently developed stability-indicating enantioselective HPLC method.In aqueous solutions, the rate of racemization increased non-linearly with increasing pH and with increasing phosphate buffer concentration. The racemization rate showed a positive slope with increasing temperature and decreasing ionic strength.The racemization rates of (+)- and (-)-DEP in the presence of cyclodextrins (CDs) did not differ significantly. CDs that were added to (+)- and (-)-DEP in a molar ratio 5:1 showed the following effects after dissolution in 10 mM phosphate buffer (final pH 6.7): sulfobutyl ether-β-CD (SBE-β-CD) and methylated-β-CD (Me-β-CD) retarded racemization; whereas hydroxypropyl-β-CD (HP-β-CD), acetyl-γ-CD (Ac-γ-CD), acetyl-β-CD (Ac-β-CD), γ-CD, and β-CD showed a weak destabilising effect. In contrast to the described CDs, α-CD distinctly accelerated the rate of racemization.The configurational stability of (+)- and (-)-DEP was also studied under physiological conditions. The half-life of racemization in heparinised human plasma was for both enantiomers determined to be approximately 23-25 min.In phosphate buffer (10 mM, pH 7.4), rac-DEP showed a high, but unselective affinity towards human α1-acid glycoprotein (orosomucoid) immobilised on silica (Chiral AGP).The rate of racemization of the free base of (-)-DEP dissolved in organic solutions generally increases with the polarity of the solvating agent. Chirality 10:307-315, 1998. © 1998 Wiley-Liss, Inc.
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    Induction of an Enantiomeric Excess in a Calix[4]arene/Bipyridine-Based chiral copper(i) complex (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 661-669 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Introduction of the non-complexing chiral (S)-2-methylbutoxy substituent close to the complexing site of a bis[(bipyridinyl)methoxy]calixarene podand resulted in the induction of an enantiomeric excess of ca. 30 % in the corresponding chiral CuI complex. Structural investigations by high-resolution NMR studies led us to propose the left-handed prohelical [CuI(bpy)2] substructure for the major enantiomer.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810316
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  • 142
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    An Advantageous Synthesis of 1D- and 1L-1,2,3,5/4-Cyclohexanepentol (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 688-694 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compounds D-10 and L-10 were prepared from 1 in eight steps and in a combined overall yield of 41-49%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810319
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  • 143
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    Orbital-Symmetry Effects in Bimolecular Electron-Transfer Reactions: Back Electron TransferDedicated to Prof. Nicholas J. Turro on the occasion of his 60th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 670-675 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Previous work has indicated orbital-symmetry effects upon forward electron transfer in bimolecular systems, with magnitude similar to that encountered in rigid monomolecular systems. The present work, which employs back electron transfer, supports and extends these earlier findings.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810317
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  • 144
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    New Axially Chiral Bis(dihydrooxazoles) as Ligands in Stereoselective Transition-Metal Catalysis (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 676-687 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The new, axially chiral bis(4,5-dihydrooxazoles) 4 have been synthesized in a straightforward manner, starting from the substituted, racemic 1,1′-biphenyl-2,2′-dicarboxylic acids 1 and optically active amino alcohols 2. The adducts were resolved by medium-performance liquid chromatography (MPLC; see Scheme 1). Formation of Cu1 complexes of 4 was followed by 1H-NMR spectroscopy. The catalytic behavior of these complexes has been investigated by asymmetric cyclopropanation of styrene with ethyl diazoacetate. Beside the influence of steric factors, a significant electronic effect on asymmetric induction could also be observed (see Scheme 2 and Table).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810318
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  • 145
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    Glycosylidene Carbenes part 26Part 25: [1]. The intramolecular F…HO hydrogen bond of 1,3-diaxial 3-fluorocyclohexanols (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 695-717 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: According to ab-initio calculations, the CF…HO H-bond in 1,3-diaxial 3-fluorocyclohexanol is characterized by d(F…H) = 2.08 Å, and ∠ (F…H—O) = 138°, and by ΔE between 1.2 and 4.1 kcal/mol, depending upon the reference system. Relative to the OH stretching frequency of axial cyclohexanol, the OH stretching frequency of 1,3-diaxial 3-fluorocyclohexanol is shifted by Δω = 7 cm-1. The rigid fluoro diols D-4 and L-4 were prepared from tetrahydroxy-p-benzoquinone in 11 steps and 1% overall yield. The IR spectrum of 4 in CCl4 soln. is characterized by Δν = 7 cm-1 for the axial and Δν = 44 cm-1 for the equatorial OH group. A relatively strong intramolecular CF…HO bond of 4 in CCl4 is evidenced by the large through-space coupling 5J(F,HO) of 9.3 Hz. Nevertheless, this FHO bond is disrupted in ethereal solvents, while the bifurcated O…HO bond subsists. In CCl4, the carbene generated from the glucosylidene-derived diazirine 5 reacted more rapidly with the axial OH group of D-4 and L-4 than with the equatorial one. This regioselectivity is in keeping with the weaker H-bond of the axial OH group. The regioselectivity is lower in ethers, but its solvent dependence does not parallel solvent basicity. This is not satisfactorily explained by the differential interaction of the ether solvents with the axial and equatorial OH groups, as evidenced by the solvent dependence of their chemical shift, but must also reflect the different interaction of the solvents with the carbene derived from 5, leading to ylides. The lower solvent dependence of the anomeric selectivity for the glycosidation of the equatorial OH group is a consequence of the coordination of the intermediate oxycarbenium ion with O—C(1) and O—C(3) rather than with the solvent. Under conditions of competitive glycosylation in CCl4, the fluoro alcohol D-22 reacted more slowly than the alcohol L-24, evidencing the intramolecular F…HO H-bond.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810320
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  • 146
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    An Alternative Synthesis of the Enantiomerically Pure Competitive NMIDA Antagonists SDZ 220-581 and SDZ EAB 515 (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 729-733 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An alternative synthesis of enantiomerically pure SDZ 220-881 (1a) and SDZ EAB 515 (1b) starting from L-Z-tyrosine is described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810322
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  • 147
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    Synthesis of Bridgehead-Functionalized Bicyclo[3.2.1]octanes via intramolecular titanium- and tributylstannane-induced pinacol coupling (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 734-743 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the enantiomerically pure, bridgehead-functionalized bicyclo[3.2.1]octanes 11 and 16, containing a conformationally fixed trihydroxypropyl (aminodihydroxypropyl) unit, as well as the X-ray structure of 11 are described. These compounds are of interest as sugar surrogates in the preparation of DNA analogs. Compounds 11 and 16 became available in 10 and 12 steps, respectively, and in an overall yield of 11 and 4% from D-arabinose via a highly stereoselective pinacol coupling as the key step.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810323
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  • 148
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    Reaction of N,N'-dimethyl-2-nitroethene-1, 1-diamine with α,ß-unsaturated acyl isothiocyanates: Preparation of 1,3-thiazin-4-one and 4-nitro-1,2-thiazol-5(2H)-imine derivativesPresented in preliminary form at the ‘XII seminario cientifico’, centro nacional de investigaciones cientificas, habana, cuba, 1995 (M. I. G. T.) and at the ‘tenth international conference on organic synthesis’, Bangalore, India, 1994 (D. M. A.). (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 718-728 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of N,N'-dimethyl-2-nitroethene-1,1-diamine (8) with α,ß-unsaturated acyl isothiocyanates 9 affords 3,3-diamino-2-nitroacrylthioamides 10 (Scheme 2) in moderate-to-good yields. Cyclization of 10 under acidic conditions gives 1,3-thiazin-4-one derivatives of type 11. Oxidative cyclization of 10 with diethyl azodicaboxylate leads to 4-nitro-1,2-thiazol-5(2H)-imine derivatives 12.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810321
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  • 149
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    Diastereo- and Regioisomeric Bicyclic Thiohydantions from Chiral 1,3-Thiazolidine-2,4-dicarboxylic acids (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 744-753 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bicyclic thiohydantoins were synthesized in a stereoselective manner by reacting (2R)/(2S)-diastereoisomer mixtures of 1,3-thiazolidine-2,4-dicarboxylic acids or their dimethyl diesters with PhNCS. 5,5-Dimethyl-1,3-thiazolidine-2,4-dicarboxylic acid with PhNCS led to a cyclization involving the C=O group at the C(2) center of the thiazolidine ring, while the acid's dimethyl diester gave cyclization involving the C=O group at C(4). In contrast, reactions involving unsubstituted 1,3-thiazolidine-2,4-dicarboxylic acid or its dimethyl diester led to thiohydantoins in which the ring closure had taken place only with the COO group at C(4). Independently of the direction of the ring closure, all reactions produce exclusively products with the (R)-configuration at C(2). The configurational assignments were based on 1H- and 13C-NMR studies, and confirmed by X-ray crystallographic analyses.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810324
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  • 150
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    Prenylated Flavanones from Monotes engleri: On-line Structure Elucidation by LC/UV/NMR (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 754-763 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The CH2Cl2 extract of Monotes engleri Gilg. (Dipterocarpaceae) showed antifungal activity against the yeast Candida albicans in our bioautographic TLC assays. After a first fractionation of the crude extract, the bioactivity was located in one of the fractions. To perform an efficient targeted isolation of the active compounds, LC/UV/MS and LC/UV/NMR analyses of the crude extract and the active fraction were performed. LC/UV/, LC/MS, and LC/NMR data (1D and 2D) allowed the identification of 1 as (2S)-2,3-dihydro-5,7-dihydroxy-{3-hydroxy-4-[(3-methylbut-2-enyl) oxy]phenyl}-4H-1-benzopyran-4-one, a new prenylated flavanone, named monoteson A. Subsequent isolation of 1 has permitted the determination of its absolute configuration on the basis of CD measurements. Theree other prenylated flavanoes 2-4 were isolated from the same extract. Compound 3 was identified as 2- (3, 5-dihydroxyphenyl) -2,3-dihydro-5, 7-dihydroxy-6, 8-bis (3-methylbut-2-enyl)-4 H-1-benzopyran-4-one, another new natural product, named monotesone B. The structures of 2 and 4 were established as selinone and lonchocarpol A, respectively. The antifungal activity against Candida albicans was determined for all compounds.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810325
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  • 151
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    Enantioselective Dicarbonylation of Styrene to Isotactic Poly[1-oxo-2-phenylpropane-1,3-diyl] with phosphinodihydrooxazole-palladium(ii) complexes: Model studies for enantioface selection preliminary communication (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 764-769 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first steps, believed to be involved in the highly enantioselective copolymerization of styrene and carbon monoxide to poly[1-oxo-2-phenylpropane-1,3-diyl] with phosphinodihydrooxazole-palladium(II) complexes, were investigated. The insertion of carbon monoxide into [Pd(Me)(P^N)(solvent)] TfO (P^N = (S)-2-[2-(5H-benzo[b]phosphindol-5-yl)phenyl]4-benzyl-4,5-dihydrooxazole (1)) and of styrene into [Pd(Me)(P^N)(solvent)] TfO were highly regioselective (alkyl and acyl substituents trans to N); moreover, the olefin insertion took place with essentially complete enantioface discrimination.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810326
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  • 152
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    Absolute rate Constants for the Addition of Malonyl Radicals to Alkenes in Solution (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 770-780 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Absolute rate constants and their Arrhenius parameters are obtained for the addition of a cyclic malonyl radical derived from Meldrum's acid to 20 mono- or 1,1-disubstituted alkenes in (±)-propylene oxide and for the addition of the open-chain di(tert-butyl)malonyl radical to six mono- or 1,1-disubstituted alkenes in 1,1,2-trichloro-1,2,2-trifluoroethane by time-resolved electron spin resonance spectroscopy. At room temperature, the radicals add at the unsubstituted C-atoms with rate constants ranging from 1.1. 105 M-1S-1 (acrolein) to 2.4. 106 M-1S-1 (1,1-diphenylethene). The frequency factors are in the narrow ranges of log (A/M-1S-1) = 8.7 ± 0.1 for the cyclic and log (A/M-1S-1) = 8.2 ± 0.2 for the open-chain malonyl species, whereas the activation energies vary from 12.9 kJ/mol (1,1-diphenylethene) to 21.7 kJ/mol (acrylonitrile). They correlate with the alkene ionization potentials and, more weakly, with the reaction enthalpy. No correlation was found between the activation energies and the alkene electron affinities. Hence, the results confirm and quantify the electrophilic nature of malonyl radicals in addition reactions.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810327
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  • 153
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    Ruzicka-Preis 1998 (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 792-792 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810329
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  • 154
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    Masthead (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810501
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  • 155
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    Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds part VIPart V: [1] Derivatives with Bidentate Ligands, Crystal Structures of tetrahedro- Decacarbonyl{μ-[1,1-bis(methylthio-χS)ethane]}tetrairidium ([Ir4(CO)10-(μ2-(MeS)2CHMe)]), tetrahedro-Tri-μ-carbonylheptacarbonyl{μ-{ethylidenebis- [diphenylphosphine]-χP:χP'}}tetrairidium ([Ir4(CO)10(μ2-(Ph2P)2CHMe)]), and tetrahedro-Tri-μ-carbonylheptacarbonyl{μ-{propane-1,3-diylbis[diphenylphosphine]-χP:χP'}}tetrairidium ([Ir4(CO)10(μ2-Ph2P(CH2)3PPh2)]) (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 781-791 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The disubstituted clusters[Ir4(CO)10(μ2-L—L)] with one edge-bridging ligand have a ground-state geometry with all COs terminal (L—L = (MeS)2CHMe, cluster 1) or with three edge-bridging COs (L—L = (Ph2P)2CHMe, cluste 2; L—L =Ph2P(CH2)3PPh2, cluster 3) in the solid state and in solution. A comparative 13C-NMR study of 1-3 shows that their respective ground-state geometries are merely relative minima of energy in the same kinetic profile of successive fluxional processes consisting of a merry-go-round of six COs about a unique trangular face and the rotation of terminal COs about one Ir-atom. The factors affecting the activation energy of the merry-go-round result from the relative bites of the bidentate ligands in the ground-state geometry, as shown by a comparison of the molecular structures of 2 and 3.
    Additional Material: 7 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810328
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  • 156
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    Synthesis of Oligonucleotides Containing 2′-Deoxyisoguanosine and 2′-Deoxy-5-methylisocytidine Using Phosphoramidite Chemistry (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 793-811 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of oligonucleotides containing 2′-deoxy-5-methylisocytidine and 2′-deoxyisoguanosine using phosphoramidite chemistry in solid-phase oligonucleotide synthesis is described. Supporting previous observations, the N,N-diisobutylformamidine moiety was found to be a far superior protecting group than N-benzoyl for 2′-deoxy-5-methylisocytidine. 2′-Deoxy-N2-[(diisobutylamino)methylidene]-5′-(4,4′-dimethoxytityl)-5-methylisocytidine 3′-(2-cyanoethyl diisopropylphosphoramidite) (1c) incorporated multiple consecutive residues during a standard automated synthesis protocol with a coupling efficiency 〉 99% according to dimethoxytrityl release. Extending coupling times of the standard protocol to ≥ 600s using 2′-deoxy-N6-[(diisobutylamino)methylidene]-5′-O-(dimethoxytrityl)-O2-(diphenylcarbamoyl)isoguanosine, 3′-(2-cyanoethyl diisopropylphosphoramidite) (7e) led to successful incorporation of multiple consecutive 2′-deoxyisoguanosine bases with a coupling efficiency 〉 97% according to dimethoxytrityl release.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810502
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  • 157
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    Selective Derivatization of the Ionophore X-206 at C(22) Maintaining Potassium Binding (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 812-827 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The insecticidal polyether antibiotic X-206 (1) complexes potassium ions using nearly all of its O-atoms either for binding to the metal ion or for participation in a H-bonding network which helps to hold X-206 in the ionophoric tertiary structure. The group OH—C(22) is not involved in these processes. It was supposed that derivatization of this group would not affect the ionophoric properties and would produce insecticidally active compounds. The chemistry leading to selective modification of OH—C(22) via the intermediate 6 was developed. The potassium-binding properties and insecticidal activities of the MeCOO—C(22) and MeO—C(22) compounds 3 and 11, respectively, confirmed that derivatization of the peripheral OH—C(22) was a valid strategy for the synthesis of biologically active compounds.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810503
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  • 158
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    A Novel Hydrocarbon, 8,10-Dimethylidenetricyclo[7.1.1.02,7]undeca-2,4,6-triene: Synthesis of benzopinane skeleton via di-π-methane rearrangement of benzonorbornadiene systemDedicated to Professor Özer Bekaroǧlu on the occasion of his 65th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 828-836 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [4 + 2] cycloadduct 17 of 2,3-dimethylidene-1,2,3,4-tetrahydro-1,4-methanonaphthalene and 4-phenyl-4H-1,2,4-triazole-3,5-dione (PTAD) was subjected to a triplet-sensitized di-π-methane rearrangement. Hydrolysis of the resulting urazol 18 gave the hydrocarbon 7. Hydrolysis of 18 at lower base concentrations led to isomeric stable semicarbazides 24 and 25, which were submitted NiO2 or MnO2 oxidation, to give the target compound 7, and oxidation products 26 and 27.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810504
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  • 159
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    Kinetic Study of the Reaction of Pyridoxal 5′-Phosphate with Hydrazino Compounds of Pharmacological Activity (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 837-844 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetics of the reaction between pyridoxal 5′-phosphate (PLP) with carbidopa, hydralazine, and isoniazid, in aqueous solution at variable pH and constant ionic strength of 0.1M was studied spectrophotometrically. The rate constants of formation and hydrolysis of the resulting Schiff base, and its stability were determined in a wide range of pH. A comparison is made of the formation rate constants with those of PLP with hydrazine. The reactivity shows the sequence isoniazid 〉 hydrazine 〉 carbidopa 〉 hydralazine in the whole range of pH studied. The Schiff bases studied are more stable than those formed by PLP and hexylamine and as stable as those described for the reactions of PLP with poly(L-lysine) or copolypeptides containing L-lysine.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810505
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  • 160
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    An Unusual Acyliminium Cyclization and Other Drawbacks during an Attempted Synthesis of a Chiral Primary α-Phosphinoalkanamine (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 845-852 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Studies towards the synthesis of a chiral primary α-phosphinoalkanamine 1a are reported. O-Activated. N-carbamate-protected phenylalaninol 3a did not undergo SN reaction with KPPh2: instead, after N-deprotonation, intramolecular substitution led to formation of the aziridine derivative 5a (Scheme 2). N-Phthalimido-protected, O-activated phenylalaninol 3b also underwent an intramolecular process on treatment with KPPh2, i.e., an unusual aryl-acyliminium cyclization furnishing the (epoxymethano)isoindolo[1,2-a]isoquinolinone 7 (Scheme 3). In a reaction with KPPh2, the N,N-dibenzyl-protected and activated phenylalaninol 3d finally yielded the intermolecular SN reaction product 2a (Scheme 4). However, debenzylation by catalytic hydrogenation turned out to be impossible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810506
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  • 161
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    Nucleotides. Part LVPart LIV: [1].. Synthesis and application of a novel linker for solid-phase synthesis of modified oligonucleotides (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 46-58 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Various bifunctional amino-protecting groups such as the phthaloyl, succinyl, and glutaryl group were investigated as potential linker molecules for attachment to solid-support materials. Pentane-1,3,5-tricarboxylic acid 1,3-anhydride (16) offered the best properties and reacted with the amino groups of differently sugar-protected adenosine (see 20 and 22), cytidine (see 29), and guanosine derivatives (see 32) to the corresponding 2-(2-carboxyethyl)glutaryl derivatives 23, 24, 30, and 33. The usefulness of the new linker-type molecules was demonstrated by the solid-support synthesis of the potentially antivirally active 3′-deoxyadenylyl-(2′-5′)-2′-adenylic acid 2′-{2-[(adenin-9-yl)methoxy]ethyl} ester (38) starting from the 2′-end with N6,N6-[2-(2-carboxyethyl)glutaryl]-9-{{2-[(4,4′-dimethoxytrityl)ethoxy]methyl}adenine (12).
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    [2+3] Cycloadditions of ‘Thiocarbonyl Ylides’ (= (Alkylidenesulfonio)methanides) and diazoalkanes with N,N′-(thiocarbonyl)diimidazole (= 1,1′-(carbonothioyl)bis[1H-imidazole])Part of the planned Ph.D. Thesis of T.G., University of Łódź.Dedicated to Professor Romuald Bartnik (University of Łódź) on the occasion of his 60th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 66-77 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Heating of a mixture of N,N′-(thiocarbonyl)diimidazole (= 1,1′-(carbonothioyl)bis[1H-imidazole]; 1) and 2,5-dihydro-1,3,4-thiadiazole 2a or 2b gave the 1,3-dithiolanes 4a and 4b, respectively, via a regiospecific 1,3-dipolar cycloaddition of the corresponding ‘thiocarbonyl methanides’ 3a,b onto the C=S group of 1 (Schemes 1 and 2). The adamantane derivative 4b was not stable in the presence of 1H-imidazole and during chromatographic workup. The isolated 1,3-dithiole 5 is the product of a base-catalyzed elimination of 1H-imidazole from the initial cycloadduct 4b. The formation of the S,N-acetal 6 can be rationalized by a protonation of the ‘thiocarbonyl ylide’ 3b followed by a nucleophilic addition of 1H-imidazole. With the diazo compounds 8a-e (Scheme 3) 1 underwent a regiospecific 1,3-dipolar cycloaddition to give the corresponding 2,5-dihydro-1,3,4-thiadiazole derivatives 9, which spontaneously eliminated 1H-imidazole to yield (1H-imidazol-1-yl)-1,3,4-thiadiazoles 10. The structures of 10a and 10d were established by X-ray crystallography. In the case of diazodiphenylmethane (8f), the initial cycloadduct 9f decomposed via a ‘twofold extrusion’ of N2 and S to give 1,1′-(2,2-diphenylethenylidene)bis[1H-imidazole] (11; Scheme 3).
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    8,19-Dimethyl-tetraepoxy[22]annulen(2.1.2.1)Ziffern in Klammern nach einem Namen geben die Zahl der Atome zwischen den Furan-Ringen an. Die Numerierung der Tetraepoxyannulene ist willkürlich; für systematische Namen, s. Exper. Teil.: ein erstes Tetraepoxy-Verbrücktes aromatisches [22]Annulen (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 93-108 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 8,19-Dimethyl-tetraepoxy[22]annulen(2.1.2.1): The First Tetraepoxy-Bridged Aromatic[22]AnnuleneBy McMurry reaction of 5,5′-ethylidenebis[furan-2-carbaldehyde] (15), a syn/anti mixture 16 of (E,E)- and (Z,Z)-8,19-dihydro-8,19-dimethyl-tetraepoxy[22]annulene is obtained. The (E/E)-isomers 16 are the first rotation- ally active noncyclic conjugated macrocycles, where the (E)-ethenediyl moieties rotate around the connecting single bonds. The dihydro-tetraepoxy[22]annulenes 16 are dehydrogenated by (Ph3C)BF4 as well as by O2 to give the tetraepoxy[22]annulene 11. The spectroscopic data support the character of 11 as an aromatic, diatropic ring system, which is rather sensitive towards O2. In the oxidation mixture obtained from 11, beside polymeric products, two compounds 19 and 20 can be isolated, carrying one and two CHO groups, respectively, resulting by oxidation of one or both Me-groups but having retained the aromatic 22π system of 11.
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  • 164
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    Stereoselective Reactions of Phthalimido-Substituted Radicals Derived from (±)-Threonine: A comparison with reactions of N-phthaloyliminium ions (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 268-284 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective reactions of phthalimido-substituted radicals derived from (±)-threonine with different radical traps are reported (Scheme 3, Table 1). A strong influence of the nature of the radical trap on the stereoselectivity was noticed. Small nucleophilic radical traps gave preferentially the syn products. The observed selectivities are explained with the A1,3 strain model and depend on steric and electronic effects (Fig. 2). Reactions with electrophilic radical traps such as diphenyl diselenide gave the anti diastereoisomers with moderate stereocontrol, presumably due to stereoelectronic effects. The same stereochemical outcome, i.e., preferential formation of the anti products, was observed for the reactions of the related N-phthaloyliminium ion (Scheme 5, Table 2). The stereochemistry of the ionic reaction is rationalized by a Felkin-Anh model (Fig. 3).
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  • 165
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    1,3-Oxathiole and Thiirane Derivatives from the Reactions of Azibenzil and α-diazo amides with thiocarbonyl compounds (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 285-302 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of α-diazo ketones 1a,b with 9H-fluorene-9-thione (2f) in THF at room temperature yielded the symmetrical 1,3-dithiolanes 7a,b, whereas 1b and 2,2,4,4-tetramethylcyclobutane-1,3-dithione (2d) in THF at 60° led to a mixture of two stereoisomeric 1,3-oxathiole derivatives cis- and trans-9a (Scheme 2). With 2-diazo-1,2-diphenylethanone (1c), thio ketones 2a-d as well as 1,3-thiazole-5(4H)-thione 2g reacted to give 1,3-oxathiole derivatives exclusively (Schemes 3 and 4). As the reactions with 1c were more sluggish than those with 1a,b, they were catalyzed either by the addition of LiClO4 or by Rh2(OAc)4. In the case of 2d in THF/LiClO4 at room temperature, a mixture of the monoadduct 4d and the stereoisomeric bis-adducts cis- and trans-9b was formed. Monoadduct 4d could be transformed to cis- and trans-9b by treatment with 1c in the presence of Rh2(OAc)4 (Scheme 4). Xanthione (2e) and 1c in THF at room temperature reacted only when catalyzed with Rh2(OAc)4, and, in contrast to the previous reactions, the benzoyl-substituted thiirane derivative 5a was the sole product (Scheme 4). Both types of reaction were observed with α-diazo amides 1d,e (Schemes 5-7). It is worth mentioning that formation of 1,3-oxathiole or thiirane is not only dependent on the type of the carbonyl compound 2 but also on the α-diazo amide. In the case of 1d and thioxocyclobutanone 2c in THF at room temperature, the primary cycloadduct 12 was the main product. Heating the mixture to 60°, 1,3-oxathiole 10d as well as the spirocyclic thiirane-carboxamide 11b were formed. Thiirane-carboxamides 11d-g were desulfurized with (Me2N)3P in THF at 60°, yielding the corresponding acrylamide derivatives (Scheme 7). All reactions are rationalized by a mechanism via initial formation of acyl-substituted thiocarbonyl ylides which undergo either a 1,5-dipolar electrocyclization to give 1,3-oxathiole derivatives or a 1,3-dipolar electrocyclization to yield thiiranes. Only in the case of the most reactive 9H-fluorene-9-thione (2f) is the thiocarbonyl ylide trapped by a second molecule of 2f to give 1,3-dithiolane derivatives by a 1,3-dipolar cycloaddition.
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    Intrinsic and Intramolecular Lipophilicity Effects in O-Glucuronides (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 330-341 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this study, we compared the lipophilicity of O-glucuronides and their aglycones. Distribution coefficients (log D) and P values of neutral species (log P) were determined by centrifugal partition chromatography (CPC) in octanol/buffer systems. Two-phase potentiometry was also used to measure the log P value of some lipophilic solutes. The experimentally determined global influence of glucuronidation on lipophilicity, obtained as the difference (decrement) log P(glucuronide) - log P(aglycone), was found to be -1.30 ± 0.16 (n = 4) for glucuronides of alcohols (methyl, menthyl, neomenthyl, and chloramphenicol O-glucuronide). The mean decrement was -2.06 ± 0.31 (n = 9) for glucuronides of phenols (phenyl, p-nitrophenyl, 1-naphthyl, 6-bromo-2-naphthyl, 4-methylumbelliferyl, 3-coumarinyl, phenolphthalein, 4′-benzophenonyl O-glucuronide, and diflunisal phenolic glucuronide). For the acylglucuronide of diflunisal and its rearrangement isomers, the mean decrement was -1.80 ± 0.08 (n = 4; range -1.7 to -1.9). Differences in through-bond proximity effects as parametrized in the CLOGP algorithm seem to account for much of this difference. Conformational factors may also play a role, although it appears modest and unassessable for the glucuronides investigated here. The results imply that in vivo glucuronidation should have a stronger influence on the excretion of phenols than on that of alcohols.
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  • 167
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    N,Se-Acetals: Preparation and use in diastereoselective radical reactionsFor a preliminary communication of this work, see [1]. (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 353-373 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new facile synthesis of N,S- and N,Se-acetals starting from aldehydes and primary amines is presented (Schemes 3-5). These acetals are used as precursors for stereoselective radical deuteration and allylation reactions (Schemes 6 and 7, Tables 1 and 2). The stereochemical outcome of the reactions depends on the radical trap and the substituents at the N-atom. Deuterations give always anti products with moderate to high selectivities. The allylation reactions give either syn or anti products with low to moderate selectivities. The observed stereoselectivities can be explained with a model based on minimization of A1,3 strain and are controlled by steric and stereoelectronic effects.
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  • 168
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    Synthesis of Chiral C2-Symmetric Binucleating Ligands (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 491-506 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of a series of chiral enantiomerically pure C2-symmetric binucleating ligands is reported. Ligands of type 1-4, which consist of a phenolic or heterocyclic unit bridging two chiral dihydrooxazole rings. are readily accessible from chiral amino alcohols. Ligands 5a and 5b are composed of a cyclic urea or thiourea unit, respectively, and two 3,4-dihydro-2H-pyrrole rings containing a stereogenic center next to the N-atom. Compounds of this type are readily assembled from ethane-1,2-diamine and an imidothioic ester derived from pyroglutamic acid. These new ligands, which can coordinate two metals in close proximity to each other, are of interest regarding possible applications in asymmetric catalysis.
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    URL: http://dx.doi.org/10.1002/hlca.19980810304
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    Structure and Properties of Transition-Metal Complexes with Chiral C2-symmetric binucleating ligands (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 507-524 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The coordination behavior of chiral binucleating ligands of type 1-4 with various transition metals has been investigated. 1H-NMR Titration experiments with zinc(II) salts gave detailed structural information about the structure of the resulting zinc complexes. Ligand 1 forms an unusual C3-symmetric dinuclear zinc complex [Zn2CIL3] (8a) which was characterized by X-ray crystallography. Treatment of complex 8a with various carboxylic acids resulted in ligand-exchange reactions. With ligand 2, a hydroxo-bridged dinuclear copper complex 15 was synthesized and its structure elucidated by X-ray analysis. Solution studies UV and 1H-NMR spectroscopy of the reaction of ligand 3 with ZnII and NII salts revealed the formation of dimeric species of the type [M2X4L2]. Ligand 4 formed well-defined dinuclear complexes with NiII and CuII salts of which the corresponding NiII complex [Ni2(AcO)2(ClO4)2L] (22a) was characterized by crystal-structure analysis.
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    Erratum (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1583-1583 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/hlca.19980810559
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  • 171
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    Nucleosides. Part LXIII.Part LXII: [1]. Acetals as new 2′-O-protecting functions for the synthesis of oligoribonucleotides: Synthesis of uridine building blocks and evaluation of their relative acid stability (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1545-1566 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A broad variety of new acyclic vinyl ethers (see 6-41) have been synthesized via the vinyl-interchange reaction of ethyl vinyl ether at room temperature using mercury(II) trifluoroacetate as a highly efficient catalyst. The appropriate vinyl ethers were reacted under acidic conditions with 3′,5′-O-silyl-protected uridine 42 to the corresponding 2′-O-(1-alkoxyethyl) derivatives 43-83 which gave, on desilylation of F- ions, in high yields the uridine-2′-O-acetal derivatives 84-124. The relative stabilities of the newly synthesized compounds under acidic and basic conditions were determined using TLC and HPLC techniques. Protected protecting groups offer the best properties for oligoribonucleotide syntheses. Interestingly, the very acid-stable acetals of the β-substituted ethyl-type 118-121 and 123 can be cleaved by a β-elimination process providing a series of base-labile acetals of potential synthetic value.
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  • 172
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    Lipase-Catalyzed Acetylation of 3-Substituted 2,3-Dihydro-1H-1,4-benzodiazepin-2-ones: Effect of temperature and conformation on enantioselectivity and configuration (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1567-1582 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantioselectivity of acetylation by vinyl acetate/AcOEt catalyzed by immobilized Candida antarctica lipase (Novozym 435) is studied for rac-3-(hydroxymethyl)-1,4-benzodiazepin-2-ones 7, 9, 14 (n = 1; number of CH2 groups in the chain at C(3)), 20 (n = 2), and for prochiral 3,3-bis(hydroxymethyl) derivative 16. Enantiomeric excess (ee [%]) is correlated with conformational properties of substrates (relative conformation, energy difference between two boat-like ground-state conformations, ring-inversion barrier) as determined by DNMR and MM2 calculations. (3S)-Enantiomers of acetates (+)-8, (+)-10, (+)-15, and (+)-21 were preferentially formed. In the case of the acetate (-)-17 (ee 90.2%), formation of the (3R)-enantiomer was favored. C(3)—OH Group with hemiaminal-like character in rac-3 (n = 0) cannot be acetylated by any of 23 tested lipases and four esterases. For racemic alcohols 7, 9, 14, and 20, preferred acetylationof the (3S)-enantiomers, present in solution in absolute (M)-conformation, was established; only in prochiral diol 16 (n = 1) the CH2OH group in the (pro-R)-position is prevalently acetylated, presumably due to the binding to the enzyme, in absolute (P)-conformation. Temperature dependence of enantioselectivity revealed inverse correlation of the E value of rac-9, and ee values for prochiral 16, with T[K], indicating prevalent contribution of the enthalpy term to enantioselection. Absolute conformation (M/P) and absolute configuration at C(3) of all products was determined by combining CD and 1H-NMR data.
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    Masthead (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810201
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    Synthesis and transformations of methyl (E)-2-(acetylamino)-3-cyanoprop-2-enoate und methyl (E)-2-(benzoylamino)-3-cyanoprop-2-enoate, versatile reagents for the preparation of polyfunctional heterocyclic systems (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 231-235 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methyl (E)-2-(acetylamino)-3-cyanoprop-2-enoate (2a), and its 2-benzoyl analog 2b ere prepared from the corresponding methyl (Z)-2-(acylamino)-3-(dimethylamino)propenoates 1 Multifunctional compounds 2 are versatile synthons for preparation of polysubstituted heterocyclic systems such as pyrroles 4, pyrimidines 5 and 6, pyridazines 7, pyrazoles 8, 9, and 11, and isoxazoles 10.
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    Preparation of N-Fmoc-Protected β2- and β3-Amino Acids and their use as building blocks for the solid-phase synthesis of β-peptidesPartially published in a preliminary communication [1]; β2 means that the NH2, group ist at C(3)(= C(β)) and the side chain at C(2) of the amino acid, and β3 indicates that both the NH2 group and the side chain are at C(3)( = C(β)). Fmoc = (9H-Fluoren-9-ylmethoxy)carbonyl.Dedicated to Professor A. I. Meyers, University of Colorado, Fort Collins, on the occasion of his 65th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 187-206 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N-Fmoc-Protected (Fmoc = (9H-fluoren-9-ylmethoxy)carbonyl) β-amino acids are required for an efficient synthesis of β-oligopeptides on solid support. Enantiomerically pure Fmoc-β3-amino acids β3: side chain and NH2 at C(3)(= C(β)) were prepared from Fmoc-protected (S)- and (R)-α-amino acids with aliphatic, aromatic, and functionalized side chains, using the standard or an optimized Arndt-Eistert reaction sequence. Fmoc-β2- Amino acids (β2 side chain at C(2), NH2 at C(3)(= C(β))) configuration bearing the side chain of Ala, Val, Leu, and Phe were synthesized via the Evans' chiral auxiliary methodology. The target β3-heptapeptides 5-8, a β3- pentadecapeptide 9 and a β2-heptapeptide 10 were synthesized on a manual solid-phase synthesis apparatus using conventional solid-phase peptide synthesis procedures (Scheme 3). In the case of β3-peptides, two methods were used to anchor the first β-amino acid: esterification of the ortho-chlorotrityl chloride resin with the first Fmoc-β-amino acid 2 (Method I, Scheme 2) or acylation of the 4-(benzyloxy)benzyl alcohol resin (Wang resin) with the ketene intermediates from the Wolff rearrangement of amino-acid-derived diazo ketone 1 (Method II, Scheme 2). The former technique provided better results, as exemplified by the synthesis of the heptapeptides 5 and 6 (Table 2). The intermediate from the Wolff rearrangement of diazo ketones 1 was also used for sequential peptide-bond formation on solid support (synthesis of the tetrapeptides 11 and 12). The CD spectra of the β2- and β3-peptides 5, 9, and 10 show the typical pattern previously assigned to an (M) 31 helical secondary structure (Fig.). The most intense CD absorption was observed with the pentadecapeptide 9 (strong broad negative Cotton effect at ca. 213 nm); compared to the analogous heptapeptide 5, this corresponds to a 2.5 fold increase in the molar ellipticity per residue!
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    Regioselectivity of the Base-Induced Ring Cleavage of 1-Oxygenated Derivatives of Cyclobutabenzene (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 251-267 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oxy anions 3 generated from 1,2-dihydrocyclobutabenzen-1-ones 1 through addition of a charged nucleophile or from 1-hydroxy-1,2-dihydrocyclobutabenzenes 2 by deprotonation with base lead to stable products through distal and/or proximal cleavage of the strained four-membered ring via benzyl carbanion 4 and/or aryl carbanion 5. A systematic study of this process reveals the relative stability of the two isomeric carbanions 4 and 5 as a key factor in determining the course of the ring-cleavage reaction. While benzyl carbanions 4 can be trapped with carbon electrophiles, attempts at trapping aryl carbanions 5 with electrophiles other than H+ failed. In protic solvents, the magnesium salt of the tertiary alcohol 2 shows an increased rate of proximal cleavage as compared to its alkali salts. From this, we conclude that, in contrast to benzyl carbanions 4, free aryl carbanions 5 are of transient existence only. Proximal C,C-bond cleavage seems to occur either through protonation of 5 from a fast, reversible equilibrium 3⇌5 in which 3 strongly predominates, or in protic solvents possibly even through a rate-limiting protonation of 3 at the aromatic C-atom, bypassing free anion 5 altogether. Thus, additional factors other than just the relative stability of isomeric carbanions 4 and 5 are of importance in determining the regiochemistry of the base-induced C,C-bond cleavage in ketones 1 and in alcohols 2.
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    URL: http://dx.doi.org/10.1002/hlca.19980810207
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    Photoaddition of Ethyl 2,2-Dimethyl-5-oxo-5,6-dihydro-2H-pyridine-1-carboxylate to 2,3-dimethylbut-2-ene (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 303-306 
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    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On irradiation (350 nm) in the presence of excess 2,3-dimethylbut-2-ene, the newly synthesized title compound 5 affords as main products the unexpected cyclopropylpyrrolidine 10 (50%) and the spiro-oxetane 9 (25%).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810210
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  • 178
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    Sulfur-Containing Heterocycles Derived by Reaction of ω-Keto Amides with Lawesson's Reagent (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1207-1214 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of ω-keto amides with Lawesson's reagent (LR: 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide) is described. Treatment of 3-keto amides (2-acylacetamides) 1 with LR gave the corresponding 3-keto thioamides 2, along with 1,2-dithiole-3-thiones 3. Treatment of 4-keto amides, 3-acyl propionamides 5, with LR yielded five-membered heterocycles, pyrroles 6 and/or 2-aminothiophenes 7. 5-Keto amides, 3-benzoyl butyramides 8, reacted with LR to give dihydrothiopyran-2-thione 9 as the sole product, but in low yield. 2-Acylbenzamides 10 also reacted with LR to afford 3-mercaptoisoindolin-2-ones 11 or dihydroisobenzothiophene-1-thiones 12.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810531
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  • 179
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    Hydrogen Isotope Effect on the Isomerization of Peroxynitrous Acid (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1201-1206 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetics of isomerization of deuterium and hydrogen oxoperoxonitrate, ONOOD and ONOOH, respectively, to trioxonitrate(1-), NO3-, and a deuteron or proton have been studied. Analysis of the pH-rate profiles result in a normal equilibrium isotope effect (KH/KD) of 3.3. The kinetic deuterium isotope effect for the isomerization reaction is 1.6 ± 0.2 between 5 and 55°. The activation enthalpies of isomerization of ONOOD and ONOOH are identical within the error of the measurement, 86.2 ± 0.5 and 86.5 ± 0.5 kJmol-1, respectively. A secondary kinetic isotope effect of 1.6 is compatible with a mechanism of isomerization where the terminal peroxide O-atom shifts to the N-atom.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810530
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  • 180
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    Partial Synthesis and Characterization of Capsokarpoxanthins and 3,6-Epoxcapsanthins (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1242-1253 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The acid-catalyzed hydrolysis of (3S,5R,6S,3′S,5′R)-5,6-epoxycapsanthin (5) led to (3S,5R,6R,3′S,5′R)-(7) and (3S,5R,6S,3′S,5′R)-capsokarpoxanthin (8). In addition, (3S,5R,8R,3′S,5′R)- (9), (3S,5R,8S,3′S,5′R)-capsochrome (10), and (3S,5R,6R,3′S,5′R)-3,6-epoxycapsanthin (6) afforded (3S,5S,6R,3′S,5′R)-capsokarpoxanthin (12) (3S,5S,8S,3′S,5′R)- (13) and (3S,5S,8R,3′S,5′S)-capsochrome (14) as well as (3S,5S,6R,3′S,5′R)-3,6-epoxyepicapsanthin (15). Compounds 5-15 were isolated in crystalline form and characterized by their UV/VIS, CD, 1H- and 13C-NMR, and mass spectra.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810535
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  • 181
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    Isolation of Carotenoids with 3,5,6-Trihydroxy-5,6-dihydro-β-end Groups from Red Paprika (Capsicum annuum) (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1233-1241 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 6-Epikarpoxanthin ((all-E,3S,5R,6S,3′R)-5,-6-dihydro-β,β-carotene-3,5,6,3′-tetrol, 5), 5,6-diepikarpoxanthin ((all-E,3S,5S,6S,3′R)-5,6-β,β-carotene-3,5,6,3′-tetrol, 13), 5,6-diepilatoxanthin ((all-E,3S,5S,6S,3′S5′R,6′S)-5,6′-epoxy-5,6,5′,6′-tetrahydro-β,β-carotene-3,5,6,3′-tetrol, 14), and 5,6-diepicapsokarpoxanthin ((all-E,3S,5S,6S,3′S,5′R)-5,6-dihydro-3,5,6,3′-tetrahydroxy-β,κ-caroten-6′-one, 15) were isolated from red spice paprika (Capsicum annuum var. longum) and characterized by their UV/VIS, CD, 1H- and 13C-NMR, and mass spectra. Our investigations demonstrate that the configuration of the 3,5,6-trihydroxy-5,6-dihydro-β-end group may differ depending on the biological source.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810534
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  • 182
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    Synthesis and Spectroscopic Characterization of 4-Chlorophenyl Isocyanate (= 1-chloro-4-isocyanatobenzene) Adducts with Amino Acids as Potential Dosimeters for the Biomonitoring of Isocyanate Exposure (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1254-1263 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Blood-protein adducts are used as dosimeter for modifications of macromolecules in the target organs where the disease develops. The functional groups of cysteine, tyrosine, serine, lysine, tryptophan, histidine and N-terminal amino acids are potential reaction sites for isocyanates. Especially the N-terminal amino acids, valine and aspartic acid of hemoglobin and albumin, respectively, are reactive towards electrophilic xenobiotics. To develop methods for the quantification of such blood-protein adducts, we treated 4-chlorophenyl isocyanate (1) with the tripeptide L-valyl-glycyl-glycine (2a) and with single amino acids yielding N-[(4-chlorophenyl)carbamoyl]valyl-glycyl-glycine (3a), N-[(4-chlorophenyl)carbamoyl]valine (3b), N-[(4-chlorophenyl)carbamoyl]aspartic acid (3c), N-(4-chlorophenyl)carbamoyl glutamic acid (3d), N-acetyl-S-[(4-chlorophenyl)carbamoyl]cysteine (3e), and N-acetyl-O-[(4-chlorophenyl)carbamoyl]serine (3f), Nα-acetyl-Nε-[(4-chlorophenyl)carbamoyl]lysine (3g). For several chemicals, it was shown that blood-protein adducts are good dosimeters of exposure and dosimeters for the target dose. The hydrolysis of the N-terminal adducts of isocyanates release hydantoins which can be separated from the rest of the protein and analyzed using GC/MS or HPLC. This was achieved with 3a. The released hydantoin could be analyzed using GC/MS. We propose to analyze the N-terminal adducts of isocyanates with blood protein to distinguish between arenamine and arylisocyanate exposure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810536
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  • 183
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    Asymmetric syn-Dihydroxylation of β-Substituted (2R)-N-(α,β-Enoyl)bornane-10,2-sultamsDedicated to the memory of Professor Vladimir Prelog (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1264-1277 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Variously β-substituted (2R)-N-[(E)-α,β-enoyl]bornane-10,2-sultams were oxidized with OsO4/4-methylmorpholine 4-oxide in a highly stereoselective manner. In all cases, the attack occurred on the C(α)-re face. The absolute configurations of all products were determined by chemical correlation. Mechanistic considerations about the reactive conformation as well as fully refined X-ray crystal structures of the dihydroxylated products are discussed. New neutral conditions for sultam acylation, applicable to the preparation of the taxol and Taxotere® C(13) side chain as well as of Cardizem® and chloramphenicol precursors, are also presented.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810537
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  • 184
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    Makrocyclische [4n + 2]-Hückel-Aromaten bis n = 9 und [4n]-Antiaromaten bis n = 10: Homologe Reihen vom Tetraepoxy[24]-annulen(2.0.2.0) zum Tetraepoxy[40]annulen(12.0.12.0) und vom ‘Tetraoxa-[22]porphyrin(2.0.2.0)’-zum ‘Tetraepoxa[38]porphyrin(12.0.12.0)’-DikationZiffern in Klammern nach einem Namen geben die Zahl der Atome zwischen den Furan-bzw. Pyrrol-Ringen an.Bei ‘Porphyrin’-Namen bezeichnet die Zahl in eckigen Klammern die Anzahl konjugierter π-Elektronen des Makrocyclus, der die Struktureinheiten =O+ - bzw. =N - beinhaltet. (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1480-1505 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Macrocyclic [4n + 2]-Hückel Aromatic Systems up to n = 9 and [4n] Antiaromatic Systems up to n = 10: Homologous Sequences from Tetraepoxy[24]annulene(2.0.2.0) to Tetraepoxy[40]annulene(12.0.12.0) and from the ‘Tetraoxa[22]porphyrin(2.0.2.0)-’ to the ‘Tetraoxa[38]porphyrin(12.0.12.0’-Dication1)2)Tetraepoxy[32]annulene(8.0.8.0) 3, tetraepoxy[36]annulene(10.0.10.0) 4, and tetraepoxy[40]annulene(12.0.12.0) 5 are synthesized and oxidized to give the ‘tetraoxa[30]-’, ‘tetraoxa[34]-’, and ‘tetraoxa[38]porphyrin’ dications 8-10. The annulenes as well as the ‘porphyrins’ are mixtures of at least two different configurational isomers, which can be analyzed by 1H-NMR techniques. Together with systems described already earlier, a complete homologous sequence from tetraepoxy[24]annulene(2.0.2.0) to tetraepoxy[40]annulene(12.1.12.0) and from ‘tetraoxa[22]porphyrin(2.0.2.0)’ to ‘tetraoxa[38]porphyrin(12.0.12.0)’ dications are available for the first time. The UV/VIS-absorptionmaxima in the annulene series are shifted to longer wavelengths with an increment of 12 nm per additional C=C bonds, and the Δδ values of the 1H-NMR spectra demonstrate a decreasing paratropic character (Δδ = 5.97 (1), 2.80 (3), and 4.23 ppm (4). The averaged λmax values of the Soret bands of the ‘tetraoxaporphyrin’ dications show a linear bathochromic shift with an increment Δδ of 58 nm per two additional double bonds, the absorptions of the most intensive Q-bands also increase linearly with an increment of ca. 160 nm. The Δδ values of the 1H-NMR spectra of the ‘tetraoxaporphyrin’ dications are increasing with the ring size (Δδ = 24.04 (7a) to 25.17 (9a) ppm), respectively, decreasing (Δδ = 21.55 (6) to 21.50 (9b) ppm) with a small maximum of 22.60 ppm for 7b, depending on the configuration of the isomers. The results confirm the antiaromatic character of the annulenes with up to 40π electrons and the aromatic character of the ‘tetraoxaporphyrin’ dications with up to 38π electrons.
    Additional Material: 18 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810553
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  • 185
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    A Novel Solution- and Solid-Phase Approach to 2,4,5-tri- and 2,4,5,6-tetrasubstituted pyrimidines and their conversion into condensed heterocycles (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 646-660 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel general synthesis of 2,4,5-tri- and 2,4,5,6-tetrasubstituted pyrimidines 5a-d and 7a, e, f, g by condensation of thiouronium salts of type 3 with (ethoxymethylidene)malononitrile (4) and [bis(methylthio)methylidene]malononitrile (6), respectively, was first established in solution (Scheme 1) and successfully transferred onto solid support by using the polymer-bound thiouronium salt 11 (Scheme 3). Further investigations were directed toward a multidirectional cleavage procedure of the 2-(alkylsulfinyl) intermediates, obtained from the 2-(alkylthio)pyrimidines 7a (Scheme 2) or 12 and 14 (Schemes 3 and 4), with different nucleophiles to form highly substituted pyrimidines. In addition, fused-heterocycle derivatives 22a--h, 24a-c, and 26a-e were generated in good-to-excellent yields by condensation of 7a, e, h with versatile isocyanates and isothiocyanates, with subsequent alkylation (Scheme 5).
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810315
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  • 186
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    Masthead (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810101
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  • 187
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    Supramolecular Chirality: Chiral hydrogen-bonded supermolecules from achiral molecular components (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1-13 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chiral supermolecules may be obtained from suitable achiral molecular constituents associated through a dissymmetrizing interaction mode. This is the case for the supermolecules I-IV formed by hydrogen-bonding association between the achiral complementary components 1a,b and 2a,b,c. The crystal structures of the supermolecular pairs I-III and of the homochiral aggregate of two ternary supermolecules IV have been determined. The structural data are discussed.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810102
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  • 188
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    Enantioselective Alternating Terpolymerization of Styrene and Ethene with Carbon Monoxide. Preliminary communication (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 35-39 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In spite of the higher reactivity of styrene with respect to ethene for the alternating copolymerization with carbon monoxide, catalyzed by chiral (dihydrooxazole)(phosphino)palladium complexes, ethene is preferentially (and randomly) enchained in terpolymerization experiments; enantioface selection for styrene is comparably high in both copolymerization and terpolymerization processes.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810104
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  • 189
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    Synthesis and Odor of Chiral Partial Structures of Khusimone. Part 3Part 2: [1].. Short communication (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 40-45 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Khusimone (1), one of the main odor-donating compounds of vetiver oil, is subject of the following study on structure/odor relationship. The omittance of the methano bridge of the tricyclic khusimone should lead to the bicyclic partial structure (-)-2. Unexpectedly, (-)-2 could not be obtained since epimerization favored (-)-16. The stereochemical key step of the synthesis of the hydrazulene nucleus is based on a highly diastereoselective conjugate addition to a chiral oxocyclopent-1-ene-1-carboxylate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810105
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  • 190
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    Synthesis and Some Transformations of 2-Acetamido-5-amino-3,4,6-tri-O-benzyl-2,5-dideoxy-D-glucono-1,5-lactam (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 865-880 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The lactam 21 was obtained in an overall yield of 72% from the hydroxy amide 16 by oxidation with the Dess-Martin periodinane, acid-catalysed isomerization of the oxidation products in toluene, whereupon 18/19 precipitated, and reductive dehydroxylation of 18/19 (Et3SiH/BF3 · OEt2; Scheme 1). The amide 16 was obtained by ammonolysis of the N-acetylglucosamine-derived lactone 15. Depending on the oxidation method, 16 yielded the keto amide 17, the hydroxy lactams 18/19, and the pyrrolidinecarboxamide 20 in widely different proportions. The pyrrolidinecarboxamide 20 was not reduced under the conditions of the reductive dehydroxylation. Hydrogenolysis of the benzyl-protected lactam 21 gave the trihydroxy lactam 22, while reduction with NaBH4/BF3 · OEt2 led to the 2-acetamidopiperidine derivative 24 (Scheme 2). Selective (tert-butoxy)carbonylation of the lactam 21 (→ 25) followed by NaBH4 reduction and acid-catalysed solvolysis in EtOH led to the α-ethoxycarbamates 28/29. Similarly, (tert-butoxy) carbonylation of 1 (→ 31) followed by reduction to 32/33 and glycosidation yielded the ethoxycarbamate 34. Treatment of the GlcNAc-derived ethyl glycosides 28/29 with Me3SiCN/BF3 · OEt2 gave the equatorial amino nitrile 30. Under similar conditions, the Glc-derived glycoside 34 led to the iminooxazolidinone 35. In the presence of a larger proportion of Me3SiCN at 5°, 34 was transformed into the axial, selectively monodebenzylated amino nitrile 36.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810508
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  • 191
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    Aldehyde-Promoted Addition of 2-(Trimethylsilyl)thiazole to α,α′-Dialkoxy Ketones: A new way to branched-chain monosaccharides (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 889-901 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of 2-(trimethylsilyl)thiazole (2-TST) with several ketones was tested in the presence or absence of aldehydes. The keto aldehyde 5 (Scheme 2) was prepared from 1 via the hydroxy aldehyde 4 in 3 steps. It reacted with 2-TST to give, after desilylation and acetylation, the bis-thiazole 6. The ketone 11, obtained from 4 in 3 steps, reacted with 2-TST to give, after desilylation, 12. The ketofuranose 17 (Scheme 3) reacted with 2-TST to yield exclusively the more stable D-gluco epimer 18. The reaction of the ketone 11 (Scheme 2) with 2-TST was faster in the presence of 1 equiv. of the keto aldehyde 5, suggesting that an aldehyde promotes the indirect and intermolecular addition of 2-TST to a ketone. We have studied the effect of several aldehydes on the rate of the reaction of the ketones 11 and 17 with 2-TST at different temperatures and at different concentrations of the ketones and of the aldehydes. Electrophilic aldehydes, and particularly 2-fluorobenzaldehyde (0.1 equiv.), promote the addition of 2-TST to electrophilic ketones.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810510
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  • 192
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    Cooperative Interactions of the Catalytic Nucleophile and the Catalytic Acid in the Inhibition of β-Glycosidases. Calculations and their validation by comparative kinetic and structural studies of the inhibition of glycogen phosphorylase b (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 853-864 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The difference between the strong inhibition of retaining β-glucosidases by the tetrazole 1 and the weak inhibition by the triazole 3 has been explained by the protonation by the enzymic catalytic acid of N(3) of 1, replaced by CH in 3. One also expects a contribution to the inhibition from the charge-dipole interaction between the enzymic catalytic nucleophile and the azole ring. The extent of this contribution was estimated from the calculated, distance-dependent heats of formation of the acetate-azole complexes. The calculations were validated by comparison of the charge-dipole interaction between phosphate and the inhibitors 1 and 3 in the glycogen phosphorylase b (GPb)-azole-phosphate complexes, as derived from differences in the Ki values for 1 and 3, while the structural invariance of the complexes was demonstrated by X-ray analysis. The difference between the charge-dipole interactions of (dihydrogen) phosphate and 1 or 3 as derived from Δ Ki is 1.1 kcal mol-1, while the calculated difference is 1.3 kcal mol-1. The calculated difference for the interaction of 1 or 3 with acetate, representing the catalytic nucleophile in β-glycosidases, is 2.0 kcal mol-1, while the differences of the binding energies as derived from the Ki values for the inhibition by 1 or 3 of different β-glycosidases range from 2.4 to 5.3 5 kcal mol-1. The calculated difference for 1 and the imidazole 6 is 2.5 kcal mol-1 in favour of 1, whereas the Ki-derived difference is 3.7 kcal mol-1 in favour of 6, equal to the calculated difference between 1 and the protonated imidazole 6. Thus, protonation by the catalytic acid and the charge-dipole interaction with the catalytic nucleophile contribute cooperatively to the binding of inhibitors possessing a trigonal anomeric centre bonded to a heteroatom.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810507
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  • 193
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    NMR Verification of Helical Conformations of Phycocyanobilin in Organic SolventsDedicated to Prof. Günther Schenck on the occasion of his 85th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 881-888 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Selective NMR decoupling and nuclear Overhauser effect (NOE) experiments with phycocyanobilin (PCB) show proton-proton interactions between the terminal rings A and D, viz. the chiral C(2) methine center and the ethyl substituent at C(18), as a result of the helical conformation of this open-chain tetrapyrrole in solution. Quantitative NOE measurements and a combination of force-field and semiempirical calculations (FSC) afford inter-proton distances across the helical gap of 4.2-4.6 (NOE) and 3.2-4.2 A° (FSC). The NOE and FSC, in conjuction with a qualitative evaluation of the steric interactions in the two optimized helices, suggest furthermore that, in solution, the helix M is somewhat more stable than P. The coexistence of at least two diastereoisomers is corroborated also by the circular dichroism (CD) spectra of PCB in MeOH/EtOH which point to a temperature-dependent equilibrium in solution, and by a considerable increase of this CD upon changing the solvent from the achiral alcohols to ethyl (-)-(S)-lactate which reflects a selective solvent-induced CD differentiating between diastereoisomers.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810509
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  • 194
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    Heterosupramolecular Chemistry: Synthetic strategies for the covalent and noncovalent assembly and organization of nanocrystals and molecules (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 902-915 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Described are the preparation of nanocrystals and the synthesis of molecules that may be covalently or noncovalently assembled in solution to yield heterosupermolecules possessing a well-defined heterosupramolecular function. Also described are preparative and synthetic methods that yield organized assemblies of heterosupermolecules possessing an addressable heterosupramolecular function. Finally, the development of these synthetic strategies to permit the covalent and noncovalent assembly and organization of a wide range of condensed phase and molecular components is outlined.
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    URL: http://dx.doi.org/10.1002/hlca.19980810511
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    Distamycin-NA: A DNA analog with an aromatic heterocyclic polyamide backbone. Part 2. Solid-phase synthesis of distamycin-NAs containing the nucleobase uracil: Unexpected solvent participation in the coupling step (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 916-931 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the Fmoc-protected amino acid 2 is presented. First attempts of amide-bond formation to the homodimer 4 in solution showed only poor coupling yields indicative for the low reactivity of the amino and carboxy groups in the building blocks 1 and 2, respectively (Scheme 1). Best coupling yields were found using dicyclohexylcarbodiimide (DCC) without any additive. The oligomerization of building block 2 adopting the Fmoc ((9H-fluoren-9-ylmethoxy)carbonyl) solid-phase synthesis yielded a mixture of N-terminal-modified distamycin-NA derivatives. By combined HPLC and MALDI-TOF-MS analysis, the N-terminal functional groups could be identified as acetamide and N,N-dimethylformamidine functions, arising from coupling of the N-terminus of the growing chain with residual AcOH or DCC-activated solvent DMF. An improved preparation of building block 2 and coupling protocol led to the prevention of the N-terminal acetylation. However, ‘amidination’ could not be circumvented. A thus isolated tetramer of 2, containing a lysine unit at the C-terminus and a N,N-dimethylformamidine-modified N-terminus, not unexpectedly, showed no complementary base pairing to DNA and RNA, as determined by standard UV-melting-curve analysis.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810512
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    γ-Peptides Forming More Stable Secondary Structures than α-Peptides: Synthesis and helical NMR-solution structure of the γ-hexapeptide analog of H-(Val-Ala-Leu)2-OHDedicated to Professor E. J. Corey on the occasion of his 70th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 983-1002 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For a comparison with the corresponding α- and β-hexapeptides H-(Val-Ala-Leu)2-OH (A) and H-(β-HVal-β-HAla-β-HLeu)2-OH (B), we have now prepared the corresponding γ-hexapeptide 1 built from the homochirally similar (S)-4-aminobutanoic acid, (R)-4-amino-5-methylhexanoic acid, and (R)-4-amino-6-methylheptanoic acid. The precursors were prepared either by double Arndt-Eistert homologation of the protected amino acids Boc-Val-OH, Boc-Ala-OH, and Boc-Leu-OH (Schemes 1 and 2), or by the superior route involving olefination/hydrogenation of the corresponding aldehydes (Boc-valinal, Boc-alaninal, and Boc-leucinal; Scheme 3). Conventional peptide-coupling methodology (EDC/HOBt) furnished the γ-hexapeptide 1 (through the intermediate γ-di- and γ-tripeptide derivatives 9-11). Analysis of NMR measurements in (D5)pyridine and CD3OH solution (COSY, TOCSY, HSQC, HMBC, ROESY) reveals that the γ-hexapeptide 1 adopts a right-handed helical structure ((P)-2.61 helix of ca. 5-Å pitch, containing 14-membered H-bonded rings) which is to be compared with the left-handed helix of the corresponding β-peptide B ((M)-31 helix of 5-Å pitch, 14-membered H-bonded rings) and with the familiar right-handed, so-called α-helix of α-peptides ((P)-3.61 helix of 5.4-Å pitch, 13-membered rings). Like the helix sense, the helix dipole reverses when going from α-, (N + → C) to β-(C + → N) to γ-peptides (N + → C). The surprising difference between the natural α-, and the analogous β- and γ-peptides is that the helix stability increases upon homologation of the residues.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810514
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    β2- and β3-Peptides with Proteinaceous Side Chains: Synthesis and solution structures of constitutional isomers, a novel helical secondary structure and the influence of solvation and hydrophobic interactions on foldingPartially mentioned in preliminary communications [1][2] and in a review article [3]. (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 932-982 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Enantiomerically pure β-amino-acid derivatives with the side chains of Ala, Val, and Leu in the 2- or 3-position (β2- and β3-amino acids, resp.), as well as with substituents in both the 2- and 3-positions (β2,3-amino acids, of like-configuration) have been prepared (compounds 8-17) and incorporated (by stepwise synthesis and fragment coupling, intermediates 24-34) into β-hexa-, β-hepta-, and β-dodecapeptides (1-17). The new and some of the previously prepared β-peptides (35-39) showed NH/ND exchange rates (in MeOH at room temperature) with τ1/2 values of up to 60 days, unrivalled by short chain α-peptides. All β-peptides 1-7 were designed to be able to attain the previously described 31-helical structure (Figs. 1 and 2). CD Measurements (Fig. 4), indicating a new secondary structure of certain β-peptides constructed of β2- and β3-amino acids, were confirmed by detailed NMR solution-structure analyses: a β2-heptapeptide (2c) and a β2,3-hexapeptide (7c) have the 31-helical structure (Figs. 6 and 7), while to a β2/β3-hexapeptide (4) with alternating substitution pattern H-(β2-Xaa-β3-Xaa)3-OH a novel, unusual helical structure (in (D5)pyridine, Fig. 8; and in CD3OH, Figs. 9 and 10) was assigned, with a central ten-membered and two terminal twelve-membered H-bonded rings, and with C=O and N—H bonds pointing alternatively up and down along the axis of the helix (Fig. 11). Thus, for the first time, two types of β-peptide turns have been identified in solution. Hydrophobic interactions of and hindrance to solvent accessibility by the aliphatic side chains are discussed as possible factors influencing the relative stability of the two types of helices.
    Additional Material: 14 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810513
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    Second-Generation Trifluoromethyl-Substituted Chiral Dendrimers Containing Triply Branched Building Blocks: CF3 as sensitive NMR probe for ‘remote’ diastereotopicityPartially published in a preliminary communication [1]. (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1003-1022 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (S)-4,4,4-Trifluoro-3-hydroxybutanoic acid was used as a starting material for the synthesis of dendritic branches (16-21, 30-32) which were attached to a chiral triol (6 or ent-6), derived from (R)-3-hydroxybutanoic acid, to give CF3-substituted dendrimers (33-36, 38-40) and dendritic compounds (37) of 1st and 2nd generation. The key steps in these syntheses are diastereoselective aldol additions of dioxanone enolates (building blocks and intermediates 6-15, 22, 23, and 25-28) and Williamson etherifications of benzylic-branch bromides with triols (intermediates and products 16-21, 28, and 30-40). The surfaces of the dendrimers are covered with MeO (33-35, 38-40) or allyloxy groups (36 and 37). The new dendrimers are characterized by NMR and mass spectroscopy. 19F-NMR Signals of the CF3 groups reveal constitutional heterotopicities caused by substituents which are separated from the F-nuclei by up to 15 bonds (through a 1,1′-biphenyl-4,4′-diyl spacer!) (Fig. 6).
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810515
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  • 199
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    Origin of the Stereoselectivity in (Ethoxycarbonyl)-, Cyano-, and Phenyl-Substituted (Arylsulfinyl)methyl Radicals (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1048-1063 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An explanation for the very high diastereoselectivity observed for the reactions of carbonyl-substituted (arylsulfinyl)methyl radicals is presented, based on experimental results and semiempirical calculations. The influence of dipole-dipole interactions, allylic 1,3-strain (A1,3 strain), allylic 1,2-strain (A1,2 strain), and coulombic interactions is discussed based on stereoselectivities observed with (alkoxycarbonyl)-, cyano-, and aryl-substituted (arylsulfinyl)methyl radicals. In the second part, the effect of solvents and Lewis acids on the stereoselectivity of reactions of (arylsulfinyl)- and (alkylsulfinyl)benzyl radicals has been examined.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19980810518
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  • 200
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    Antitumor Agents. Part 184.Part 183: [1]. Syntheses and antitubulin activity of compounds derived from reaction of thiocolchicone with amines: Lactams, alcohols, and ester analogs of allothiocolchicinoidsDedicated to Prof. Dieter Seebach on the occasion of his 60th birthday (1998)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 81 (1998), S. 1023-1037 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 7-O-Substituted analogs of deaminodeoxycolchinol thiomethyl ether were synthesized and evaluated for their inhibitory effects on tubulin polymerization in vitro. Ketone 9, a key compound in this study, was derived from thiocolchicone 6 by reaction with aniline. Reaction of compound 6 with MeNH2 or BuNH2 gave tetracyclic lactams 7 and 8, respectively. Optically active alcohols 11a and 11b were obtained from racemic 11 by chemical resolution including a separation of the comphanate diastereoisomers 12a and 12b, followed by basic hydrolysis. The (aR,7R)-configuration of 12b was verified by X-ray crystallographic analysis. Almost all racemic and optically active 7-O-acyl or 7-O-aroyl compounds had strong inhibitory effects on the tubulin polymerization reaction, with IC50 values from 1.7 to 5.1 μM. A few agents, such as the lactams 7 and 8, the camphanates 12a and 12b, the cyclohexanecarboxylates 19a and 19b, and, most notably, the (7S)-benzoate 15a, had negligible effects on polymerization, yielding IC50 values greater than 40μM. Ketone 9 showed strong inhibition of tubulin polymerization comparable to that of thiocolchicone (6). Optically active alcohol 11a and acyl esters 13a and 14a with a (7S)-configuration were more active than the (7R)-esters 13b and 14b. However, the esters 15a-17a with a (7S)-configuration were less active than the (7R)-isomers 15b-17b, in which the (7R)-benzoate 15b was at least 15-fold more inhibitory than the (7S)-isomer 15a. For the most part, the agents causing strongest inhibition of polymerization also caused the greatest inhibition of [3H]colchicine binding. NMR and optical rotatory data indicate that, in polar solvents, the equilibrium in esters with a 7-O-aroyl substituent, i.e., 15a,b, 16a,b, and 17a,b, is reversed from (aS) to (aR) or from (aR) to (aS), as compared to nonpolar solvents.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19980810516
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