bluuebunny/crossref_metadata_2025_split huggingface dataset:
A magnetic resonance study on the Mn(II)‐poly(methacrylic acid) interaction
Authors: Westra S. W. T. , Leyte J. C. 
Published: September 2010

AbstractThe interaction of Mn2+ with poly(methacrylic acid) in aqueous solutions is studied as a function of the polymer charge density using 1H‐nuclear magnetic relaxation data, Mn2+ X‐band ESR spectra and IR spectra of the polymer carboxylate groups. A model comprising at least two phases (associated and free Mn2+ ions) is necessary for a consistent interpretation of the experimental results. As no evidence for direct site binding was found, the metal ions are considered to be hydrated and mobile in both phases. In the associated phase the interaction with the charged macromolecule is seen to reduce the mobility of the fully hydrated Mn2+ ions with respect to the free, unperturbed ions.The results obtained here are compared with an earlier study of the interaction of Mn2+ with poly(acrylic acid) a polymer that is similar to poly(methacrylic acid) in many respects. If both polymers bear sufficient charge, their influence on the Mn2+ ions in the associated phase is not too different. In the region of low polymer density, where densely coiled conformations prevail for poly(methacrylic acid) a dramatic reduction of the mobility of the metal ions contrasts sharply with the moderate influence of poly(acrylic acid).
Substituted imidazoles as ligands for copper(I) and copper(II). Stabilisation of copper(I) in case of bulky substituents
Authors: Hendriks H. M. J. , Reedijk J. 
Published: September 2010

AbstractIn a search for model compounds of copper‐containing proteins several Cu(II) and Cu(I) chloride complexes with 2‐isopropylimidazole, benzimidazole, 1‐benzylimidazole and 2‐phenylimidazole have been synthesized and characterized with the help of analytical data, IR, far‐IR, EPR and electronic spectra and magnetic susceptibility measurements. The Cu(I) complexes were produced either with anionic ligands or with the neutral ligands and chloride as counter ion. Some of these Cu(I) compounds are very stable, others are re‐oxidized in moist air and molecular formulae are given for these oxidation products. It appears that Cu(I) compounds are the more stabilised when bulky substituents are present in the ligands.
Chemistry of peroxyacyl nitrates II: Reactions in alcoholic solution; formation and subsequent conversion of peroxyacid
Authors: van Swieten A. P. , Louw R. 
Published: September 2010

AbstractPeroxycapronyl nitrate 1 decomposes at equal rates in methanol and 2‐propanol, significantly faster than in cyclohexane. There appears to be no direct reaction between (aqueous) alcohol and 1, however. When (other) nucleophiles are absent, O‐NO2 homolysis is the rate‐determining step; RCO3* radicals abstract hydrogen from solvent to give peroxycarboxylic acid RCO3H (2). This largely survives reaction in 2‐propanol, but in methanol it is converted into methyl caproate. This process is found to be acid‐catalysed.Peroxycaproic acid 2 also appears to react rapidly with NO2 ‐ whether in alcoholic or cyclohexane solution ‐ very probably via a free‐radical mechanism.Rates and products of reaction of 1 in (aqueous) tert‐butanol closely resemble those in cyclohexane.
Studies on polypeptides XXX. Replacement of histidine‐12 by β‐(4‐pyridyl)‐L‐alanine in RNase S': (Preliminary communication)
Authors: Hoes C. , Hoogerhout P. , Bloemhoff W. , Kerling K. E. T. 
Published: September 2010

Abstract[β‐(4‐pyridyl)‐L‐alanine12]RNase S‐peptide 1–14 was synthesized by the solid‐phase method. The peptide was found to bind strongly to RNase S‐protein; this binding was accompanied by specific changes in the UV spectrum of the pyridyl chromophore. The RNase S' analogue showed appreciable activity (28–34%) when acting on RNA; lower activities (0.4–4%) were observed using CpC and 2′,3′‐CMP as the substrate.
Metal catalysis in oxidation by peroxides. Part 5. On the mechanism of vanadium(V) catalysed oxidation by hydrogen peroxide
Authors: di Furia Fulvio , Modena Giorgio 
Published: September 2010

AbstractThe oxidation of p‐chlorophenyl methyl sulphide and of a series of phenyl substituted aryl methyl sulphides with hydrogen peroxide in the presence of catalytic amounts of bisacetylacetonato‐oxovanadium(IV) [VaO(acac)2] in ethanol, at 25°C, has been studied. The reaction affords the parent sulphoxide in quantitative yield. Oxidation rates are little affected by substitution in the phenyl ring of the sulphide. Comparison with vanadium catalysed oxidation of sulphides by tert‐butyl hydroperoxide shows that the Va(V)‐H2O2 system is much more effective than Va(V)‐ButO2H system. The initial rates of the reaction, which is first‐order in sulphide and in vanadium catalyst, show a rather complicated dependence on the initial concentration of the peroxide. Initial rates are found to increase with increasing [H2O2]0 at low peroxide initial concentration, then decrease at higher [H2O2]0; both the positive and the negative dependence show a fractionary kinetic order. Also on the ground of a kinetic investigation of the Va(V) catalysed autodecomposition of H2O2, which proceeds at measurable rate only at much higher Va(V) concentrations than those employed in the oxidation experiments, the involvement of two peroxyvanadium(V) species in the oxidation mechanism, corresponding to the equilibrium addition of one and two molecules of H2O2 to Va(V), respectively, is suggested. Whereas the mono‐adduct is believed to be a strong oxidant, the di‐adduct appears to behave, in our system, as a much weaker oxidizing agent. This is likely related to the fact that, in the conditions adopted, the di‐adduct may be largely ionized to its monoanionic form.
Photoisomérisation des quelques cyclohexadiènes et heptatriènes. Photocycloaddition des hexatriènes
Authors: Courtot Pierre , Rumin René , Salaün Jean‐Yves 
Published: September 2010

AbstractPhotolytic ring opening of cis and trans‐1,5‐diphenyl‐6‐méthylcyclohexa‐1,3‐dienes yields a unique primary triene with a geometry determined by the cyclodiene preferred conformation in the ground state.Irradiation of 3 isomers of 1,5‐diphenylhepta‐1,3,5‐triene with a cis‐central double bond (Z, Z, E; E, Z, E and E, Z, Z) yields with a complete stereospecificity 3 bicyclo[3.1.0]hexenes (respectively endo‐Me‐endo‐Ph; endo‐Me‐exo‐Ph and exo‐Me‐exo‐Ph). Kinetic measurements have shown that these internal photocycloadditions are of the type [4πa + 2πa], which is forbidden by Woodward‐Hoffmann rules. In all of the three cases studied by us, orbital factors seem anyway to play a major role compared to steric constraints (an endo‐endo photoproduct is formed).A kinetic study of the relative cis‐trans photoisomerization rates of carbon double bonds of the eight isomers of 1,5‐diphenylhepta‐1,3,5‐triene has been performed.
The oxidation of 7‐(p‐X‐phenyl)pteridin‐4‐ones (X = Me, H, Br, CN, NO2) with free and immobilized xanthine oxidase
Authors: Tramper J. , Nagel A. , van der Plas H.C. , Müller F. 
Published: September 2010

AbstractBovine milk xanthine oxidase was used as a catalyst for the oxidation of various 7‐(p‐X‐phenyl)pteridin‐4‐ones into 7‐(p‐X‐phenyl)lumazines. On a preparative scale the derivatives with X = Br, CN and NO2 were most conveniently oxidized with the free enzyme in a solution continuously fed with concentrated substrate. The lumazines obtained separated during the reaction and were easily isolated in good yield (> 90%) and high purity by filtration. The compounds with X = H and OMe, on the other hand, were more conveniently oxidized with the immobilized enzyme, i.e. xanthine oxidase adsorbed to 1‐octyl‐substituted Sepharose 4B. Immobilized xanthine oxidase was used either as a suspension in a stirred batch reactor which was continuously fed with concentrated substrate, or packed in a column through which substrate solution was recirculated. After completion of the reaction the filtrate of the suspension, or the concentrated recirculation solution, respectively, were acidified and the product was collected in good yield (> 90%) and high purity by filtration. The rate of oxidation was found to be influenced by the substituent X. The more electron attracting X, by either resonance or induction, the lower the reaction rate. Accordingly, a negative reaction constant ρ of ∼ −0.5 was calculated from the data for the free and immobilized enzyme. The enzyme‐substrate complex formation, as reflected in the Michaelis constant Km, is largely determined by the hydrophobicity of the phenyl group.
Synthesis and structures of 2,4,5‐trialkyl‐2,3,4,5‐tetrahydropyrimidines and 2,5,6,8,9‐pentalkyl‐1,3,7‐triazabicyclo[3.3.1]non‐3‐enes
Authors: Oude‐Alink B. A. M. 
Published: September 2010

AbstractThe reaction products resulting from deammoniation of 2,4,6‐trialkylhexahydro‐1,3,5‐triazine (1a‐c) derived from n‐alkyl aldehydes (propanal, butanal and pentanal) have been identified as 2,4,5‐trialkyl‐2,3,5‐tetrahydropyrimidines and 2,5,6,8,9‐pentalkyl‐1,3,7‐triazabicyclo[3.3.1]non‐3‐enes.
Convenient preparations of ω‐substituted picoline N‐oxides utilising triphenylpyridinium intermediates
Authors: Katritzky Alan R. , El‐Mowafy Azzahra M. , Patel Ranjan C. 
Published: September 2010

Abstract3‐ and 4‐Picolylamines are converted by 2,4,6‐triphenylpyrylium cation into pyridinium salts which can be N‐oxidised in high yield. The resultant N‐oxopyridinium salts react with pyridine and with sodium thiophenoxide to give ω‐substituted 3‐ and 4‐picoline N‐oxides in high overall yields.
3‐Methoxypropen‐2‐yl formate, a selective reagent for the formylation of alcohols or phenols
Authors: Scheeren J. W. , Bats H. 
Published: September 2010

AbstractAn improved synthesis is given for the selectively amino formylating reagent, 3‐methoxypropen‐2‐yl formate. The compound can also be used for selective formylation of either alcohols or phenols by using an acid or tertiary amine as a catalyst.
MO‐SCF calculations on protonated butatriene and 1,2,3‐pentatriene
Authors: Mirejovsky D. , Drenth W. , van Duijneveldt F. B. 
Published: September 2010

AbstractEnergies are calculated for the cations obtained by protonation of butatriene and 1,2,3‐pentatriene at various positions. The relative stabilities of the various ions are discussed. The ion of lowest energy is an allylic cation obtained by rotation around a CC bond after or concomitant with protonation. Experimentally, instead of this ion the next higher in energy is formed. The latter is a propargylic ion, formed by proton attack at the terminal carbon atom. Apparently, rotation is not appreciable in the transition state of the experimental protonation.
A method for the quantitative determination of juvenile hormones in insects
Authors: Huibregtse‐Minderhoud L. , van der Kerk‐Van Hoof A. C. , Wijkens P. , Biessels H. W. A. , Salemink C. A. 
Published: September 2010

AbstractA method to determine the content of the juvenile hormones in insects as low as 0.2 nanogram per sample was developed. The juvenile hormones were transformed in their 10‐hepta‐' fluorobutyrate‐11‐methoxy‐derivatives and simultaneously determined by capillary gas chromatography with electron capture detection.
Photoreactions of o‐vinyltriphenylethylenes II. Photochemistry of 1,1‐diphenyl‐2‐(o‐vinylphenyl)‐ and 1,1‐diphenyl‐2‐(o‐propenylphenyl)ethylene
Authors: Sindler‐Kulyk M. , Laarhoven W. H. 
Published: September 2010

AbstractIrradiation of 1,1‐diphenyl‐2‐(o‐vinylphenyl)ethylene (3) leads stereoselectively to endo‐4,5‐diphenylbenzobicyclo[3.1.0]hex‐2‐ene (6). The analogous reaction of the homologous (o‐propenylphenyl)ethylene derivative (4) seems to be less stereoselective, as products 10 and 11 could be isolated. In both cases a photodehydrocyclisation product (7 and 12, respectively) is also found. Possible mechanisms for the photocyclisation reactions are discussed, and the photochemical behaviour of 3 and 4 is compared with that of other o‐vinylstilbenes, previously investigated.
The photosensitized oxidation of the 19‐norsteroids: 17α‐19‐norpregn‐5‐en‐20‐yne‐3β,17‐diol, 3β‐chloro‐17α‐19‐norpregn‐5‐en‐20‐yn‐17‐ol and 17α‐19‐norpregn‐5‐en‐20‐yn‐17‐ol
Authors: Peters J. A. M. , van Vliet N. P. , Zeelen F. J. 
Published: September 2010

AbstractPhotosensitized oxygenation of 19‐norsteroids with a 5–6 double bond proceeds with high stereoselectivity to give, after reduction of the initially formed hydroperoxides, 5α‐hydroxy 6‐enes as the major products. Higher amounts of 6β‐hydroxy 4‐enes, the minor products, are formed than in the case of cholesterol. These results are in agreement with the “ene” mechanism proposed by Schenck and Nickon and not with the peroxirane mechanism proposed by Dewar and Thiel.
Synthesis, characterisation and complex formation of N,N,N′,N′‐tetrakis‐(2‐benzimidazolylmethyl)‐1,2‐ethanediamine, a new hexadentate ligand
Authors: Hendriks H. M. J. , ten Bokkel Huinink W. O. , Reedijk J. 
Published: September 2010

AbstractThe synthesis and complex formation of a new hexadentate, nitrogen‐donor ligand are described. The ligand N,N,N′,N′‐tetrakis(2‐benzimidazolylmethyl)‐1,2‐ethanediamine (EDTB) is prepared by condensation of 1,2‐benzenediamine and ethylenediaminetetraacetic acid. This chelate ligand appears to be able to solvate first‐row transition‐metal ions in an octahedral manner, resulting in species M(EDTB) (anion)2 and M2(EDTB) (anion)4. The products have been analysed and characterised by spectroscopic and magnetic techniques (EPR, NMR, ligand‐field and IR).During recent years much interest has arosen in the bioinorganic chemistry of first‐row transition‐metal ions, in particular with respect to the relationships between the activity and structural properties of metalloproteins, such as hemocyanin, tyrosinase, laccase, cytochrome‐c‐oxidase, superoxide dismutase and carbonic anhydrase.1–3Nitrogen‐donor ligands such as imidazoles of polypeptidyl histidines have been shown to play an important rǒle in the coordination of the metal ions in such proteins. 4,5 Furthermore, the stereochemistry imposed by the rigid structure of the protein seems to be responsible for some of the characteristic properties and the activity of metalloproteins.6
Studies on polypeptides XXXI. An approach to the chemical semisynthesis of ribonuclease A analogues. Part I: The preparation of N‐acetimidoyl‐S‐protein
Authors: Hoogerhout P. , Bloemhoff W. , Kerling K. E. T. 
Published: September 2010

AbstractThe limited proteolysis of acetimidated ribonuclease A by subtilisin has been investigated. Evidence of the cleavage being restricted to the Ala20–Ser21 bond is presented. N8ε‐acetimidyl‐S‐protein, an intermediate for the chemical semisynthesis of ribonuclease A analogues, can be obtained in an over‐all yield of 46% starting from ribonuclease A.A new method to follow the subtilisin‐catalyzed cleavage reaction of ribonuclease A and derivatives, applying reverse‐phase high‐performance liquid chromatography, is reported.
X‐ray structure determination of the phosphite triester of methyl β‐D‐ribopyranoside; CNDO‐2 calculation
Authors: Bellaart A. C. , van Aken D. , Buck H. M. , Stam C. H. , van Herk A. 
Published: September 2010

AbstractThe phosphite triester (6), the O,O,O‐thiophosphate triester (7), and the phosphate triester (8) of methyl β‐d‐ribopyranoside were prepared. Crystal structures of 6 and 7 were determined by X‐ray diffraction. Quantum‐chemical calculations were performed on 6. They clearly demonstrate that the positive charge on phosphorus is relatively high. This high charge is a result of the strain in the ester moiety of 6.
On the formation of allenes in the steroid series IV. Synthesis and structure of several 3‐methoxy‐19‐nor‐1,3,5(10)‐17(20), 20‐pregnapentaenes, substituted at C(21)
Authors: van Dijck L.A. , Lankwerden B.J. , Vermeer J.C.G.M. 
Published: September 2010

AbstractPairs of steroid 17(20),20‐allenes, epimeric at C(21), have been obtained by LiAlH4(D4)/AlCl3 reduction of 17α/β‐hydroxy‐17β/α‐alkynyl steroids, confirmed by means of optical rotation differences and NMR spectroscopy. In all cases the reduction proved to be fully stereospecific, confirming the cis‐SN2′ mechanism, postulated in part II1.In addition, the NMR spectra of 17‐(trans‐1‐alkenyl) 17‐hydroxy compounds, obtained as by‐products or especially prepared, have been analysed. Finally the structure of a dideuterioallene, tentatively described as a 16β,21α‐dideuterio‐17(20),20‐allene in part III2, has been proved by full analysis of the 300 MHz spectra of allenic model compounds.
Reactions d'addition nucléophile sur les cétones. Rǒle des facteurs stériques à longue distance dans les réactions d'addition de l'ion sulfite et de l'hydroxylamine
Authors: Boyer B. , Lamaty G. 
Published: September 2010

AbstractRate constants have been measured for the addition of sulphite ion and hydroxylamine on a series of sterically hindered cyclohexanones as well as for the decomposition of the bisulphite adducts. The observed differences in reactivity mau be regarded as resulting from the intervention of the “steric compression effect” due to the axial substituents. Calculated δΔG≠ values confirm the existence of such an effect and the results are consistent with the intermediate formation of a “reactant‐like” transition state in these reactions.
A combined study of vicinal and geminal carbon‐proton spin‐spin coupling constants as a tool in conformational analysis. The conformation of 2,6‐dichloro‐1,4‐oxathianes
Authors: Spoormaker Tom , de Bie Marius J. A. 
Published: September 2010

Abstract13C FT single resonance spectra are obtained for two isomers of 2,6‐dichloro‐1,4‐oxathiane; one of the isomers is a racemic mixture. From a combined study of the various long‐range carbon‐proton coupling constants the conformations of both isomers could be determined. Each isomer exists in two conformers which are in a rapid interconversion equilibrium. Contrary to predictions based on the “anomeric‐effect”, cis‐2,6‐dichloro‐1,4‐oxathiane in the liquid phase is predominantly in the cis‐di‐equatorial chair form.
Thiophilic additions of functionalized carbanions to sulfines
Authors: Loontjes J. A. , van der Leij M. , Zwanenburg B. 
Published: September 2010

AbstractThe reaction of three types of sulfines with functionalized carbanions is described. Di‐p‐tolylsulfine 1 reacts with excess of MeSOCH2Na at the sulfine sulfur atom (thiophilic addition) to give Tol2CHS(O)CH2SOMe (Tol = p‐MeC6H4). Similarly, acetonitrile with excess of sodium amide adds thiophilically. N,N‐Diethyl χ‐lithio‐n‐butyramide and dilithio‐n‐butyric acid also form thiophilic addition products. Methyl p‐tolyl sulfone in the presence of BuLi and excess of t‐BuOK affords the adduct Tol2CHS(O)CH2SO2Tol, whereas lithiomethyl p‐tolyl sulfone furnishes alkenesulfone Tol2CCHSO2Tol. It is presumed that the primary adduct rearranges via a thiocarbonyl ylide and an episulfide. From α,α‐dilithiobenzyl cyanide Tol2CC(Ph)CN is obtained by a similar rearrangement. α‐Lithioisopropyl p‐tolyl sulfone with sulfine 1 gives the olefin Tol2CCMe2 which arises from a Ramberg‐Bäcklund type rearrangement of the primary thiophilic adduct. Similarly, p‐tolyl(phenylthio)sulfine 2 and p‐tolyl(phenylsulfonyl) sulfine 3 give thiophilic addition products with some of the carbon nucleophiles mentioned above. The mechanism of the thiophilic reaction is discussed.
Synthesis, crystal structure and properties of the spin singlet six‐coordinated Ni(II) compound Ni(Et2dtc)2(AsI3)2
Authors: Willemse J. , Cras J. A. , Bosman W. P. , Steggerda J. J. 
Published: September 2010

AbstractThe reaction of Ni(R2dtc)2 (R  Et, Bu) with AsI3 yields a product which could be characterised as Ni(R2dtc)2(AsI3)2. The crystal structure of the compound with R  Et [space group P21/C, Z = 2, a = 8.428(1), b = 19.692(2), c = 9.194(1) Å, β = 102.24(2)°] revealed that the nickel atom is coordinated by four dithiocarbamato sulfur atoms and two arsenic atoms in pseudo‐octahedral geometry. Magnetic susceptibility measurements show these six‐coordinated Ni(II) compounds to have a spin singlet ground state.
Thin‐film dye sensitization and impurity effects on TiO2 and SrTiO3 electrodes for the photoelectrolysis of water
Authors: Mackor A. , Schoonman J. 
Published: September 2010

AbstractSingle crystals of TiO2 and SrTiO3 are sensitized by thin films of a ruthenium surfactant dye, which is able to sustain catalytic oxidation of water upon irradiation with visible light. Calculated turnover numbers exceed 2000. Doping of the crystals with niobium does not improve the suitability of these semiconductors as photoanodes, contrary to results obtained with polycrystalline Nb‐doped TiO2.
The amination of 3‐bromo‐2‐ethoxy‐1,5‐naphthyridine. A reinvestigation
Authors: van den Haak H. J. W. , van der Plas H. C. 
Published: September 2010

AbstractThe reaction of 3‐bromo‐2‐ethoxy‐1,5‐naphthyridine with KNH2/NH3 was reinvestigated. The procedure, described in the literature for the preparation of the title compound does not lead to this compound but to the isomeric 3‐bromo‐1‐ethyl‐1,5‐naphthyridin‐2(1H)‐one. Reaction of this compound with KNH2/NH3 yields 3‐ and 4‐amino‐1‐ethyl‐1,5‐naphthyridin‐2(1H)‐one, the latter being the main product. The title compound was prepared on reacting 2,3‐dibromo‐1,5‐naphthyridine with sodium ethoxide. A mixture of 3‐ and 4‐amino‐2‐ethoxy‐1,5‐naphthyridine was obtained on amination of 3‐bromo‐2‐ethoxy‐1,5‐naphthyridine. In both cases the intermediacy of the respective 3,4‐didehydro compounds was proposed.
Acidity constants of some aromatic astatine compounds
Authors: Visser Gerard W. M. , Diemer Eduard L. , Kaspersen Frans M. 
Published: September 2010

AbstractThe acidity constants of [211At]‐astatobenzoic acids, ‐phenols, ‐anilines and 5‐astatouracil have been determined by measurement of the distribution coefficients between water and an organic phase as a function of the pH. On the basis of the acidity constants of the phenols and anilines the σm and σp constants for astatine were calculated as +0.25 and +0.18, respectively. From these values it can be concluded that the field (F) effect of astatine is weaker than that of iodine, whereas the resonance (R) effect is about the same.
Synthesis of the 2‐ethyl‐3‐methylsuccinic acidsviaa stereospecific malonic ester alkylation
Authors: van Leeuwen F. F. , Noordam A. , Maat L. , Beyerman H. C. 
Published: September 2010

AbstractReaction of (−)‐(S)‐methyl 2‐bromopropionate (4) (obtained from l‐alanine) with dibenzyl ethylmalonate gave stereospecifically in four steps ( + )‐(2R,3S)‐2‐ethyl‐3‐methylsuccinic acid (8a) and its diastereoisomer (8b). The malonic ester alkylation proceeded with Walden inversion. The chiroptical properties of the acids and the intermediates are in conformity with the known absolute configurations.
Studies on polypeptides XXXII: Solid‐phase synthesis of RNase S‐peptide 1‐14 analogues; replacement of histidine‐12 by β‐(2‐pyridyl)‐L‐alanine and β‐(4‐pyridyl)‐L‐alanine
Authors: Hoes C. , Raap J. , Bloemhoff W. , Kerling K. E. T. 
Published: September 2010

AbstractThe synthesis of two analogues of the N‐terminal tetradecapeptide of ribonuclease A (RNase S‐peptide 1‐14), in which the active site histidine‐12 residue is replaced by β‐(2‐pyridyl)‐L‐alanine and β‐(4‐pyridyl)‐L‐alanine, has been accomplished by the solid‐phase method.The enantiomers of β‐(2‐pyridyl)alanine were obtained by chymotryptic hydrolysis of its racemic Nα‐acetyl ethyl ester. The Boc derivative of both L‐amino acids has been synthesized.