bluuebunny/crossref_metadata_2025_split huggingface dataset:
Isoform‐speciﬁc quantiﬁcation of endothelins in HUVEC culture supernatants by on‐line high‐performance liquid chromatography/electrospray mass spectrometry
Authors: Carrascal Montserrat , Schneider Klaus , Peralta Carmen , Escolar Gines , Gelpi Emilio , Abian Joaquin 
Published: July 2004

AbstractA method for the quantitative analysis of endothelin peptides in human umbilical vein endothelial cell (HUVEC) culture supernatants is reported. The analysis is isoform‐speciﬁc and employs solid‐phase extraction and subsequent HPLC fractionation followed by HPLC‐ESIMS analysis.The peptide vasoactive‐intestinal‐contractor (VIC) was used as internal standard for the HPLC‐ESIMS analysis. Linearity of calibration curves was from 50 fmol to 25 pmol. The limit of detection of the HPLC‐ESIMS step using a buffer matrix was estimated at 50 fmol (S/N > 3). The overall limit of detection for supernatants of HUVEC was 500 fmol/mL. In HUVEC culture supernatants only ions of endothelin‐1 (ET1) were observed. Basal levels were determined to be 1.8 ± 0.3 pmol/mL. Quantitative results obtained for ET1 were in agreement with those obtained by using a standard addition method and by an ELISA method. Copyright © 2004 John Wiley & Sons, Ltd.
A more sensitive MS detector does not obviously lead to a more sensitive assay: experiences with ES‐285
Authors: Stokvis Ellen , Rosing Hilde , López‐Lázaro Luis , Schellens Jan H. M. , Beijnen Jos H. 
Published: July 2004

AbstractIn this paper the transfer of an existing method for the quantitative determination of the anticancer agent ES‐285 in human plasma using liquid chromatography tandem mass spectrometry on an API 365 to an API 3000 mass spectrometer is described. The transfer appeared not to be straightforward. Problems arose resulting from carry‐over and interferences. In addition, due to the expansion of the calibration range, data ought to be weighted with a different factor to increase the accuracy of the lower concentrations. After ﬁnding appropriate solutions for these problems, the lower limit of quantitation could be lowered from 10 to 1 ng/mL for ES‐285 in human plasma. The usefulness and necessity of the modiﬁed assay was demonstrated by analysis of plasma samples from a patient receiving a low dosage of the drug. Copyright © 2004 John Wiley & Sons, Ltd.
Cubane‐Type Mo3CoS4 Molecular Clusters with Three Different Metal Electron Populations: Structure, Reactivity and Their Use in the Synthesis of Hybrid Charge‐Transfer Salts
Authors: Feliz Marta , Llusar Rosa , Uriel Santiago , Vicent Cristian , Coronado Eugenio , Gómez‐García Carlos J. 
Published: July 2004

AbstractHeterodimetallic cubane‐type complexes coordinated to diphosphanes [Mo3CoS4(dmpe)3Cl4]+ ([1]+) (dmpe=1,2‐bis(dimethylphosphanyl)ethane), [Mo3CoS4(dmpe)3Cl4] (1) and [Mo3CoS4(dmpe)3Cl3(CO)] (2) with 14, 15 and 16 metal electrons, respectively, have been prepared from the [Mo3S4(dmpe)3Cl3]+ trinuclear precursor using [Co2(CO)8] or CoCl2 as cobalt source. Cluster complexes [1]+ and 1 are easily interconverted chemically and electrochemically. The CoCl distance increases upon electron addition and substitution of the chlorine atom coordinated to cobalt with CO only takes place in presence of a reducing agent to give complex 2. Structural changes in the intermetallic distances agree with the entering electrons occupying an orbital which is basically MoMo non‐bonding and slightly MoCo bonding. Magnetic susceptibility measurements for [1]+ and 1 are consistent with the presence of two and one unpaired electrons, respectively and therefore with an “e” character for the HOMO orbital. Oxidation of 1 with TCNQ results in the formation of a charge transfer salt formulated as [1]+[TCNQ]− with alternate layers of paramagnetic cluster cations and also paramagnetic organic anions. There is no magnetic interaction between layers and the thermal variation of the magnetic susceptibility has been modelled as a S=${{ 1/2 }}$ TCNQ antiferromagnetic chain plus a S=1 cluster monomer with zero field splitting.
Theoretical Study of the Electrocyclic Ring Closure of Hydroxypentadienyl Cations
Authors: Nieto Faza Olalla , Silva López Carlos , Álvarez Rosana , de Lera Ángel R. 
Published: July 2004

AbstractAt the 6‐311G* level of theory, DFT methods predict that the rearrangement of 1,4‐dihydroxy‐5‐methylpentadienyl cation 1 (R = Me) to protonated trans‐3‐hydroxy‐2‐methylcyclopent‐4‐en‐1‐one 2, an intermediate step in the Piancatelli reaction or rearrangement of furfuryl carbinols to trans‐2‐alkyl(aryl)‐3‐hydroxycyclopent‐4‐en‐1‐one, is a concerted electrocyclic process. Energetic, magnetic, and stereochemical criteria are consistent with a conrotatory electrocyclic ring closure of the most stable out,out‐1 isomer to afford trans‐2. Although the out,in‐1 isomer is thermodynamically destabilized by 6.84 kcal mol−1, the activation energy for its cyclization is slightly lower (5.29 kcal mol−1 versus 5.95 kcal mol−1). The cyclization of the isomers of 1 with the C1‐hydroxy group inwards showed considerably higher activation energies than their outwards counterparts. in,out‐1, although close in energy to out,out‐1 (difference of 1.57 kcal mol−1) required about 10 kcal mol−1 more to reach the corresponding transition structure. The value measured for the activation energy of in,in‐1 (17.32 kcal mol−1) eliminates the alternative conrotatory electrocyclization of this isomer to provide trans‐2. Geometric scrambling by isomerization of the terminal C1C2 bond of 1 is also unlikely to compete with electrocyclization. The possibility to interpret the 1→2 reaction as a nonpericyclic cationic cyclization was also examined through NBO analysis, and the study of bond lengths and atomic charges. It was found that the 1→2 concerted rearrangement benefits from charge separation at the cyclization termini, an effect not observed in related concerted electrocyclic processes, such as the classical Nazarov reaction 3→4 or the cyclization of the isomeric 2‐hydroxypentadienyl cation 5.
Using Noncovalent Intra‐strand and Inter‐strand Interactions To Prescribe Helix Formation within a Metallo‐supramolecular System
Authors: Childs Laura J. , Pascu Mirela , Clarke Adam J. , Alcock Nathaniel W. , Hannon Michael J. 
Published: July 2004

AbstractThe effect of inter‐strand and intra‐strand interactions is explored in a metallo‐supramolecular system in which the metal–ligand coordination requirements may be satisfied by more than one different supramolecular architecture. This is achieved by introducing alkyl substituents onto the spacers of readily prepared bis(pyridylimine) ligands. The alkyl substituents induce twisting within the ligand strand and this intra‐strand effect favours formation of helical architectures. The alkyl substituents also introduce inter‐strand CH⋅⋅⋅π interactions into the system. For the smaller methyl group these are most effectively accommodated in a trinuclear circular helicate architecture. A solution mixture of dinuclear double‐helicate and trinuclear circular helicate results from which, for copper(I), the trinuclear circular helicate crystallises. The CH⋅⋅⋅π interactions endow the circular helicate with a bowl‐shaped conformation and the triangular unit aggregates into a tetrahedral ball‐shaped array. Low‐temperature NMR studies indicate that the CH⋅⋅⋅π interactions also confer a bowl‐shaped conformation on the triangle in solution. The larger ethyl groups can sustain intra‐strand CH⋅⋅⋅π interactions in the lower nuclearity double‐helical system and this is the unique architecture for that ligand system in both solution and the solid state. Crystal structures are described for both the copper(I) and silver(I) complexes. Thus we show that intra‐strand interactions may be used to induce helicity within this system, while the nuclearity of the array can be prescribed by the inter‐strand interactions.
A General and Versatile Approach to Thermally Generated N‐Heterocyclic Carbenes
Authors: Nyce Gregory W. , Csihony Szilard , Waymouth Robert M. , Hedrick James L. 
Published: July 2004

AbstractThe synthesis of N‐heterocyclic carbene (NHC) adducts by condensation of diamines with appropriately substituted benzaldehydes is described. This simplified approach provides the NHC adduct without first having to generate the carbene followed by its protection. These adducts undergo thermal deprotection to generate N‐heterocyclic carbene in situ. Adduct decomposition temperatures were investigated as a function of catalyst structure by using thermal analysis and spectroscopic techniques. Importantly, unlike adducts derived from chloroform, the new pentafluorobenzene‐based adducts are more readily prepared and are stable at room temperature. The utility of these adducts as organic catalyst precursors for living ring‐opening polymerization (ROP) of lactide, transesterification reactions, and the synthesis of N‐heterocyclic carbene ligated organometallic complexes is also described.
Cylindrical Micelles from the Aqueous Self‐Assembly of an Amphiphilic Poly(ethylene oxide)–b‐Poly(ferrocenylsilane) (PEO‐b‐PFS) Block Copolymer with a Metallo‐Supramolecular Linker at the Block Junction
Authors: Gohy Jean‐François , Lohmeijer Bas G. G. , Alexeev Alexander , Wang Xiao‐Song , Manners Ian , Winnik Mitchell A. , Schubert Ulrich S. 
Published: July 2004

AbstractA supramolecular AB diblock copolymer has been prepared by the sequential self‐assembly of terpyridine end‐functionalized polymer blocks by using RuIII/RuIIchemistry. By this synthetic strategy a hydrophobic poly(ferrocenylsilane) (PFS) was attached to a hydrophilic poly(ethylene oxide) (PEO) block to give an amphiphilic metallo‐supramolecular diblock copolymer (PEO/PFS block ratio 6:1). This compound was used to form micelles in water that were characterized by a combination of dynamic and static light scattering, transmission electron microscopy, and atomic force microscopy. These complementary techniques showed that the copolymers investigated form rod‐like micelles in water; the micelles have a constant diameter but are rather polydisperse in length, and light scattering measurements indicate that they are flexible. Crystallization of the PFS in these micelles was observed by differential scanning calorimetry, and is thought to be the key behind the formation of rod‐like structures. The cylindrical micelles can be cleaved into smaller rods whenever the temperature of the solution is increased or they are exposed to ultrasound.
Self‐Assembled Metallocycles with Two Interactive Binding Domains
Authors: Chang Sung‐Youn , Jang Hye‐Young , Jeong Kyu‐Sung 
Published: July 2004

AbstractFive metallocycles 1 a–e have been self‐assembled from S‐shaped bispyridyl ligands 2 a–e and a palladium complex, [Pd(dppp)(OTf)2] (dppp=1,3‐bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including 1H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen‐bonding sites based on pyridine‐2,6‐dicarboxamide units. The binding properties of the metallocycles with N,N,N′,N′‐tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and 1H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. 1H NMR studies on 1 a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1 a and G, showing maximum complexation at ∼0.33 mol fraction of the metallocycle 1 a in CDCl3. The binding constants K1 and K2 are calculated to be 1600 and 1400 M−1 (±10 %), respectively, at 25 °C in CDCl3, indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl3, for comparison purposes the binding properties of the metallocycles 1 b–e were investigated in a more polar medium, 3 % CD3CN/CDCl3. 1H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles.
A New Phototransformation of Methoxycarbonyl‐Substituted (E,Z,E)‐1,3,5‐Hexatrienes: Easy Access to Ring‐Annelated 8‐Oxabicyclo[3.2.1]octa‐2,6‐diene Derivatives
Authors: von Essen Regina , von Zezschwitz Paultheo , Vidović Denis , de Meijere Armin 
Published: July 2004

AbstractOn attempting photochemically induced electrocyclizations of the previously reported 1,6‐bismethoxycarbonyl‐ or 1,6‐bistrimethylsilyl‐substituted ring‐attached (E,Z,E)‐1,3,5‐hexatrienes4 b,4 cand5 b,5 c, equilibrium mixtures of the starting materials and their diastereomers, the corresponding (E,Z,Z)‐hexatrienes4 b,4 cand5 b,5 cwere obtained. The desiredtrans‐disubstituted ring‐annelated cyclohexadienes9 band10 bwere formed by subsequent thermal 6π‐electrocyclization of the (E,Z,Z)‐hexatrienes4 band5 bin good yields (77–83 %). Upon treating the bissilyl‐substituted hexatrienes (E,Z,E)‐5 cor (E,Z,Z)‐5 cunder the same conditions, 6π‐electrocyclizations did occur, but the primary products immediately isomerized to a large extent, and mixtures of the cyclohexane‐annelated cyclohexadienes10 c–12 calong with the dehydrogenation products13,14 cwere formed. When the bismethoxycarbonyl‐substituted hexatriene (E,Z,E)‐5 bwas irradiated for an extended period of time (4.5 h), the gradual formation of the oxabicyclo[3.2.1]octa‐2,6‐diene17 bby a formal intramolecular hetero‐Diels–Alder reaction was observed and17 bcould be isolated in up to 69 % yield. To explore the scope of this new photochemical reaction, the new ring‐attached (E,Z,E)‐hexatrienes4 a,5 aand6 bwere synthesized by twofold Heck reactions from 1,2‐dibromocycloalkenes1–3(59–66 %). While irridiation of the cyclopentene‐attached 1,3,5‐hexatrienes only led to decomposition, the cyclohexene‐ and cycloheptene‐attached hexatrienes gave the hetero‐Diels–Alder products or other photoproducts depending on the size of the cycloalkene moiety and the nature of the alkoxycarbonyl substituents at the vinyl termini. The photoreaction products17 band18 bwhich are bicyclic acetals, underwent cleavage upon treatment with a Lewis acid such as Me3SiOTf to give the ring‐annelated methoxycarbonyl‐substituted troponecarboxylates21 band22 b.
A Novel Dimerization Mode of a Cyclic Ketene Imine
Authors: Langhals Elke , Huisgen Rolf , Polborn Kurt 
Published: July 2004

AbstractThe strained seven‐membered cyclic ketene imine 9, obtained by cycloaddition of thiocarbonyl ylide 6 with 2,3‐bis(trifluoromethyl)fumaronitrile (7), underwent base‐catalyzed dimerization at room temperature on treatment with KCN in acetonitrile or with proton sponge in acetonitrile or CDCl3. Two diastereoisomeric dimers, (6SR,3′RS)‐13 and (6SR,3′SR)‐13, were formed in 1:1 ratio in high yield. X‐ray analysis revealed a deep‐seated structural change which is unrelated to known dimerization pathways of ketene imines. In 13, one of the seven‐membered rings is opened, and attached to the second unit by a thioimidate group. An ionic chain reaction with a formal fluoride ion as transfer agent offers a rationalization.
Solid‐Phase Synthesis of Dihydrovirginiamycin S1, a Streptogramin B Antibiotic
Authors: Shaginian Alex , Rosen Marissa C. , Binkowski Brock F. , Belshaw Peter J. 
Published: July 2004

AbstractWe describe the first solid‐phase synthesis of dihydrovirginiamycin S1, a member of the streptogramin B family of antibiotics, which are nonribosomal‐peptide natural products produced by Streptomyces. These compounds, along with the synergistic group A components, are “last line of defense” antimicrobial agents for the treatment of life‐threatening infections such as vancomycin‐resistant enterococci. The synthesis features an on‐resin cyclization and is designed to allow production of streptogramin B analogues with diversification at positions 1′, 1, 2, 3, 4, and 6. Several synthetic challenges known to hinder the synthesis of this class of compounds were solved, including sensitivity to acids and bases, and epimerization and rearrangements, through the judicious choice of deprotection conditions, coupling conditions, and synthetic strategy. This work should enable a better understanding of structure–activity relationships in the streptogramin B compounds, possible identification of analogues that bypass known resistance mechanisms, and perhaps the identification of analogues with novel biological activities.
A New Color of the Synthetic Chameleon Methoxyallene: Synthesis of Trifluoromethyl‐Substituted Pyridinol Derivatives: An Unusual Reaction Mechanism, a Remarkable Crystal Packing, and First Palladium‐Catalyzed Coupling Reactions
Authors: Flögel Oliver , Dash Jyotirmayee , Brüdgam Irene , Hartl Hans , Reißig Hans‐Ulrich 
Published: July 2004

AbstractAddition of lithiated methoxyallene to pivalonitrile afforded after aqueous workup the expected iminoallene 1 in excellent yield. Treatment of this intermediate with silver nitrate accomplished the desired cyclization to the electron‐rich pyrrole derivative 2 in moderate yield. Surprisingly, trifluoroacetic acid converted iminoallene 1 to a mixture of enamide 3 and trifluoromethyl‐substituted pyridinol 4 (together with its tautomer 5). A plausible mechanism proposed for this intriguing transformation involves addition of trifluoroacetate to the central allene carbon atom of an allenyl iminium intermediate as crucial step. Enamide 3 is converted to pyridinol 4 by an intramolecular aldol‐type process. A practical direct synthesis of trifluoromethyl‐substituted pyridinols 4, 10, 11, and 12 starting from typical nitriles and methoxyallene was established. Pyridinol 10 shows an interesting crystal packing with three molecules in the elementary cell and a remarkable helical supramolecular arrangement. Trifluoromethyl‐substituted pyridinol 4 was converted to the corresponding pyridyl nonaflate 13, which is an excellent precursor for palladium‐catalyzed reactions leading to pyridine derivatives 14–16 in good to excellent yields. The new synthesis of trifluoromethyl‐substituted pyridines disclosed here demonstrates a novel reactivity pattern of lithiated methoxyallene which is incorporated into the products as the unusual tripolar synthon B.
Generation of Cationic [Zr‐{tert‐Butyl Enolate}] Reactive Species: Methyl Abstraction versus Hydride Abstraction
Authors: Lian Bing , Toupet Loïc , Carpentier Jean‐François 
Published: July 2004

AbstractTreatment of the neutral methyl–Zr–enolate [Cp2Zr(Me){O(tBuO)CCMe2}] (1) with one equivalent of B(C6F5)3 or [HNMe2Ph][B(C6F5)4] as a methyl abstractor in THF at 0 °C leads to the selective formation of the free ion pair complex [Cp2Zr(THF){O(tBuO)CCMe2}]+ [anion]− (2) (anion=MeB(C6F5)3−, B(C6F5)4−), which is relevant to the controlled polymerization of methacrylates. Cation 2 rapidly decomposes at 20 °C in THF with release of one equivalent of isobutene to form the cationic Zr–carboxylate species [Cp2Zr(THF)(O2CiPr)]+ (3), through a proposed intramolecular proton transfer process from the tert‐butoxy group to the enolate. The reaction of 1 with one equivalent of B(C6F5)3 or [HNMe2Ph][B(C6F5)4] in CH2Cl2 leads to the direct, rapid formation of the dimeric μ‐isobutyrato–Zr dicationic species [{Cp2Zr[μ‐(O2CiPr)]}2]2+ (4), which gives 3 upon dissolution in THF. Contrastingly, when [Ph3C][B(C6F5)4] is used to generate the cationic Zr–enolate species from 1 in CD2Cl2, a 15:85 mixture of dicationic complexes 4 and [{Cp2Zr[μ‐(O2CC(Me)CH2)]}2]2+[B(C6F5)4]${{{- \hfill \atop 2\hfill}}}$ (5‐[B(C6F5)4]2) is obtained quantitatively. The formation of 5 is proposed to arise from initial hydride abstraction from a methyl enolate group by Ph3C+, as supported by the parallel production of Ph3CH, and subsequent elimination of methane and isobutene. In addition to standard spectroscopic and analytical characterizations for the isolated complexes 2–5, complexes 4 and 5 have also been structurally characterized by X‐ray diffraction studies.
Enantioselective renal excretion of albendazole metabolites in patients with neurocysticercosis
Authors: Lanchote V.L. , Takayanagui O.M. , Mateus F.H. 
Published: July 2004

AbstractThe present study investigates the urinary excretion of the enantiomers of (+)‐ and (–)‐albendazole sulfoxide (ASOX) and albendazole sulfone (ASON) in 12 patients with neurocysticercosis treated with albendazole for 8 days (7.5 mg/kg/12 h). Serial blood samples (0–12 h) and urine (three periods of 8 h) were collected after administration of the last dose of albendazole. Plasma and urine (+)‐ASOX, (–)‐ASOX, and ASON metabolites were determined by HPLC using a chiral phase column (Chiralpak AD) with fluorescence detection. The pharmacokinetic parameters (P < 0.05) for (+)‐ASOX, (–)‐ASOX, and ASON metabolites are reported as means (95% CI); amount excreted (Ae) = 3.19 (1.53–4.85) vs. 0.72 (0.41–1.04) vs. 0.08 (0.03–0.13) mg; plasma concentration‐time area under the curve, AUC0–24 = 3.56 (0.93–6.18) vs. 0.60 (0.12–1.08) vs. 0.38 (0.20–0.55) μg.h/ml, and renal clearance ClR = 1.20 (0.66–1.73) vs. 2.72 (0.39–5.05) vs. 0.25 (0.13–0.37) l/h. Sulfone formation capacity, expressed as the Ae ratio ASON/ASOX + ASON, was 2.21 (1.43–2.99). These data point to enantioselectivity in the renal excretion of ASOX as a complementary mechanism to the metabolism responsible for the plasma accumulation of (+)‐ASOX. The results also suggest that the metabolite ASON is partially eliminated as a reaction product of the subsequent metabolism. Chirality 16:520–525, 2004. © 2004 Wiley‐Liss, Inc.
Ionen und Ionenbindung: Fehlvorstellungen hausgemacht! Ergebnisse empirischer Erhebungen und unterrichtliche Konsequenzen
Authors: Hilbing Claus , Barke Hans‐Dieter 
Published: July 2004

AbstractKINDER UND JUGENDLICHE beobachten ihre Umgebung meist sehr genau und gelangen zu Erklärungen und Vorstellungen, die vom wissenschaftlichen Standpunkt aus gesehen zwar nicht angemessen sind, aber auf die Schüler bezogen als respektable Denkleistungen eingestuft werden müssen. Aus diesem Grund nennt man solche ursprünglichen Schülervorstellungen auch nicht falsch, sondern bezeichnet sie etwa als alternativ, vorläufig oder lebensweltlich. Solche Vorstellungen wurden vielfach empirisch erhoben und sind uns z. B. bezüglich der Stoffumwandlung, der Verbrennung, der Luft und anderer Gase oder erster Teilchenvorstellungen bekannt [1].
Computer und Multimedia im Chemieunterricht heute. Eine Einordnung aus didaktischer und lerntheoretischer Sicht
Authors: Eilks Ingo , Flintjer Bolko , Krilla Bodo , Möllencamp Hartwig , Wagner Walter 
Published: July 2004

AbstractIM BEITRAG wird der Einsatz von Computer und Multimedia im Chemieunterricht aus didaktischer und lerntheoretischer Sicht diskutiert. Eine ausführliche Version des Textes mit einer umfassenden und weiter führenden Literatursicht ist auf der Seite der Fachgruppe Chemieunterricht verfügbar. Diese umfasst auch eine Betrachtung der lerntheoretischen Grundlagen, eine Übersicht über aktuelle Multimedia bezogene Forschung und Hinweise auf eine Vielzahl von unterrichtspraktischen Beiträgen.