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  • Physical Chemistry  (3,728)
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  • 1
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 1 (1988) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 2
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 1 (1988), S. 123-131 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photolysis of N-alkyl-N-(3-aryl-3-butenyl) ureas (1) in acetonitrile gave cyclization products, 3-aryl-3-methyl-pyrrolidines, in good yields, whereas irradiation of 1 in methanol afforded methanol adducts as well as the cyclization products. Both the reactions are singlet reactions, and the cyclization is presumed to proceed via 1,6-hydrogen transfer from exciplexes with charge transfer character.
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  • 3
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    Journal of Physical Organic Chemistry 1 (1988), S. 143-151 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The dependence of the catalytic efficiencies of sodium carboxymethylamylose (NaCMA) for the hydrolysis of N-lauryl-3-acetoxypyridinium iodide (1) and p-nitrophenyl dodecanoate (3) on its degree of substitution (D. S. = 0·00, 0·12, 0·18, 0·24, 0·29 and 0·35) and on the pH values of the solutions (pH = 7·32, 7·80, 8·10 and 9·30) have been studied. At fixed D. S. values, the observed hydrolysis rates of 1 and 3 increase with increasing concentrations of NaCMA and follow saturation kinetics. At fixed concentration of NaCMA, the rates increase with decreasing D. S. values until they reach maxima at D. S. = 0·00. Furthermore, at any D. S. value the catalytic efficiency increases with increasing pH values of the solutions. All these results indicate that the hydroxyl groups are actually the principal catalyzing groups.
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  • 4
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 1 (1988) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 5
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 1 (1988), S. 191-195 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The photochemical reaction of MAQO with various aromatic amines were studied by ESR. The results show that nitroxide radicals are stable productrs of the photooxidation of both diphenylamines and phenylamines. The photolyzed phenothiazine does not yield nitroxide as the final product, instead it gives the neutral radical as the stable final product.
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  • 6
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    Journal of Physical Organic Chemistry 1 (1988), S. 209-223 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The equilibrium acidities of phenylacetonitrile, and 20 of its m- and p-substituted derivatives have been measured in Me2SO solution. Their pKa′s plot linearly with those of the corresponding anilines. Combination of the pKa′s of these acids with their oxidation potentials, Eox(HA), and those of their conjugate bases, Eox(A-), provide an estimate of the acidities of the corresponding radical cations. The pKHA+ values for ArCH2CN+., where Ar is Ph, 1- and 2-naphthyl, and 9-anthryl, are -32, -18·5, -17·5, and -11, respectively, compared to 21·9, 20·85, 20·65, and 19·8 for the corresponding ArCH2CN acids. Acidities of PhCH(Me)CN+., Ph2CHCN+., 9-CN-FlH+., and 9-CN-XnH+. are -33, -35, -25, and -27, respectively, compared to 23·0, 17·5, 8·3, and 13·6 for the corresponding acids from which they were derived. The homolytic bond dissociation energies (BDEs) for the benzylic C—H bonds in these arylacetonitriles, estimated by combining pKHA with Eox(A-), fall in the range of 69 kcal/mol for 9-CN-XnH to 82 kcal/mol for PhCH2CN. For GC6H4CH2CN+. radical cations the acidities are decreased, relative to G=H, when G is an electron donor substituent and increased when G is an acceptor. The BDEs of the benzylic C—H bonds in GC6H4CH2CN are weakened by up to 4 kcal/mol by para donors and strengthened by up to 1·2 kcal/mol by m- or p-acceptors. The significance of these changes in BDEs with regard to the use of σ. scales and the ΔAOP method for estimating substituent effects on radical stabilities is discussed.
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  • 7
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    Chichester : Wiley-Blackwell
    Journal of Physical Organic Chemistry 1 (1988), S. 281-285 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The thermal and photochemical decompositions of 5 have been studied. Both reactions lead to CBr2 transfer in good to high yields. With the 2-pentenes as substrates, CBr2 transfer is stereospecific in the classical singlet carbene manner.
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  • 8
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    Journal of Physical Organic Chemistry 1 (1988), S. 305-308 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Decomposition of the tosylhydrazone salts formed from cubyl carboxaldehyde and homocubanone, potential precursors of cubylcarbene (5) and homocubanylidene (6), leads in each case to products derived from 6. It is suggested that homocub-1(9)-ene (7) is the active ingredient in the formation of 6 from both precursors. The hydrazones formed from N-aziridylamines are useful photochemical sources of 6, and presumably other carbenes. Flash vacuum pyrolysis of the tosyl hydrazone salts and hydrazones at 600°C leads to indene.
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  • 9
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    Journal of Physical Organic Chemistry 1 (1988), S. 317-332 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Light- and heat-induced decompositions and reactions with nucleophiles of 6,6,8,8-tetramethyl-2-selena-3,4-diaza-7-oxabicyclo[3.3.0]octa- 1(5),3-diene have been studied. In contrast with the conversion to the cyclopentyne derivative (9), selenium containing intermediates (6a, 6b, and 7) were efficiently trapped using several reagents to give various kinds of organoselenium compounds. Of particular note is the isolation of a stable selenirane derivative (16) obtained by the cycloaddition of the photochemically generated selenirene intermediate (7) with furan. The character and reactivity of the intermediates are also discussed.
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  • 10
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    Journal of Physical Organic Chemistry 1 (1988), S. 359-362 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reaction rates for solvolysis of a mustard analogue are accurately correlated by the solvatochromic equation without inclusion of a nucleophilicity term, and thus the equation is shown to reveal mechanistically significant information.
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  • 11
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    Journal of Physical Organic Chemistry 2 (1989), S. 26-34 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A set of 4-monosubstituted cyclopentenes, , were synthesized and their relative rates (kX/kH) for bromination and chlorination were determined in methanol, ethanol and acetic acid at 25 °C by competitive method. log(kX/kH) for most of the substituents can be correlated by means of Taft's equation, log(kX/kH) = ρI σI + C. In methanol ρI, Br2 = -2·91, ρI, Cl2 = -0·49, in ethanol ρI, Br2 = -3·07, ρI, Cl2 = -0·70 and in acetic acid ρI, Br2 = -1·64, ρI, Cl2 = -0·65. The presence of C(〈0) is due to a constant steric effect. The deviation of X = H is ascribed to the absence of the steric effect and that of X = CO2Me and CO2Et is accounted for in terms of anchimeric assistance. For chlorination no anchimeric assistance was observed.
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  • 12
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    Journal of Physical Organic Chemistry 2 (1989), S. 43-50 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The cyclic vinyl ether dihydro-1,4-dioxin is converted to its cyclic hemiacetal hydration product, 2-hydroxy-1,4-dioxane, in aqueous solution by an acid-catalyzed reaction for which kH+ = 1·80 × 10-5 M-1 S-1 at 25°C. This reactivity and the solvent isotope effect kH+/kD+ = 2·2 show that the reaction occurs by rate-determining proton transfer from catalyst to substrate and not by a pre-equilibrium mechanism as recently proposed.2
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  • 13
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    Journal of Physical Organic Chemistry 2 (1989), S. 57-88 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The cation radical vinylcyclobutane (VCB) rearrangement is found to be a reaction of substantial scope, synthetic utility, and exceptional kinetic facility. In conjuction with cation radical cyclobutanation, it constitutes an effective method for net (indirect) Diels-Alder addition to electron rich dienophiles. Reactions can be carried out with either aminium salt or photosensitized electron transfer (PET) initiation and are powerfully facilitated by ionizable substituents such as p-anisyl, phenylthio, and phenoxy at the 2-position of the vinylcyclobutane. The intramolecularity of the reaction is clearly established and in four discrete systems preferred sr (suprafacial/retention) stereochemistry is observed. A theoretical basis for sr stereochemistry in the cation radical VCB rearrangement is advanced. The transition state for the reaction is considered to be similar to that for the direct cation radical Diels-Alder cycloaddition, another cation radical pericyclic reaction which converges on the same product. This model of the VCB rearrangement transition state is used to rationalize the strong rate-retarding effect of a Z-methyl substituent attached to the vinyl group and of a methyl substituent at the 4-position of the vinylcyclobutane ring cis to the vinyl substituent.
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  • 14
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    Journal of Physical Organic Chemistry 2 (1989), S. 110-116 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The transamination reaction of α-amino acids with glyoxylic acid as catalyzed by copper(II) ions was investigated kinetically in an aqueous medium at pH 5·0 and 30·0°C. L-Phenylalanine transferred its amino group to glyoxylic acid most readily among seven different amino acids used here in the single-walled bilayer vesicle formed with N,N-dihexadecyl-Nα-[6-(trimethylammonio)hexanoyl]-L-histidinamide bromide (N+C5His2C16). Such rate enhancement was found to originate from the cooperative trifunctional catalysis: a coordination effect exercised by copper(II) ions, a general acid-base catalysis by the imidazolyl group of the lipid, and a hydrophobic field effect provided by the bilayer vesicle. Lack of any of the three functions failed to give out significant rate enhancement. As regards correlation between the reactivity and the nature of α-amino acids, the copper(II)-catalyzed transamination was progressively enhanced as hydrophobicity of the α-amino acid was increased in the N+C5His2C16 vesicle.
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  • 15
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    Journal of Physical Organic Chemistry 2 (1989) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 16
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    Journal of Physical Organic Chemistry 1 (1988), S. 119-121 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Reduction of aromatic aldehydes by benzeneselenol is found to be promoted by molecular oxygen to give corresponding alcohols in good yields. No reduction took place without oxygen.A free radical process involving SH2 reaction at the selenium atom is proposed where the phenylseleno radical attacks the selenium atom of selenohemiacetal, the adduct of benzeneselenol to aldehyde, to give the ketyl radical which then abstracts hydrogen from benzeneselenol. The intermediacy of selenohemiacetal is supported by a quantitative reduction of α-methoxybenzyl phenyl selenide, which is used as a model compound of the intermediate.
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  • 17
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The dependency of the wrapping-up capability of sodium carboxymethylamylose (NaCMA) on its degree of substitution (D. S. = 0·00, 0·12, 0·18, 0·24, 0·29, 0·35 and 0·41) have been studied, using two guest species, iodine and cetyl-trimethylammonium bromide (CTAB). The λmax values of NaCMA-iodine helical inclusion complexes decrease with increasing D. S. values and the amounts of encapsulated iodine by NaCMA as measured by amperometric titrations also decrease with increasing D. S. values. With CTAB as the substrate, the largest number of binding sites, n, and the dissociation constants Kd have been determined by the method of surface tension versus the CTAB concentration plots. The results show that n decreases while Kd increases with increasing D. S. values. All these observations point to the fact that the wrapping-up capability of NaCMA decreases with increasing degrees of substitution. The results are discussed in terms of host-guest and host-solvent hydrophobic-lipophilic interactions as well as intramolecular hydrogen-bonding.
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  • 18
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    Journal of Physical Organic Chemistry 1 (1988), S. 161-167 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The temperature-dependence of the kinetic isotope effects for branched reactions proceeding via a common intermediate has been simulated by calculations. It is shown that, under certain conditions, anomalously small isotope effects on the Arrhenius preexponential factors, as well as unusually large observed isotope effects, may originate from the branching.
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  • 19
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    Journal of Physical Organic Chemistry 1 (1988), S. 153-160 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The structures of the allyl anion (1) and two fluoro-substituted derivatives, perfluoro (2) and 1,1-difluoro (3) have been calculated by ab inito molecular orbital theory. Geometries were gradient optimized, and force fields and an MP-2 correlation correction were determined at stationary points. The calculations were done with a double zeta basis set augmented by d functions on carbon (DZ + Dc). Final self-consistent field (SCF) and MP-2 energy calculations were done with the DZ + Dc basis set augmented by diffuse functions. The ground state of 1 is the planar allyl anion (C2v). The rotation barrier in 1 is 21·1 kcal/mol at the MP-2 level. The cyclopropyl carbanion is 27·0 kcal/mol higher in energy at the MP-2 level. The perfluoroallyl anion is not planar, and the rotated structure is 25·7 kcal/mol more stable than the C2v structure at the MP-2 level. The lowest energy structure on the potential energy surface for 2 is the perfluorocyclopropyl carbanion which is 27·8 kcal/mol more stable than the C2v structure at the MP-2 level. The 1,1-difluoroallyl anion is also not planar. Here the most stable structure on the potential energy surface is the rotated allyl anion which is 8·3 kcal/mol more stable than the all-planar allyl anion structure.
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  • 20
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 2,2′-Diselenocyanato-1,1′-binaphthyl(1), the first selenium-containing binaphthyl derivative, has been synthesized in optically active form the corresponding diamine by diazotization followed by the reaction with potassium selenocyanate. Its molecular structure is determined by X-ray diffraction method. It is revealed that selenium atoms have hypervalent penta-coordination in the crystal.
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  • 21
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    Journal of Physical Organic Chemistry 1 (1988), S. 185-190 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: From the 1H-NMR spectra of the various pyrrole compounds, 1-substituted 2,5-dimethylpyrroles having two substituent groups on the carbon adjacent to the nitrogen atom were found to be very much hindered compounds. 1-[(1-Substituted)-2-phenylethyl]-2,5-dimethylpyrroles showed the restricted rotation, and the phenyl group of the most stable conformer was in close proximity to the pyrrole group due to dipole interaction. These NMR observations were supported by force field conformational analysis.
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  • 22
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    Journal of Physical Organic Chemistry 1 (1988), S. 333-349 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: New series of platinum complexes of cyclopropenylidenes of the types of PtX2(CP)2 and trans-PtX(PBu3)2(CP) have been synthesized, where CP is di-t-butylcyclopropenylidene (BCP) or bis(diisopropylamino)cyclopropenylidene (ACP). The 13C-NMR chemical shifts, and 13C-195Pt coupling constants (1JPtC) for the complexes are discussed in comparison with those values derived from closely related series of compounds, trans-PtCl(PR3)2L; L — —CH3, —C6H5 and —C≡CBu-t. An excellent linear relationship through the origin was obtained between 1JPtC and the formal ‘s’ % character of the carbon directly bonded to Pt for the series trans-PtCl(PR3)2L in which the Pt—C bond is regarded as a pure σ-linkage, whereas 1JPtC deviates largely from this relationship when pπ—dπ bonding interaction possibly exists in the Pt—C bond. The NMR data suggest the strong nmr trans-influence of the cyclopropenylidenes and that in the Pt—CP bond the σ-interaction is appreciable but the π-interaction is negligible.
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  • 23
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    Journal of Physical Organic Chemistry 2 (1989), S. 1-14 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The mechanism of aromatic nucleophilic substitutions by amines in protic solvents is well established; on the contrary the mechanism/s of the reactions in aprotic solvents is/are still subject of controversy. The present paper describes several systems for which fourth-order kinetics (third-order in amine) were observed. A mechanism is proposed to account for this as well as other observation such as: overall negative energies of activation, quadratic dependence of kA with non-nucleophilic tertiary bases, spectacular effects of hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) catalysts, etc. Other alternative mechanisms are also discussed.
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  • 24
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The kinetics and mechanisms of the reactions between 1-phenylethyl benzenesulfonates (1-PEB) with N,N-dimethylanilines are investigated in methanol at 35·0°C. Reactivity and selectivity trends were found to be similar to those for the reactions of 1-PEB with anilines, but the magnitudes of cross interaction constants, ρXZ, between substituents X in the nucleophile and Z in the leaving group were substantially smaller indicating no hydrogen-bond bypass bridge formation in the transition state. However, the magnitude of ρXZ suggested a direct electrostatic interaction between the reaction centers in the nucleophile and leaving group in the frontside nucleophilic attack with a loose transition state structure.
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  • 25
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    Journal of Physical Organic Chemistry 2 (1989), S. 89-92 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Spectral characteristics of several simple substituted B,B-bis(mesityl)pyrroloboranes are reported which support a theoretical treatment by Bonacic-Koutecky and Michl (J. Am. Chem. Soc. 107, 1765 (1985)) describing the excited states of simple aminoboranes as an example of twisted internal charge transfer. In the aminoboranes the pyrrolo moiety functions as the electron donor group and the empty p-orbital of the boron atom as the acceptor.
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  • 26
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    Journal of Physical Organic Chemistry 2 (1989) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
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    Topics: Chemistry and Pharmacology , Physics
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  • 27
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    Journal of Physical Organic Chemistry 2 (1989), S. 183-186 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The harmonic vibrational frequencies were calculated analytically at the 6-31** level for azetidine using the GAUSSIAN 82 program. The results strongly indicate the presence of several errors in a recent assignment of the fundamentals of azetidine based on a normal coordinate analysis and a revised assignment is suggested. It is concluded that reliable vibrational data for azetidine in the gas phase are needed in order to resolve the remaining ambiguities in the interpretation of the spectra.
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  • 28
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    Journal of Physical Organic Chemistry 2 (1989), S. 205-213 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Mechanism of the Grignard reactions of aromatic ketones in THF was studied by spectroscopic and kinetic methods. The stable radical intermediates generated in the initial electron transfer from Grignard reagent to ketones are in a state of aggregated dimer of corresponding ion-radical pairs; in which two ketone anion radicals are bridged by a dimer di-cation of Grignard reagent. Subsequent alkyl radical transfer from dimeric Grignard reagent cation moiety to ketone anion radical aggregated each other are promoted by a participation of another neutral Grignard reagent. Proposed mechanism by present authors is able to explain well addition products/reduction products ratios in the Grignard reactions.
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  • 29
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    Journal of Physical Organic Chemistry 2 (1989), S. 232-242 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Characteristic vector analysis of a set of six physical and empirical parameters of 103 commonly used organic solvents (bp, ∊r, μ, nD, ETN, and δ) gives four vectors describing 95% of the total data variability. Non-hierarchical cluster analysis, applied to our results, leads to ten separate classes of organic solvents.
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  • 30
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    Journal of Physical Organic Chemistry 1 (1988), S. 103-114 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The effect of solvent additives on the course of TiO2 photocatalyzed oxygenation of α-methylstyrene has been studied. While the addition of small amounts of nonhalogenated alcohols to TiO2 powders suspended in acetonitrile was found to decrease the rate of photooxygenation, added halogenated alcohols increase the rate of reaction. In addition, the solvent additives affect the observed product distribution.
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  • 31
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    Journal of Physical Organic Chemistry 1 (1988), S. 115-117 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A novel oxygen induced reduction of α,β-unsaturated carbonyl compounds is discovered. The reduction of the carbon-carbon double bond of α,β-unsaturated carbonyl compounds by benzeneselenol was caused by an introduction of molecular oxygen into the reaction system. This reduction is likely to proceed via a radical chain pathway involving an SH2 type reaction between a phenylseleno radical and a 1,2-adduct of benzeneselenol to the carbonyl group of the α,β-unsaturated carbonyl compound to give an allylic radical which absracts a hydrogen atom from benzeneselenol to form the reduction product.
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  • 32
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    Journal of Physical Organic Chemistry 1 (1988), S. 169-178 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The development of positive charge β to silicon in a saturated five-membered ring has been studied in the cis- and trans-2-(trimethylsilyl)cyclopentyl trifluoroacetates. The cis substrate solvolyzes in 97% trifluoroethanol at 25° C about 5 times faster than the analogous six-membered ring, after correction for differences in ring strain. The trans substrate solvolyzes about 360 times more slowly than the analogous six-membered ring. These changes are in agreement with a hyperconjugative mechanism for interaction between the silyl group and the developing positive charge. The expected cosine-squared dependence of hyperconjugation on the Si—C—C—X dihedral angle suggests that the cis dihedral angle is reduced somewhat from the 60° in the six-membered ring, and the trans dihedral angle is reduced substantially from the 180° in the six-membered ring.
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  • 33
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    Journal of Physical Organic Chemistry 1 (1988) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 34
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    Journal of Physical Organic Chemistry 1 (1988), S. 247-257 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rate of hydrolysis of the aromatic vinyl ether o-carboxy-α-methoxy-β,β-dimethylstyrene was found to be accelerated 25-fold by ionization of its carboxylic acid group, but the effective molarity which may be calculated if all of this rate acceleration is ascribed to intramolecular general acid catalysis is only EM = 1 · 1 m. This is similar to the small effective molarities found before for intramolecular catalysis by carboxylic acid groups of aliphatic vinyl ethers, which shows that, unlike the situation in other intramolecular reactions, e.g. ketone enolization, the extra rigidity of aromatic over aliphatic systems does not improve the efficiency of intramolecular catalysis in vinyl ether hydrolysis.It is suggested that this behaviour is the result of reduced conjugation between the vinyl ether group and the aromatic ring in the transition state of the vinyl ether hydrolysis reaction, which retards the rate and offsets any improvement effected by increased rigidity of the aromatic system.
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  • 35
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    Journal of Physical Organic Chemistry 1 (1988), S. 287-298 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Ab initio STO-3G and 6-31G minimized geometries of 5-oxo- (9), 6-oxo-7-oxabicyclo[2.2.1]hept-2-yl cation (12) and 7-oxabicyclo[2.2.1]hept-2-yl cation (15) were obtained. The energy barriers for their Wagner-Meerwein rearrangements to the more stable 5-oxo- (11), 6-oxo-3-oxabicyclo[2.2.1]hept-2-yl (13) and 3-oxabicyclo[2.2.1]hept-2-yl (17) cations, respectively, have been evaluated and compared with those calculated for the rearrangement of the 5-oxo (18) and 6-oxobicyclo[2.2.1]hept-2-yl cations (20). In agreement with experimental data, the ‘true migratory aptitude’ of an acyl group is higher than that of β-oxoalkyl group in competitive Wagner-Meerwein rearrangments that are ‘energetically unbiased’. The ease of the acyl group 1,2-shift toward an electron-deficient center is related to the electron-donating ability of the carbonyl group due to favorable n(CO) ↔ σ σ ↔ p(C+) hyperconjugative interaction.
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  • 36
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    Journal of Physical Organic Chemistry 1 (1988), S. 299-303 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Iodide ion promotes the free radical reaction of isopropylmercury idide with bromotrichloromethane to yield isopropyl bromide with rate enhancement in the order of 104. The reaction involves electron transfer from i-PrHgI2- to the trichloromethyl radical in a long kinetic chain process. Iodide ion also promotes the free radical chain conjugate addition of tert-butylmercury chloride to α,β-unsaturated ketones, esters, phosphonate esters and sulfones. Competitive reactivity studies indicate that lithium di-tert-butylcuprates or tri-tert-butylzincates react with 2-cycloalkenones by a mechanism involving attack by tert-butyl radicals. No evidence for radical attack is observed for the corresponding n-butyl ate complexes of copper or zinc.
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  • 37
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    Journal of Physical Organic Chemistry 1 (1988), S. 197-207 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: ESR spectra of l-cyano-2-(trimethylsilyl)ethyl and related radicals were recorded. Temperature dependent ESR spectra of these radicals revealed that electron-releasing trimethylsilylmethyl and electron-withdrawing cyano groups synergetically functioned to stabilize the radical centers due to σ-π captodative effects.
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  • 38
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The excellent linearity (R2 = 0·997) of a plot of pKa values for 17 m- and p-substituted benzyl phenyl sulfones, GC6H4CH2SO2Ph, vs. those for the corresponding arylacetonitrile, GC6H4CH2CN, demonstrates that substituent solvation and substituent solvation assisted resonance (SSAR) effects for p-CN, p-COPh, and p-SPh are nearly identical in these two substrates. The PhSO2 group in PhCH2SO2Ph increases the BDE of the α-C—H bond by 2 kcal/mol, relative to toluene. The α-C—H bonds in GC6H4CH2SO2Ph sulfones are stabilized by 1-2 kcal/mol by acceptor G′s (m-CN, p-CN, m-CF3, p-CF3), but weakened by 1 and 5 kcal/mol, respectively, by donors (p-OMe and p-NMe2). The GC6H4CH2SO2Ph+. radical cation with G = H has a pKHA+. = -25. Its acidity is increased when G is an acceptor by as much as 9 to 10 kcal/mol (G = 3-CN, 3-CF3, 4-CF3, 4-NO2), but is decreased when G is a donor by as much as 33 kcal/mol (G = NMe2). When G = 4-SPh the radical cation is stabilized, relative to G = H, by a larger amount (25 kcal/mol) than when G = 4-OMe (18 kcal/mol). Structural changes along the series PhCH2SO2Ph, 2-naphthyl-CH2SO2Ph, 9-anthryl CH2SO2Ph cause negligible changes in the acidities of these acids, but sizable decreases in the acidities of the corresponding radical cations. Introduction of a phenylsulfonyl group into the methyl group of 9-methylanthracene or the 9-position of fluorene or xanthene increase the BDEs by 3, 2, and 7 kcal/mol, respectively. These effects of PhSO2 groups are compared and contrasted with those of CN groups.
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  • 39
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    Journal of Physical Organic Chemistry 1 (1988), S. 267-273 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The PE spectra of four stable bicyclic triaziridines with trifluoromethyl, methyl, phenyl, spirocyclopentane and spirocyclohexane substituents are measured. From a comparison of these experimental data with MNDO calculations the energies and the interactions of the nitrogen lone-pair orbitals were obtained. The trifluoromethyl group may be reliably simulated by a fluorine atom. The ionization potentials are unexpectedly high and so are the cyclovoltammetric anodic potentials. The MNDO valence electron densities are in agreement with the comparatively small differences in the chemical shifts of the differently substituted nitrogens of the triaziridine rings.
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  • 40
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    Journal of Physical Organic Chemistry 1 (1988), S. 275-280 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: In order to examine the substituent effect of ethylenedithio and trimethylenedithio groups on electron acceptors, the two title compounds were prepared in one step from the corresponding quinones. These species behave as weak π-electron acceptors owing to the electronic and sterical effects of the fused groups. Only the bis(ethylenedithio) derivative formed a charge-transfer complex with tetrathiotetracene, which showed a moderate electric conductivity of 2·0 × 10-2 S cm-1. In addition, both species oxidized copper, sodium, and potassium iodides to give the corresponding deeply colored metal salts, most of which were semiconductive. The naphtho analogs were similarly studied, but hardly served as electron acceptors.
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  • 41
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    Journal of Physical Organic Chemistry 1 (1988), S. 259-265 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 1-(Phenylazo)-1-(2-pyridyl)ethyl hydroperoxide, 1, 1-(phenylazo)-1-(2-furyl)ethyl hydroperoxide, 2, phenylazo(2-furyl)methyl hydroperoxide, 3, 1-(phenylazo)-1-(4-anisyl)ethyl hydroperoxide, 4, were synthesized in moderate yield by autoxidation of the phenylhydrazones in benzene. The ionic oxidation of benzyl methyl sulfide in benzene by 1-4 yielded the sulfoxide and the metastable α-azo hydroxides in essentially quantitative yield. The reaction was of the first order in α-azo hydroperoxide and sulfide, respectively. The relative reactivity series found was: 1(1.0) 〈4(1.4) 〈 phenylazo(4-anisyl)methyl hydroperoxide 5 (2·9) 〈2 (3·8) 〈 3 (9·6). α-Methyl substitution was found to slow the rate of oxygen-atom trasfer by a factor of 2 to 2.5. The low relative reactivity of 1 was opposite that expected based on electronic effects. Competitive intramolecular hydrogen bonding of the hydroperoxy proton to the pyridyl nitrogen in 1 accounted for the observed result.
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  • 42
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    Journal of Physical Organic Chemistry 1 (1988), S. 241-245 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The rates of solvolysis of l-aryl-l-phenyl-2,2-dimethylpropyl(1) p-nitrobenzoates and of aryldiphenylmethyl-p-nitrobenzoates (2) were measured in 80% acetone. An inverse order of reactivity, kp-CF3/km- CF3 〉 l was observed for 1 but not for 2. The X-ray crystallogoraphic study of the parent compounds, l,l-diphenyl-2,2-dimethylpropyl-p-nitrobenzoate (1b) and triphenylemethyl benzoate (3b), and of 2-phenyl-2-propyl-p-nitrobenzoate (4) indicated that in the highly congested system 1 both phenyl rings were nonplanar, whereas the phenyl rings in 3 and 4 were essentially planar.
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  • 43
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    Journal of Physical Organic Chemistry 2 (1989), S. 103-109 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A preliminary theoretical study of the mechanisms for the reactions of the perchlorofluoroethanes CF2ClCCl3 (1), CF2ClCCl2F (2) and CF3CCl3 (3), with nucleophiles has been carried out by the MNDO method, following the experimentally suggested process shown in Scheme 1. The unlikely chlorophilic attack in the first step of Scheme 1 has been shown to be feasible for 1, 2 and 3 by analysis of the MO interactions. The second step has been found to be affected by the anionic hyperconjugation which stabilizes the anions CF2ClCCl2- (4), CF2ClCClF- (5) and CF3CCl2- (6) and would make reactions (2) (the second step) unfeasible in gas phase, but in solution reaction (2) may still easily occur for 4 and 5.
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  • 44
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    Journal of Physical Organic Chemistry 2 (1989), S. 146-160 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The alkaline hydrolysis of several alkylphosphonates and alkylphosphonyl chlorides has been investigated by molecular mechanics calculations (MM2, 1985 version). The difference of the steric energies (ΔE) between tetracoordinate substrate and pentacoordinate transition state of phosphorus compounds represents the activation energy (ΔE≠) in hydrolysis. The change of ΔE for various alkyl groups relative to methyl group (ΔΔER) is suggested as a measure of the steric effect of substituents. Thus the correlation analysis involving log k and ΔΔER of the branched alkyl group gives good results and it is reasonable to anticipate that analogous treatment using ΔΔER for the straight chain alkyl group is not satisfactory owing to the minor contribution of steric effect of the latter. However, the multiple regression analysis of log k with ΔΔER and Taft's σ* provides very good results. As shown by us, for the hydrolytic reactions studied, the proposed ΔΔER is much better than Taft's Es and Charton's ν, the commonly used well-known steric parameters in the chemistry of carbon compounds.
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  • 45
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    Journal of Physical Organic Chemistry 2 (1989), S. 161-176 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Cyclobutanediyl (2) has been studied in both its singlet and triplet states by ab initio electronic structure theory. The triplet, which is the ground state of the molecule, exists in both C2h and C2v forms which interconvert via a Cs transition state. For the singlet, only a C2h form is found. It passes, via a Cs transition state, onto the C2v surface on which bicyclobutane (3) is the only minimum. The ring-flipping (inversion) process in 3 includes the singlet biradical as an intermediate, and involves a novel, non-least motion path similar to one previously proposed by Gassman. Semiclassical periodic orbit theory indicates that the various minima on both the singlet and triplet surfaces can interconvert via quantum mechanical tunneling.
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  • 46
    ISSN: 1075-2617
    Keywords: FMDV ; structure ; NMR spectrocopy ; NEO constraints ; RGD (Arg-Gly-Asp) ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solution structure of a 20 amino acid long peptide corresponding to the region 141-160 of the envelope protein Vp1 from foot-and-mouth disease virus (FMDV) serotype A, variant A, has been determined by a combination of NMR experiments and computer calculations. The peptide contains both the immunodominant epitope as well as the sequence (RGD) used by the virus to bind the cell receptor in the initial stages of infection. These two sites have been shown to partially overlap.One hundred and thirty-five NMR distance constraints were used to obtain a set of 11 structures by distance geometry, minimization and molecular dynamics simulations. These structures were divided into two homogeneous families based upon backbone superimposition. The first and most populated family was characterized by a backbone RMS of 1.5±0.4 Å, the second by a backbone RMS of 0.8±0.2 Å. The two families had similar structural features and differed mainly in the backbone angles of G149. In the larger of the two families these angles favoured the formation of a loop comprising residues 147 to 152 and stabilized by a H-bond between the NH of D147 and the CO of A152. In the second family, where this bond was absent, the peptide adopted in this region the shape of an irregular helix. The C-terminal half of the peptide (152-159) was similar in both families and largely helical. Similar structural features were also found within the VRGDS sequence (144-148) which was assigned to a β-turn type IV. The features of the two families of structures were found to be different from those of the recently published X-ray structure of the antigenic loop of a chemically modified form of FMDV. Proposals accounting for these differences are provided which take into account the dual activity of the 141-160 sequence (i.e. antibody binding and cell invasion through receptor binding).
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  • 47
    ISSN: 1075-2617
    Keywords: FMDV ; NMR spectrocopy ; RGD (Arg-Gly-Asp) ; NEO constraints ; structure-activity correlation ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solution structure of a peptide corresponding to the VP1 region 141-160 of foot-and-mouth disease virus (FMDV) serotype A variant USA has been studied by NMR and computer calculations and compared with the results from a study on a highly homologous peptide deriving from serotype A, variant A. The two peptides differ in their serological behaviour and contain the immunodominant epitope of the virus which partly overlaps with its receptor binding region. Distance constraints, derived both from 2D and 3D homonuclear NMR and 2D-heteronuclear NMR experiments, were combined with DG calculations to yield 50 structures. After refinement through EM and restrained molecular dynamics simulations the selected structures shared several general features. In particular the 151-158 region was a helix in all cases while a large loop similar to that found in peptide A but comprising less residues and stabilized by an H-bond between the side chains of D147 and S150 was found in the majority of structures. A further loop, common to all structures, was identified around the RGD sequence (145-147). This was different from that found in the corresponding region of peptide A as were the conformations of the individual residues within the RGDX sequence.The different structural features shown by the two peptides were rationalized in terms of the S148 (peptide A) to F148 (peptide USA) mutation. The second mutation, that at position 153 (L in A, P in USA) did not appear to affect the structure of the peptide significantly although the different dimensions of the loop in the central region and the type of H-bond stabilizing it could be potentially ascribed to this second mutation.All criteria used pointed to different structural features for the two peptides consistent with their serological behaviour.
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  • 48
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    Journal of Peptide Science 2 (1996), S. 125-133 
    ISSN: 1075-2617
    Keywords: SPPS ; labelled peptides ; cellular localization ; amphipathic peptide ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report the solid-phase synthesis by the Fmoc strategy of a peptide containing a cysteamide group at its C-terminus. This peptide was subject to further modifications including the linkage of fluorophores, namely lucifer yellow and coumarin respectively, at the C- and/or N-terminals. After incubation with living cultured cells these two probes were localized and it is concluded that the post-synthesis modifications can strongly modify the localization of the peptide.
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  • 49
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    Journal of Peptide Science 2 (1996) 
    ISSN: 1075-2617
    Keywords: Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 50
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    Journal of Peptide Science 2 (1996), S. 106-116 
    ISSN: 1075-2617
    Keywords: all parallel helix assemblies ; helix transition ; 310-/Α-HELICES ; two conformers ; water associated with non-polar helices ; X-ray crystallography ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The role of end groups in determining stereochemistry and packing in hydrophobic helical peptides has been investigated using an α-aminosobutyric acid (Aib) containing model nonapeptide sequence. In contrast to the Boc-analogue, Ac-(Aib-Val-Ala-Leu)2-Aib-OMe crystallizes with two independent molecules in a triclinic cell. The cell parameters are: space group P1, a=10.100(2)Å, b=15.194(4) Å, c=19.948(5) Å, α=63.12(2)°, β=88.03(2)°, γ=88.61(2)°, Z=2, R=7.96% for 5140 data where |Fo|〉3σ(F). The two independent molecules alternate in infinite columns formed by head-to-tail hydrogen bonding. The helices in the two independent molecules are quite similar to each other but one molecule is rotated ≍123° about its helix axis with respect to the other. All the helical columns pack parallel to each other in the crystal. Replacement of the bulky Boc group does not lead to any major changes in conformation. Packing characteristics are also similar to those observed for similar helical peptides.
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  • 51
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    Journal of Peptide Science 2 (1996), S. 117-124 
    ISSN: 1075-2617
    Keywords: peptides ; neuropeptide ; antimicrobial agent ; skin secretion ; frogs ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The granular dorsal glands of the giant tree frog Litoria infrafrenata contain five peptides including caerulein (a known neuropeptide), and four new peptides named frenatins 1 (MH+ = 1140 Da), 2 (1423), 3 (2180) and 4 (2493). The amino acid sequences of the frenatins are detailed: their structures do not correspond to those of peptides isolated from other amphibians or animals. Frenatin 3, Gly-Leu-Met-Ser-Val-Leu-Gly-His-Ala-Val-Gly-Asn-Val-Leu-Gly- Gly-Leu-Phe-Lys-Pro-Lys-Ser-(OH), has wide spectrum antimicrobial properties.
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  • 52
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    Journal of Peptide Science 2 (1996), S. 141-156 
    ISSN: 1075-2617
    Keywords: gramacidin A ; channel forming peptides ; Peptide conformational analysis ; cicular dichroism ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The relation between the various spatial structures of the gramicidin A channels and their ionic conductance has been studied. For this aim, various conformations of the peptide were pre-formed in liposomal bilayer and after subsequent fusion of liposomes with planar lipid bilayer the measured channel conductance was correlated with gramicidin structures established in liposomes. To form the single-stranded π6.3π 6.3 helix the peptide and lipid were co-dissolved in TFE prior to liposome preparation. THF and other solvents were used to form parallel (↑ ↑ π π) and antiparallel (↑ ↓ π π) double helices. Conformation of gramicidin in liposomes made by various phosphatidylcholines was monitored by CD spectroscopy, and computer analysis of the spectra obtained was performed. After fusion of gramicidin containing liposomes with planar bilayer membranes from asolectin, the histograms of single-channel conductance were obtained. The histograms had one or three distinct peaks depending on the liposome preparation. Assignment of the structure of the channel to conductance levels was made by correlation of CD data with conductance histograms. The channel-forming analogue, des(Trp-Leu)2-gramicidin A, has been studied by the same protocol. The channel conductances of gramicidin A and the shortened analogue increase in the following order: ↑ ↓ π π 2 ↑ ↑ π π 〈 π 6.3π6.3. Single-channels formed by double helices have higher dispersity of conductance than the π6.3π6.3 helical channel. Lifetimes of the double helical and the π6.3π6.3 helical channels are very close to each other. The data obtained were compared with theoretically predicted properties of double helices [1].
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  • 53
    ISSN: 1075-2617
    Keywords: HIV ; peptide library ; immune response ; cytokines ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The hypervariable domain of the HIV gp120, the V3 loop domain, represents a target for neutralizing antibodies and for HIV vaccine strategies. In this study, we have investigated in murine species the potential cross-reactivity of immune responses elicited by immunization either with individual V3 peptides, derived from distinct HIV sequences (BRU, RF, SF2, MN and ELI sequences), or with a V3 combinatorial peptide library.We observed that individual V3 peptides are immunogenic but elicit a specific B- and T-cell immune response that is mainly restricted to the sequence of the immunizing peptide. In particular, T-cell responses that depend on T-cell receptor recognition of peptides bound to the molecules encoded by the major histocompatibility complex were significantly influenced by small differences in the peptide amino acid sequence. The combinatorial V3 peptide library, previously described as B- and T-cell immunogens, induced a more broadly reactive immune response, specially when T-cell cytokine secretion was used as a readout for restimulation of T-cells with individual V3 peptides.These data suggest that amino acid variations in the sequence of an antigenic peptide could lead to the induction of different transducing signals in the primed T-cell population and to the activation of T-cells with distinct cytokine secretion properties. These observations may have implications in the understanding of antigenic variability and in the design of vaccine strategies.
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    Journal of Peptide Science 2 (1996), S. 157-164 
    ISSN: 1075-2617
    Keywords: sideropore ; peusdobactin ; peusdomycin ; solution-phase peptide synthesis ; unusual amino acids ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Pseudobactin is a structurally complex and physiologically important siderophore (microbial iron chelator) from Pseudomonas putida- fluorescens. Various fragments of the unusual peptide component of pseudobactin listed below were prepared by solution-phase peptide synthesis.L-Lys· D-threo-β-OH Asp· L-Ala· D-allo-Thr· L-AlaL-Lys· D-threo-βOH Asp· L-Ala· D-allo-ThrD-threo-β-OH Asp· L-Ala· D- allo-Thr· L-Ala· D-N-OH-cycloOrnD-threo-β-OH-Asp· L-Ala· D-allo-Thr· L-AlaL-Ala· D-allo-Thr· L- Ala· D-N-OH-cycloOrnA class of related peptides named pseudomycins have shown promising antifungal activity. To examine if these peptide fragments above would elicit similar activity, the fragments were tested and found to have no antifungal activity in limited bioassays.
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  • 55
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    Journal of Peptide Science 2 (1996) 
    ISSN: 1075-2617
    Keywords: Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 56
    ISSN: 1075-2617
    Keywords: Nformylmethionyl peptides ; human neutrophils ; chemotaxis ; superoxide anion generation ; lysozyme release ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formylpeptides formyl-methionyl-Nmethylleucyl- phenylaline methyl ester [for-Met-(NMe)Leu-Phe-OMe] 1, formyl-methionyl-2-aminotetralin-2-carboxyl-phenylalanine methyl ester [for-Met Act-Phe-OMe] 2, formyl-methionyl-1,2,3,4-terahydroisoquinoline-3-carboxl- phenylalanine methyl ester [for-Met-Tic-Phe-OMe] 3 and formyl- methionyl-2-aminoxy-4-methylvaleryl-phenylalanine methyl ester [for-Met-OLeu-Phe-OMe] 4 were synthesized in order to investigate the role of the amide bond at position 2 on biological activities on human neutrophils. Only analogue 2, which keeps the NH group at position 2, was found to retain activity though sterically encumbered.
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  • 57
    ISSN: 1075-2617
    Keywords: NPY ; conformational analysis (CD, NMR) ; molecular dynamics ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Neuropeptide Y (NPY), a peptide amide comprising 36 residue has been shown to act as a potent vasoconstrictor. In order to shed light on the structural requirements for the biological activities with respect to the different prerequisites for affinity to the NPY receptor subtypes Y1 and Y2, in the present study the syntheses and conformational analyses of two C-terminal segments, NPY(18-36) and NPY(13-36), are described.The results obtained by CD measurements, two-dimensional NMR spectros copy and a conformational refinement of the NMR-derived structure by molecular mechanics simulations support the findings of previously published structure -activity relationship studies for biologically active and selective compounds. In particular, the α-helical conformation as well as an appropriate exposure of the side chains of the critical C-terminal dipeptide within NPY(18-36) are in agreement with the prerequisites proposed for Y2 receptor binding of that segment.
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  • 58
    ISSN: 1075-2617
    Keywords: antimicrobial peptides ; hydrophobic residue ; ion-channels ; pores ; α-helix ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The design of cecropin-melittin hybrid analogues is of interest due to the similarities in the structure of the antimicrobial peptides cecropin and melittin but differences in their lytic properties. We suspected that a hydrophobic residue in position 2 of milittin (Ile8 in the hybrid) plays an important role in the activity of the 15-residue hybrid, KWKLFKKIGAVLKVL-NH2, [CA(1-7)M(2-9)NH2] and have now examined its role in the analogue toward five test bacteria. Deletion of Ile8 reduced activity, and it was not restored by lengthening to 15 residues by addition of another threonine at the C-terminus. Replacement of Ile8 by a hydrophobic leucine maintained good activity and Ala8 was equally active for four organisms, although less active against Staphylococcus aureus. Replacement by the hydrophilic Ser8 strongly reduced potency against all five organisms. Deletion of Leu15 decreased activity, but addition of Thr16 maintained good activity. The presence of hydrophobic residues appears to have a significant effect on the process of antibacterial activity. These peptide analogues showed voltage-dependent conductance changes and are capable of forming ion-pores in planar lipid bilayers. The antibacterial action of the peptides is thought to be first an ionic interaction with the anionic phosphate groups of the membrane followed by interaction with the hydrocarbon core of the membrane and subsequent reorientation into amphipathic α-helical peptides that form pores (ion-channels), which span the membrane. The analogue also showed an increase in α-helicity with an increase in hexafluoro 2-propanol concentration.
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  • 59
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    Journal of Peptide Science 2 (1996), S. 233-239 
    ISSN: 1075-2617
    Keywords: NMR ; chemical shift estimation ; restrained refinement ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For flexible peptides, nuclear Overhauser Effects (NOE) experiments do not provide enough information to ensure a correct definition of their solution structure. The use of distance constraints, derived from the knowledge of proton chemical shifts, is developed to restrict the number of possible conformations. In the case of flexible molecules, randomization appears as an important factor of the correct estimation of the chemical shifts from the 3D structure. The refinement of the solution structure of the highly flexible AVP-like parallel dimer is described to illustrate this process.
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  • 60
    ISSN: 1075-2617
    Keywords: mucin glycopeptides ; tumour associated antigen ; cancer ; MHC Class II binding ; glycopeptide synthesis ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of peptides and glycopeptides derived by amino acid and glycosyl amino acid scans through the self peptide from CBA/J mouse haemoglobin Hb (67-76), VITAFNEGLK, was synthesized by multiple column peptide synthesis (MCPS). Investigation of glycopeptide binding to the mouse major histocompatibility class II molecule Ek showed that glycans in position 72 did not interfere with the binding to Ek. Immunization experiments revealed that glycopeptides with the glycan in position 72 were immunogenic. Therefore a series of N-linked and O-linked glycopeptides with the glycan attached in the position 72 either to serine, threonine or asparagine was synthesized by MCPS. The glycan structure was furthermore varied with respect to monosacc haride component, size of oligosaccharide, anomer configuration and stereoche mistry of essential hydroxyl groups in order to investigate the specificity of the interaction with the T-cell receptor. Easy synthesis of ready to use Ser and Thr building blocks corresponding to mucin core 1, the Tn-antigen and its β-anomer were developed using trichloroacetimidates as glycosyl donors and reduction with in situ acetylation of the azide containing glycosylation products. Synthesis of an α-linked GlcNAc-Thr building block was achieved by glycosylation of Fmoc-Thr-OPfp with 2-azido-2-deoxy-3,4,6-tri-O-acetyl-D- glycopyranosyl trichloroacetimidate as a glycosyl donor. Other building blocks were obtained by previously described procedures.
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  • 61
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    Journal of Peptide Science 2 (1996), S. 195-211 
    ISSN: 1075-2617
    Keywords: peptide absorption ; dipeptide carrier ; brush border membrane ; M-cell, Caco-2 cell ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Today there is considerable interest in oral peptide delivery. However, oral administration of peptides is limited by a low bioavailability and a high variability in plasma levels. A review is given of the literature describing the major barriers in peptide absorption, the basic mechanisms of intestinal peptide transport, the experimental models and the pharmaceutical approaches currently used in the investigation of peptide and protein absorption processes.
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  • 62
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    Journal of Peptide Science 2 (1996), S. 252-260 
    ISSN: 1075-2617
    Keywords: melittin immunogenicity ; antigen recognition ; membrane involvement ; haptenic position ; late IgG responses ; cellular dynamics ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Melittin peptides carrying 2,4-dinitro-6-carboxyphenyl (Dncp) haptenic groups regularly evoked anti-hapten IgG responses in mice or guinea pigs when the hapten was C-terminally attached. Single haptens on the N-terminal helix in several positions gave poor or no responses in the early stages but adequate titres after prolonged immunization. Peptides with Dncp at the C-terminus as an invariant feature and a second Dncp in various positions along the peptide chain did not fail to produce adequate responses. The hampering effect is not due to a defect at the T-cell level but involves the recognition step on the B-cell. It is implied that the haptenic interaction with the paratope of the recognizing immunoglob ulin on the B-cell involves the cell membrane in an important way. It is also suggested that late antibody responses should not be overlooked during the development of proteinaceous immunogens for vaccination.
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  • 63
    ISSN: 1075-2617
    Keywords: substance P ; agonist ; conformational constraint ; backbone cyclization ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Novel backbone-to-side chain and backbone-to-backbone cyclic analogues of substance P (SP) were prepared by solid-phase synthesis and screened for biological activity. An analogue containing a thioether- lactam ring between positions 9 and 11 showed an EC50 value of 20nM toward the neurokinin 1 (NK-1) and was inactive toward the NK-2 and NK-3 receptors. On the other hand, in a multiple backbone cyclic peptide library of similar analogues, in which the sulphur was excluded from the ring, very low activity was detected. The activity was re-evaluated and was found to be even lower (EC50=0.11 mM) than the previously published data. These results indicate that the thioether moiety has a crucial role in receptor activation. The results also show tolerance of the NK-1 receptor, but not NK-2 or NK-3, to cyclization of the C-terminal portion of the SP6-11 hexapeptide.
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  • 64
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    Journal of Peptide Science 2 (1996), S. 245-251 
    ISSN: 1075-2617
    Keywords: enkephalin ; neurotensin ; opioid receptor affinity ; multivalent ligand ; liposome ; fluorescence microscopy ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A multivalent ligand system was constructed by coimmobilization of two kinds of peptide ligands, enkephalin and neurotensin derivatives having a dioctadecyl group, on dimyristoylphosphatidylcholine (DMPC) liposomes. The enkephalin derivatives are Tyr-D-Ala-Gly-Trp-Leu- (Sar-Sar-Pro)n-[N(C18H37)2] (Enk3nD, n=0, 1, 2), where a dioctadecyl group was connected to the C-terminal side of enkephalin directly or through a hydrophilic and flexible spacer chain of different lengths. The neurotensin derivatives are Ac-Glu[N(C18H37)2]-(Sar-Sar-Pro)n-Arg-Arg-Pro-Tyr-Ile-Leu-OH (D3nNT, n=0, 1, 2, 3). The derivatives were spontaneously immobilized on DMPC liposomes by overnight incubation. The receptor affinity of the enkephalin derivatives became significantly higher upon immobilization on liposomes. The highest affinity was obtained for the δ receptor by Enk6D immobilized on DMPC liposomes. This affinity is higher than that of enkephalinamide. Neurotensin derivatives coimmobilized with large amounts of Enk3D on DMPC liposomes show higher affinity than the neurotensin derivatives immobilized alone. The effect of Enk3D on the receptor affinity of the coimmobilized neurotensin derivative disappeared by the addition of [Ala2, MePhe4, Gly-ol5]enkephalin (DAGO). Therefore, the receptor affinity of a peptide hormone is altered by immobilization on DMPC liposomes and by coimmobilization with other peptide hormones. It was confirmed by fluorescent microscopy that the multivalent ligand system binds to receptors without release of the bound ligands from DMPC liposomes.
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    Journal of Peptide Science 2 (1996) 
    ISSN: 1075-2617
    Keywords: Chemistry ; Biochemistry
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    Topics: Chemistry and Pharmacology
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  • 66
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    Journal of Peptide Science 2 (1996), S. 271-275 
    ISSN: 1075-2617
    Keywords: peptide synthesis ; cysteine ; racemization ; enantiomeric resolution ; capillary electrophoresis ; gas chromatography ; Chemistry ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new method has been developed for the rapid determination of D-cysteine contents in synthetic peptides. It is based on the reduction of cystine residues, when present, with tris- alkylphosphines, selective derivatization of the cysteine residues with 4-vinylpyridine, followed by acid hydrolysis of the (4-pyridylethyl)cysteine -peptides. Baseline enantiomeric resolution of theD,L-S-β-(4-pyridylethyl)cysteine, and thus quantification ofD- enantiomer contents at levels ≤1%, is easily achieved by capillary zone electrophoresis exploiting the host-guest complexation principle with crown ethers or by gas chromatography on chiral glass capillary columns upon conventional derivatization of the hydrolysate. The acid-stability of the (4-pyridylethyl)cysteine derivative prevents racemization via thiazoline intermediates and allows for standardization of the acid hydrolysis-dependent racemization.
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  • 67
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    Journal of Physical Organic Chemistry 1 (1988) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 68
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photoresponse of the poly(vinyl chloride) membranes, which contain spirobenzopyran and crown ether, covered with a urease layer was studied in the presence and absence of urea. In the absence of urea, UV light irradiation induced more than 160 mV of membrane potential change, whereas the photoresponse decreased with an increase in the concentration of urea in the solution. The effects of such operating variables as crown loading and pH and ionic strength in the aqueous phase on the potentiometric response were also elucidated in the presence of urea. The results were explicated using the fact that the local concentration on NH4+ and H+ ions changed as a result of the urease-catalyzed decomposition reaction of urea.
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  • 69
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    Journal of Physical Organic Chemistry 2 (1989) 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
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  • 70
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    Journal of Physical Organic Chemistry 1 (1988), S. 351-357 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The title reaction is an instance of a simple nucleophilic attack of neutral nitrogen on aromatic sp2 carbon. The kinetic data (obtained in dimethyl sulphoxide) are compared to the results reported by the literature for the same reaction of aliphatic amines. Some possible interactions (substrate/nucleophile, substrate/salt) preceding the attack of the nucleophile are discussed.
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  • 71
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    Journal of Physical Organic Chemistry 2 (1989), S. 15-25 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvolysis of 2-X-2-phenylpropane (1-X) in 25 vol% acetonitrile in water at 25°C produces 2-hydroxy-2-phenylpropane (1-OH) and 2-phenylpropene (3). The carbocationic intermediate discriminates between different nucleophiles; azide anion, acetate anion, and methanol are more efficient nucleophiles than water, kN3/kH2O = 42 kOAc/kH2O = 3, and kMeOH/kH2O = 2·9 (ratio of second-order rate constants). The fraction of the elimination product 3 increases with increasing basicity of the leaving group X as well as by addition of general bases. The Brønsted parameter for this catalysis is small, β = 0·13, with substituted acetate anions. The kinetic deuterium isotope effect for the dehydronation of the intermediate has been measured (assuming the reaction from intermediate to alcohol is insensitive to isotopic substitution) employing the hexadeuterated substrate d6-1-X as k3H/k3d6 = 3·5 ± 0·2 for the chloride 1-Cl with acetate anion, and, without added base, 3·1 ± 0·2 for the acetate 1-OAc, and 3·1 ± 0·2 for the p-nitrobnzoate 1-PNB, respectively, and ∼5 for the protonated methyl ether 1-OMeH+. The variation in isotope effect with change in leaving group is discussed in terms of elimination from contact ion pairs and ‘free’ carbocation. The overall kinetic isotope effect for the solvolysis was found to be kobsH/kobsd6 = 1·31 (1-OMeH+), 1·38 (1-OAc), 1·40 (1-PNB), and 5·7 (1-OH2+). These isotope effects consist of the isotope effect k12H/k12d6 for the formation of the substitution product 1-OH and k13H/k13d6 for production of the olefin 3. It is concluded that the latter isotope effect is enlarged owing to a branched mechanism in which the deprotonation of the carbocationic intermediate competes with formation of the substitution product. As large an isotope effect as k13H/k13d6 ∼6·5 has been measured for 1-OMeH+.
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  • 72
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: IR spectroscopy is presented as a convenient means to monitor the formation of acyl (and alkyl) cations from corresponding acid chlorides, with Lewis acids at low temperature in the solid state. Phenylacetyl chloride is co-deposited (under vacuum at -173°C with antimony pentafluoride on a KBr window), as in the ‘molecular beam’ method of cation generation. The initial IR spectrum of the deposition shows (in addition to starting materials) that a small amount of phenylacetyl cation (2274 cm-1) has been formed. Warming the solid matrix to -123°C Promotes the smooth conversion of acid chloride to acyl cation. Ultraviolet irradiation (through quartz windows) at -123°C to -73°C facilitates this conversion but does not result in the loss of carbon monoxide from the acyl cation to form benzyl cation. When benzyl chloride is used in place of phenylacetyl chloride, there is no noticeable difference between IR spectra taken before and after warming and/or photolysis of the solid deposition.
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    Journal of Physical Organic Chemistry 2 (1989), S. 93-102 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Organocerium(III) reagents reacted with α,β-unsaturated carbonyl compounds to give 1, 2-addition products (allylic alcohols) in good to high yields. The reaction was studied from a mechanistic point of view by the use of (E)- and (Z)-1-(4′-methoxyphenyl)-3-phenyl-2-propen-1-ones and 4,4-ethylenedioxy-2,6-dimethyl-2,5-cyclohexadienone as the probe compounds. A polar pathway was suggested for the reaction with the former enones. On the other hand, the operability of single electron transfer processes was demonstrated in the reaction with the latter probe compound.
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  • 74
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Bis(1,2-diaryl-1,2-ethylenedithiolato)metal(0) complexes (1; metal = Ni, Pd, and Pt) react with quadricyclane (Q) to give selectively 1:1 adducts which are identical with the adducts between 1 and norbornadiene (NBD). In the adducts, two sulfur atoms of the different dithiolato ligands are added to one of the double bonds of NBD. The reactions of 1 with Q are 103-104 times faster than those with NBD. The more electropositive dithiolatometal complexes react faster with Q and NBD. The adducts are dissociated to the free dithiolatometal complexes and NBD by UV-irradiation (254-436 nm). Upon irradiation with 254 nm light, the photodissociation occurs before the decomposition of the dithiolatometal complexes.
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  • 75
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    Journal of Physical Organic Chemistry 2 (1989), S. 117-130 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Gas phase ion/molecule reactions have been used to probe the structure of ions obtained by electron impact upon 1-(diphenylmethylene)cyclopropane and 2,2-diphenyl-1-methylenecyclopropane. The resulting ions of molecular composition C16H14 (m/z 206+) were reacted with charge transfer reagents (6·9 〈 IP 〈 8·8eV), giving evidence for the presence of isomeric ions with different reactivities. The less reactive ion is identified as a trimethylenemethane species (TMM+) in which one cyclopropane bond is broken; the more reactive ions are assigned as vertical ions in which the cyclopropane ring is unaffected. The vertical ions have recombination energies of 8·44 ± 0·05eV, whereas TMM+ has one of 7·41±0·05eV. The TMM+ fraction is not constant; it increases with increasing IP of the reagent. This is attributed to a reagent-catalyzed isomerization of the vertical ion to TMM+. In addition, the reagent ions are observed to undergo a unique reaction with the neutral methylenecyclopropane derivatives: electron transfer and ring opening to yield TMM+. These findings limit the application of the customary equilibrium measurements as a method to determine the substrate IP.
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    Journal of Physical Organic Chemistry 2 (1989), S. 177-182 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 4-O2NC6H4OCH2CH2NHPh undergoes clean base-catalyzed Smiles photorearrangement to 4-O2NC6H4N(Ph)CH2CH2OH in dimethyl sulfoxide-water (25:75). A linear plot of φ-1 vs [OH-]-1 indicated that the limiting quantum yield at high [OH-] is 0·12 and that uncatalyzed photorearrangement does not occur. Rate constants for the deprotonation step were determined for a variety of bases having conjugate acid pKa values in the range 5-10. When plotted according to the Brønsted Catalysis Law, these data gave a non-linear plot approaching slopes of zero and unity above and below a pKa of 6-7. This indicates that the proton-donating intermediate in this photo-Smiles rearrangement is the zwitterion diradical (pKa ≈ 7) rather than the Meisenheimer complex (pKa ≈ 2).
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    Journal of Physical Organic Chemistry 2 (1989), S. 225-231 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The gas-phase pyrazole elimination of N-alkyl pyrazoles has been studied using MNDO semi-empirical molecular orbital (MO) theory with complete geometry optimization of all stationary points. We found that the activation energies (Ea) of the concerted processes are around 80 kcal/mol, 25 Kcal/mol higher than experimental values. But the differences in Ea between compounds with different substituents are in good agreement with the experimental ones.
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    Journal of Physical Organic Chemistry 2 (1989), S. 255-262 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The photostimulated reaction of neopentyl halides with different nucleophiles by the SRN1 mechanism of nucleophilic substitution has been studied. Neopentyl halides do not react with carbon nucleophiles, diethylphosphite, diphenylphosphonite and azide ions, but they react with arsenide and selenide ions. The photostimulated reaction of neopentyl bromide with diphenylarsenide ions gave only the straightforward substitution product neopentyldiphenylarsine. On the other hand, the photostimulated reaction of bromobenzene with dineopentyl arsenide ions gave three arsines: dineopentylphenylarsine, neopentyldiphenylarsine and triphenylarsine. Neopentyl chloride reacts under irradiation with diphenylphosphide ions giving good yields of the substitution product.
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  • 79
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    Journal of Physical Organic Chemistry 2 (1989) 
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    Keywords: Organic Chemistry ; Physical Chemistry
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  • 80
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    Journal of Physical Organic Chemistry 4 (1991), S. 361-366 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Vibrational analysis calculations have been conducted in an effort to understand the origins of apparently anomalous relationships reported between kH/kT and kD/kT kinetic isotope effects. The actual isotopic substitutions used in previous work are more complex than a simple comparison between H/T and D/T isotope effects. The relationships between the actual isotope effects determined in these studies amount to tests of two standards of conventional isotope effect theory, the rule of the geometric mean (or the lack of isotope effects on isotope effects) and the Swain - Schaad rule. Model calculations illustrate the importance of violations of the rule of the geometric mean (over Swain - Schaad deviations) arising in models that incorporate both explicit reaction-coordinate coupling of two isotopic sites and reaction-coordinate tunneling. Implications for experimental studies of turneling using apparent Swain - Schaad exponents are discussed.
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  • 81
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    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Electrochemical and spectrophotometric studies of the pyridoxal-5′ -phosphate hexylamine Schiff base (PHSB) in strongly basic media were carried out. The equilibrium constant and rate constants of the formation and hydrolysis of the unprotonated imine from the unprotonated pyridoxal-5′ -phosphate and free amine were determined by linearscan cyclic voltammetry. The acid-base dissociation constant of the imine group was obtained by spectrophotometric measurements. Absorption bands for the free imine and the conjugate acid are characterized by a log-normal distribution. These studies are for general application to the quantitative characterization of non-enzymatic model compounds.
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  • 82
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    Journal of Physical Organic Chemistry 4 (1991), S. 387-398 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The tendency of ion-pair intermediates generated by treatment of trans-10-bromo-10, 11-dihydro-11-hydroxy-5H-dibenzo [a, d] cycloheptene (5) with HBr to release Br2 giving 5H-dibenzo [a, d,] cycloheptene (1) has been evaluated. The observed 9:1 ratio between ion-pair collapse to the trans-dibromide (7) and Br2 release to olefin 1, as compared with the 3:7 ratio found with the 5H-dibenz [b, f] azepine-5-carbonyl chloride system, has been interpreted as indicating a much lower propensity for reversibility of ion-pair formation in the bromination of 1. The structural parameters obtained by x-ray diffraction of the dibromide 7 show for this compound an angle strain at C-10 and C-11 analogous to that found in trans-10, 11-dibromo-10, 11-dihydro-5H-dibenz [b, f,] azepine-5-carbonyl chloride. Olefin 1 adds Br2 in 1,2-dichloroethane at 25°C according to a third-order rate law, with k3 = 30(3) M-2 s-1. About 10% of 5-bromo-5H,-dibenzo [a, d] cycloheptene, arising from collapse of a dibenzo [a, d] tropylium ion intermediate (9), is formed in this reaction besides the expected trans-dibromide 7. Spectral evidence for the formation of 9, arising by rearrangement of a first formed weakly bridged ionic intermediate in the reaction of 1 with Br2, is presented. the results obtained with the 5H-dibenzo [a, d,] cycloheptene system have been rationalized on the basis of the formation of very weakly bridged intermediates having a much lower tendency to revert back to olefin and Br2 with respect to the corresponding symmetrically bridged bromonium ions.
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  • 83
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    Journal of Physical Organic Chemistry 4 (1991), S. 404-412 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Treatment of o-3-(but-3-enyloxy)benzene (1) with anions -SPh, -PPh2 and -PO(OEt)2 in ammonia, acetonitrile or dimethyl sulphoxide gives both direct substitution and cyclized products. The formation of the latter confirms the intermediacy of the o-3-(but-3-enyloxy)phenyl radical (2) and confirms that the reactions follow the SRN1 pathway. The kinetics of the coupling of the aryl radical 2 were determined by comparison with the known rate of cyclization of 2. Similar experiments with the neopentyl bromide, 6-bromo-5,5-dimethylhex-1-ene, and its 3-oxa derivative also gave cyclized and uncyclized products, thus confirming that the SRN1 mechanism applies to the reactions. Approximate rate constants for the coupling of a neopentyl radical with the nucleophiles -SPh and -PPh2 have thus been determined.
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  • 84
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    Journal of Physical Organic Chemistry 4 (1991), S. 459-462 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Photolysis of biphenyl-3,4′ -diazide in a rigid glassy matrix at 77 K yields quintet state biphenyl-3,4′ -dinitrene with zero-field splitting parameters of |D/hc- = 0.153 cm-1 and |E/hc| = 0.019 cm-1 determined by electron spin resonance spectroscopic studies. Curie plot studies are consistent with assigning the quintet to be the ground state in this species. This finding confirms qualitative connectivity-based predictions for this general connectivity type of openshell system, and is in qualitative agreement with spectral INDO-CI computational predictions for both planar and twisted geometries of the dinitrene.
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  • 85
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    Journal of Physical Organic Chemistry 4 (1991), S. 420-438 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A series of diterpenoids whose C-ring was an α-bromocyclohexanone showed a discontinuity in the relationship between the Br—C—C=O torsion angle and the Br…O contact distance at about 50°, below which the Br…O distance remained effectively constant at about 3.0 Å. This effect is caused by the non-compressibility of the van der Waals radii of the Br and O atoms. Analysis of the parameters for all compounds in the Cambridge Data File containing an α-bromocyclohexanone ring confirmed this phenomenon. Once the Br…O distance approaches 3.0 Å, the Br—C—C and C—C=O angles are forced open to accommodate the Br…O compression strain as the torsion angle is further reduced. An approximate value of the Br…O distance, d in Å, can be estimated for any torsion angle, ø, by the empirical equation d2 = R - S cos φ + T cos2 φ where R = 11.73, S = 3.62 and T = 0.75. There is a linear relationship between the Br…O separation and both the infrared vibration frequency v and the ultraviolet absorption wavelength δmax of the C=O group: the greater the distance, the lower is the frequency v and longer the wavelength δmax. Thus measurement of the ultraviolet and infrared spectra of a compound containing an α-bromocyclohexanone system can yield information about the Br…O distance and hence the conformation of the six-membered ring. There are also systematic trends in the effect on the spectroscopic properties of the keto group that accompany changing the halogen from fluorine through to iodine. When the halogen is axial, its electronegativity has a maximum influence on the C=O vibration frequency by a ‘through-bond’ inductive process. When the halogen is equatorial, it exerts a steric compression on the non-bonded electrons in the 2p orbital of the oxygen atom. This enhances hybridization to the sp2 state and this simultaneously strengthens the C=O bond and enlarges the n → π* energy gap. This ‘through-space’ influence is simply a process of steric compression, dependent on the size of the halogen atom. It accounts well for all of the systematic trends in both infrared and ultraviolet frequencies of the C=O group when the halogen is equatorial and eclipses the C=O bond.
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  • 86
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    Journal of Physical Organic Chemistry 4 (1991) 
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    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 87
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    Journal of Physical Organic Chemistry 4 (1991), S. 467-478 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The previously developed general method (IDME), which includes calculation of electrostatic and solvation effects on conformational equilibria, has been extended to molecules containing double bonds and conjugated systems. It was applied in connection with MM2 studies to the calculation of the charge distributions and dipole moments, and to the conformational energies of some β-substituted cyclohexenes, exo-methylenecyclohexanes and benzocycloheptenes. Conformational equilibria and dipole moments were well calculated. It was concluded that electrostatic interactions do not play a major role in determining conformational preferences in these compounds. On the other hand, the energies of the preferred conformations of the ten-membered ring in (E)-3β-acetoxy-5,10-seco-1(10)-cholesten-5-one are poorly calculated without the IDME procedure, and fairly well with it.
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  • 88
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Two mechanistic pathways have been proposed for halogen atom transfer from the benzylic positions of halomethylarenes to triorganotin radicals. These are direct atom abstraction, which might involve an extremely polar transition state, and single electron transfer followed by bond cleavage. AM1 semi-empirical calculations have been utilized to model the rate-determining step of these processes. A wide range of related families of compounds have been studied, including substituted halomethylbenzes, selected halomethyl-substituted polycyclic aromatic hydrocarbons and oxygen- and nitrogen-containing chloromethyl-substituted heteroaromatic systems. Although these calculations are relatively simple, the present results corroborate the view that chlorine and bromine atom transfer from the benzylic position to triorganotin radicals involves a direct atom abstraction in the rate-determining step whereas reduction of the corresponding iodides proceeds via an electron-transfer mechanism.
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  • 89
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    Journal of Physical Organic Chemistry 4 (1991), S. 516-522 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydrolysis of dimethyl aryl phosphates, (MeO)2P(O)OAr where the ArO group is derived from N-methyl-8-hydroxyquinolinium or 4-hydroxy-(N,N,N-trimethylanilinium) ion, proceeds with the cleavage of the P—OAr and Me—O bonds. The effect of external factors on the relative contributions of those two reaction pathways was studied. The increase in temperature favours, owing to the difference in activation entropies, the reaction at carbon. The addition acetone to the aqueous reaction medium slows down the reaction at carbon more rapidly than the reaction at phosphorus, indicating higher hydration requirements of the dealkylation transition state. In water-triffuoracetic acid mixtures only the P—OAr bond cleavage is acid catalysed; the catalytic effect is superimposed on the general, decelerating medium effect. Specific nucleophilic catalysis was demonstrated for the Me—O bond fission by thiosulphate ion (ca 60-fold acceleration), and for the reaction at phosphorus by fluoride ion (ca 500-fold acceleration).
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  • 90
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    Journal of Physical Organic Chemistry 4 (1991) 
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    Keywords: Organic Chemistry ; Physical Chemistry
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  • 91
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    Journal of Physical Organic Chemistry 4 (1991), S. 523-525 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: 1,2-Bis(phenylseleno)benzene reacted with equimolar amount of bromine to yield a trigonal bipyramidal (TB) adduct, in contrast to the molecular complex (MC) structure of selenanthrene with bromine. Whereas the structure of 1,4-(PhSeBr2)2C6H4 was (TB, TB) at the two selenium atoms, that of the 1,2-derivative was demonstrated to be (TB, MC), the first example of an MC structure of PhSeC6H4Y with Br2 [Y = 2-PhBr2Se(TB)]. Both electronic and steric effects of the group must play an important role in MC formation. Rapid structural exchange in this tetrabromide, (TB, MC) ⇄ (MC, TB), was also observed.
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  • 92
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    Journal of Physical Organic Chemistry 4 (1991), S. 536-540 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Empirical linear relationships between standard Gibbs free energy, ΔG298°, and parachor and between standard entropy, S298°, and parachor are presented for the aliphatic series H(CH2)nX with X = CH3, CH=CH2, C≡CH, Cl, OH, SH, SCH3 and SC2H5. In the series with X = OH and SH significant deviations of points for n = 4 and 5 are observed. INDO quantum-chemical analysis points to the formation of cyclic conformers of butan-1-ol, pentan-1-ol and their corresponding thio analogues. The formation energy of the cyclic conformers (higher for thiols than for alcohols) is proportional to the sum of deviations from the linearity of the points under discussion.
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  • 93
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    Journal of Physical Organic Chemistry 4 (1991), S. 527-535 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Solvation is the main driving force of electrophilic bromination since it is impossible to obtain a bromonium ion from bromine and alkene in the gas phase, whereas it is a very fast reaction in solution. The role of a protic solvent in this addition was investigated experimentally by extended Winstein-Grunwald relationships, kinetic solvent isotope effects and R, the rate ratios in two solvents of similar ionizing powers but different nucleophilicities. It is shown that electrostatic medium effects and electrophilic assistance to bromide ion departure are the main rate-determining factors of the reaction. These two contributions are roughly independent of the double bond substituents. Nucleophilic solvent assistance to positive charge development is also found; however, it provides only a small acceleration, the magnitude of which depends on alkene structure. This nucleophilic solvent involvement is annulled when crowded substituents inhibit approach of the solvent to the cationic part of the transition state or when positive charge is delocalized by conjugated electron-donating groups. These several solvent roles are identical in nature and in magnitude with those observed in heterolytic solvolysis. In halogenated solvents, the driving force of bromination arises from catalysis by a second bromine molecule which assists heterolysis of the bromine-bromine bond, leading to the bromonium-tribromide ion pair. Similar halogen catalysis occurs also in some solvolyses. Finally, return is also found in both reactions; reversible formation of bromonium ions is observed when their nucleophilic attack, the productforming last step, is made energetically difficult either by steric inhibition or by poor nucleophilicity of the trapping nucleophiles. Similarities and differences between electrophilic bromine addition and limiting solvolysis are discussed in terms of respective intermediate stabilities and heats of formation.
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  • 94
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    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Direct irradiations of 2-chloro-6,7 : 8,9-dibenzotricyclo [3.2.2.02,4] nona-6,8-diene (5-Cl) were conducted in cyclohexane, acetic acid and wet acetonitrile. The products are 1-methylfluoranthene (8) in all three solvents, the allylic chlorides 8-chloro-7-methylene-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5-diene (3-Cl) and 7-chloromethyl-2,3 : 5,6-dibenzobicyclo [2.2.2] octa-2,5,7-triene (2-Cl) in acetic acid and acetonitrile and solvolysis product amides in wet acetonitrile. Compound 5-Cl had previously been shown to be the product of triplet sensitization of 2-Cl and 3-Cl, so that the singlet reaction reverses that of the triplet. The formation of 8 from 5-Cl was quenched with piperylene, whereas that of the allylic chlorides was not. Quantum yields of products and singlet lifetimes in the three solvents were measured. The solvent effects are discussed. Deuterium-labeling results on the formation of 5-Cl from 2-Cl and of 8 from 5-Cl are reported.
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  • 95
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    Journal of Physical Organic Chemistry 3 (1990), S. 17-22 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A large number of 1,n-bis(alkylthio)alkanes, R′S(CH2)nSR″, was investigated by pulse radiolysis in aqueous solutions. The sulphur-centred radical cations, which were obtained on oxidation of the dithia compounds by ·OH radicals, are stabilized via 2σ/1σ* three-electron interaction between two sulphur atoms. Intramolecular stabilization is the only process observed for n = 2-4, whereas for longer chain compounds with n 〉 4 some intermolecular sulphur-sulphur interaction is also indicated. The stability and yields of the three-electron-bonded species also depend on the nature of the substituents R′ and R″, and for the intermolecular systems are a function of solute concentration. The results corroborate earlier investigations and demonstrate, in particular, that the optical parameters are a very sensitive measure of changes in electron density and structure of the three-electron-bonded systems.
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  • 96
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    Journal of Physical Organic Chemistry 3 (1990), S. 23-34 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The stereochemistry and mechanism of solvolysis of optically active tert-butylphenylphosphinothioic-O-trifluoromethanesulphonate (1) in solvents of different ionizing power were studied. It was found that in solvents of high ionizing power and low nucleophilicity 1 ionizes with the formation of a phosphathiacylium cation (2) as the reaction intermediate. Product resulting from the reaction of 2 with anisole was isolated and characterized.
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  • 97
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    Journal of Physical Organic Chemistry 3 (1990), S. 35-37 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Studies on the stereochemistry of silver-ion-assisted solvolysis of a new optically active tert-butylphenylphosphinothioic iodidate demonstrated that the reaction proceeds with inversion of configuration at the phosphorus atom. Evidence for phosphathiacylium cation formation as a reactive intermediate is presented.
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  • 98
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    Journal of Physical Organic Chemistry 3 (1990), S. 38-40 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Studies on the photochemically induced hydrogen-tritium exchange in o-methyl-substituted phenyl ketones are reported. It is shown that the tritium exchange can be beneficial for probing transient photoenol formation, particularly when photochemical side-reactions are occurring.
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  • 99
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Alkaline hydrolysis of diethyl alkylphosphonates and alkyl diethylphosphinates was studied in aqueous dimethyl sulphoxide. The behaviour of these acyclic phosphorus esters is very similar to that of some cyclic phosphorus esters. Rate constants were measured at three temperatures over a 30°C range, and the activation functions of the reaction were estimated. The basic hydrolytic process was proved to be a bimolecular AE reaction. Multiple regression analyses involving rate constants and substituent parameters gave, as a rule, poor results using steric constants derived from carbon compounds. However, the substituent steric effect on the rate of hydrolysis of phosphonates and phosphinates studied correlated very well with ΔΔE, representing the difference in steric energy calculated by molecular mechanics. It is attributed to the various degrees of susceptibility of carbon and phosphorus atoms to the steric hindrance of the substituents. Moreover, Newman's rule of six for the hydrolytic reaction of carboxylates was completely eliminated in phosphorus ester hydrolysis.
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  • 100
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    Journal of Physical Organic Chemistry 3 (1990), S. 62-68 
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The free energy dependence of the kinetic deuterium isotope effect of proton-transfer reactions in solution is usually analysed in terms of the Marcus model. By means of modern computer techniques, the more general quantumstatistical mechanical model is also amenable to routine application. The differnces in the basic assumptions of the two approaches concerning the kinetic isotope effect are discussed. Reaction series for which both the Brønsted relation and the free-energy dependence of the kinetic isotope effect are available are analysed by means of the two models using one set of parameters for both free-energy relations.
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