bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optisch aktive 3‐Amino‐2H‐azirine als Bausteine für enantiomerenreine αα‐disubstituierte α‐Aminosäuren: Synthese von Isovalin‐Synthonen und Einbau in ein Trichotoxin‐A‐50‐Segment
Authors: Bucher Christoph B. , Heimgartner Heinz 
Published: December 2004

Optically Active 3‐Amino‐2H‐azirines as Synthons for Enantiomerically Pure αα‐Disubstiuted α‐Amino Acids: Syntheses of Isovaline Synthons and a Segment of Trichotoxin A‐50The synthesis of a novel 3‐amino‐2‐methyl‐2‐[2‐(phenylsulfonyl)ethyl]‐2H‐azirine derivative 12 with a chiral substituent at the amino group is described. Chromatographic separation of the diastereoisomer mixture gave pure diastereoisomers which, after an electrochemical cleavage of the phenylsulfonyl group, yielded the (S)‐ and (R)‐isovalin (Iva) synthons 13a and 13b, respectively. The absolute configuration of the precursor molecule 12b was established by X‐ray crystallography. The Iva synthons were successfully used in the synthesis of the C‐terminal pentapeptide Z‐Leu‐Aib‐(R)‐Iva‐Gln‐Valol of the peptaibole Trichotoxin A‐50 and its epimer.
Synthesis of Monodisperse Macromolecular Bicyclic and dendritic compounds from (R)‐3‐hydroxybutanoic acid and benzene‐1,3,5‐tricarboxylic acid and analysis by fragmenting MALDI‐TOF mass spectroscopy
Authors: Seebach Dieter , Herrmann Guide F. , Lengweiler Urs D. , Amrein Walter 
Published: December 2004

AbstractAs previously shown, oligo‐ and poly(β‐hydroxyalkanoates) have a high tendency to form lamellar crystallites with ca. 50‐Å thickness which corresponds to chain lengths of 16 units (Fig. 1). To have monodisperse model compounds, we have now prepared bicyclic derivatives with three parallel (27–29) or two parallel and an antiparallel chain (68–70) consisting of up to 16 3‐hydroxybutanoate (3‐HB) units. We also prepared dendritic compounds (71–75, 82–85) containing oligo(3‐HB) chains which cannot possibly be arranged as in the lamellae; the branching units were prepared from trimesic acid (= benzene‐1,3,5‐tricarboxylic acid). So far, none of the prepared samples formed crystals or contained crystalline domains which would have been suitable for single‐crystal or powder‐diffraction X‐ray analysis. The terminally deprotected dendrimers (74, 75, and 85) are multi‐anionic (up to 12 peripheral CO2H groups) and biodegradable. The macromolecular HB derivatives (molecular weight up to 10150 Da) have been fully characterized by IR, 1H‐ and 13C‐NMR, [α]D, and elemental analysis. Especially important is the analysis by mass spectrometry with the MALDI‐TOF technique (Fig. 2), proving that the products are monodisperse; application of a new variation of this MS method (post source decay = PSD or fragment analysis by structural time of flight = FASTTM) allows for the observation of metastable fragment ions and, thus, is a tool for structural oligomer analysis (Fig. 3).
Organonickel(II) Complexes with Anionic N‐Donor Ligands. Hydration of coordinated nitriles at a nickel(II) site
Authors: Sánchez Gregorio , Ruiz Francisco , Garcia Joaquín , Carmen de Arellano M. Ramírez , López Gregorio 
Published: December 2004

AbstractThe hydroxo complex (Bu4N)2[Ni2(C6F5)4(μ‐OH)2]reacts with 2,3,4,5,6‐pentafluoro benzenamine (C6F5‐NH2), 1,3‐diaryltriaz‐1‐enes (ArNHN=NAr, Ar = Ph, 4‐MeC6H4, 4‐MeOC6H4), 7‐aza‐1H‐indole (= 1H‐pyrrolo[2.3‐b]pyridine; Hazind),N‐phenylpyridin‐2‐amine(pyNHPh), andN‐phenylpyridine‐2‐carboxamide (py‐CONHPh) at room temperature in acetone to give the binuclear complexes (Bu4N)2[Ni2(C6F5)4(μ‐C6F5NH)2] (1) and (Bu4N)2[{Ni(C6F5)2} 2(μ‐OH)(μ‐azind)] (2) and the mononuclear complexes Bu4N[Ni(C6F5)2(ArN3Ar)] (3–5), Bu4N[Ni(C6F5)2(pyNPh)] (6), and Bu4N[Ni(C6F5)2(pyCONPh)] (7). The hydroxo.complex (Bu4N)2[{Ni(C6F5)2‐(μ‐OH)}2] promotes the nucleophilic addition of water to pyridine‐2‐carbonitrile, 2‐aminoacetonitrile, and 2‐(dimethylamino)acetonitrile, and complexes8–10containing pyridine‐2‐carboxamidato, 2‐aminoacetamidato and 2‐(dimethylamino)acetamidato ligands are formed. Analytical (C, H, N) and spectroscopic (IR,1H and19F‐NMR, and FAB‐MS) data were used for structural assignments. A single‐crystal X‐ray diffraction study of (Bu4N)2[{Ni(C6F5)2}2(μ‐OH)(μ‐azind)] (2) established the binuclear nature of the anion; the two Ni‐atoms are bridged by an OH group and a 7‐aza‐7H‐indol‐7‐yl group, but the central NiONiNCN ring is not planar, the dihedral angle between the NiONi and NiNCNNi planes being 84.4°.
Synthesis of N‐{[(9H‐Fluoren‐9‐yl)methoxy]carbonyl}‐Protected (Fmoc) β‐Amino Acids (= homo‐α‐amino acids) by direct homologation
Authors: Ellmerer‐Müller Ernst P. , Brössner Dagmar , Maslouh Najib , Takó Andreas 
Published: December 2004

AbstractThe successful application of the Arndt‐Eistert protocol starting from commercially available N‐{[(9H‐fluoren‐9‐yl)methoxy]carbonyl}‐protected (Fmoc) α‐amino acids leading to enantiomerically pure N‐Fmoc‐protected β‐amino acids in only two steps and with high yield is reported.
Stability and Lability of Dicopper Double‐Stranded Helicates in Solution
Authors: Carina Riccardo F. , Williams Alan F. , Piguet Claude 
Published: December 2004

AbstractStability constants have been measured for a series of ligands based on a 2,2′‐(pyridine‐2,6‐diyl)bis[1H‐benzimidazole] unit which forms dinuclear double‐stranded helical complexes with copper(I). Variation of different structural parameters confirms the importance of the coordinate bond, the stacking interactions, and the weakly bridging pyridine units observed by X‐ray crystallography. The stabilities of the complexes depend strongly on the solvent, and in MeCN, which is a good solvent for copper(I), the complexes are less stable and assemble in a stepwise manner. The interconversion of the enantiomers may be followed by 1H‐NMR and takes place on a millisecond time scale around room temperature. The trends in lability are similar to those found for the stability of the complexes.
Diastereo‐ and Regioisomeric Bicyclic Thiohydantions from Chiral 1,3‐Thiazolidine‐2,4‐dicarboxylic acids
Authors: Miskolczi István , Zékány András , Rantal Ferenc , Linden Anthony , Kövér Katalin E. , Györgydeák Zoltán 
Published: December 2004

AbstractBicyclic thiohydantoins were synthesized in a stereoselective manner by reacting (2R)/(2S)‐diastereoisomer mixtures of 1,3‐thiazolidine‐2,4‐dicarboxylic acids or their dimethyl diesters with PhNCS. 5,5‐Dimethyl‐1,3‐thiazolidine‐2,4‐dicarboxylic acid with PhNCS led to a cyclization involving the CO group at the C(2) center of the thiazolidine ring, while the acid's dimethyl diester gave cyclization involving the CO group at C(4). In contrast, reactions involving unsubstituted 1,3‐thiazolidine‐2,4‐dicarboxylic acid or its dimethyl diester led to thiohydantoins in which the ring closure had taken place only with the COO group at C(4). Independently of the direction of the ring closure, all reactions produce exclusively products with the (R)‐configuration at C(2). The configurational assignments were based on1H‐ and13C‐NMR studies, and confirmed by X‐ray crystallographic analyses.
Enantioselektive Synthese von α‐substituiertenN‐Methyl‐sulfonamiden. Vorläufige Mitteilung
Authors: Enders Dieter , Thomas Christian R. , Raabe Gerhard , Runsink Jan 
Published: December 2004

AbstractThe first asymmetric α‐alkylations of lithiated sulfonamides bearing the chirality information within the amine moiety under high asymmetric inductions (de 83–95%) are described. Racemization‐free acidic hydrolysis led to the title compounds11in acceptable overall yields and with high enantiomeric purity (ee 91− ≥ 98%;Scheme 2). As a novel chiral auxiliary, the primary amine (S,S)‐or (R,R)‐2was synthesized employing the classicalErlenmeyerphenylserine synthesis (Scheme 1).
Analysis of biomaterials deposited on soft contact lenses
Authors: Wedler Frederick C. 
Published: December 2004

AbstractFormation of opaque deposits on the anterior (air) surface of hydrophilic soft contact lenses is a problem worthy of investigation by all concerned. These deposits have been analyzed for biomaterials by chemical, biochemical, electrophoretic, and immunological techniques. Qualitative and quantitative chemical colorimetric tests revealed the presence of variable amounts of protein (5–10 μg/lens), carbohydrate (1.0–1.2 μg/lens), and phospholipids (0.01–0.05 μmole/lens). Cholesterol and glucose were not present at detectable levels. Fluorescent antibody tests with appropriate controls gave positive tests for albumin, lysozyme, γ‐G‐globulin, and α1‐lipoprotein in the deposits, all proteins presents in tear fluid. Deposits were most effectively removed from the lenses by the combination of heat, sodium dodecyl sulfate (SDS) detergent, and the thiol reagent dithiothreitol (DTT). SDS‐denatured protein migrated on polyacrylamide gels with electrophoretic patterns corresponding to molecular weights for those proteins detected by the above antibody tests. The nature of the bonding interactions of biomaterials to the lenses was probed by chemical reagents used to remove them, employed singly and in all possible combinations. Urea, guanidine hydrochloride, potassium thiocyanate, potassium perchlorate, hydroxylamine, and EDTA were much less effective than SDS and DTT. These data suggest that apolar interactions plus disulfide bonds may be important in stabilizing the deposit structure, and point to improved cleaning procedures.
In vitro clot formation on the polyelectrolyte complexes of sodium dextran sulfate with chitosan
Authors: Fukuda Hiroshi , Kikuchi Yasuo 
Published: December 2004

AbstractThe solution of charged sodium dextran sulfate (molecular weight 6000) co‐reacted with that of oppositely charged chitosan to form a water‐insoluble hydrous complex, whose clot‐inhibiting property was sensitive to such reaction conditions as the order of mixing, the kind of acid species used for the adjustment of pH, and the concentration of hydrogen ion at which the preparation of polyelectrolyte complex was carried out, although all the polyelectrolyte complexes were prepared from the same starting materials. In order to clarify the relationship between the molecular conformation of the polyelectrolyte complex and its clot‐inhibiting property, ir spectroscopic studies, in vitro blood‐clotting tests, color reaction with toluidine blue, elemental analyses, and so on, were performed. Consequently, it was suggested that the polyelectrolyte complexes consisting of sodium dextran sulfate of lower molecular weight are more clot‐inhibiting than those with higher molecular weight. It was also found that polyelectrolyte complexes prepared under the mixing order of sodium dextran sulfate solution added to chitosan solution are also more clot‐inhibiting than those prepared under the reverse mixing order, chitosan solution added to sodium dextran sulfate solution. Furthermore, it has become apparent that the anticlotting polyelectrolyte complexes have some characteristic ir absorption bands, and that they displayed color reaction with toluidine blue. A new possibility may be anticipated from our experimental results, providing important information relating to realizing a functional polymer, antithrombogenic material, whose property may be ordered.
The effect of surface charge on arterial thrombosis
Authors: Helmus Michael N. , Gibbons Donald F. , Jones Richard D. 
Published: December 2004

AbstractThe amount of thrombus formed on the surface of implants of random copolymers of (L‐glutamic acid co‐L‐leucine) implanted in the femoral and carotid arteries of dogs is related to composition and the degree of ionization. When the initial surface concentration of unionized glutamic acid is greater than 10%, the surface is completely covered with thrombus; the thrombus is, however, limited in thickness to ∼600 μ. For surface concentration of unionized glutamic acid less than 10%, the amount of thrombus is a linear function of the degree of ionization. When 10% of the total surface sites consists of ionized glutamic acid residues, there is no thrombus, and only formed elements adhere to the surface. Thrombus covered surfaces tended to show decreased thrombus formation by seven days post implantation. The presence of thrombus enhanced the rate of development of an endothelial‐like cell layer on the surface, while on thrombus‐free implants, large‐diameter flattened monocytes, some of which formed confluent patches, covered only small areas of the implant surface.
Morphological assessment of hepatoma cells (HepG2) microencapsulated in a HEMA‐MMA copolymer with and without Matrigel
Authors: Babensee Julia E. , De Boni Umberto , Sefton Michael V. 
Published: December 2004

AbstractHepatoma cells (HepG2), an anchorage‐dependent cell line, were microencapsulated in a HEMA‐MMA polyacrylate membrane to which the cells do not ad here. This environment was altered by the coencapsulation of Matrigel, a reconstituted extracellular matrix derived from the Engelbreth‐Holm‐Swarm (EHS) mouse tumor basement membrane, to provide sites for cell attachment. The effect on the cells of these two capsule microenvironments during a 2‐week in vitro culture period was assessed by examining the spatial arrangement, morphology, and viability of the cells using light microscopy and scanning electron microscopy (SEM). In preparation for microscopy, dissolution of the polymer was prevented by the use of frozen sections embedded in a water‐soluble compound. Similarly, freeze cleavage of conductively stained capsules permitted SEM observation of the capsule interior along with ultrastructural detail of the cells. In the absence of Matrigel, cells in HEMA‐MMA capsules were found to form aggregates in intracapsular pockets with central necrosis occurring at day 7 in large aggregates. The coencapsulation of HepG2 cells with Matrigel, resulted in an initially uniform distribution of essentially individual cells with aggregates appearing later within the Matrigel. Many cells within these capsules had remained viable when examined up to day 14 with only limited cellular necrosis, implying a favorable environment for microencapsulated HepG2 cells. © 1992 John Wiley & Sons, Inc.
Parallel processing on minicomputers: A powerful tool for quantum chemistry
Authors: Seeger Rolf 
Published: December 2004

AbstractIt is shown that performance of quantum chemistry program systems on minicomputers may be enhanced by a factor between 2.5 and 2.8 utilizing microprogramming techniques and a computer architecture allowing four processor activities to operate in parallel, i.e., memory access, basic arithmetic operations, and two high‐speed 64‐bit floating‐point arithmetic units.
A comparison of selected force constants derived from ab initioSCF calculations on β‐hydroxyacrolein. Acrolein, performic acid, and formic acid
Authors: Bock Charles W. , Trachtman Mendel , George Philip 
Published: December 2004

AbstractHarmonic force fields have been calculated for the planar hydrogen‐bonded ring conformer of β‐hydroxyacrolein, cCc, which is the most stable, and the chain conformer, cCt, generated by 180° rotation of OH about the CO bond axis. The equilibrium structure obtained using the 4‐31 G basis set with full geometry optimization was employed in each case. Selected force constants for the bonds directly concerned in the formation of the ring from the chain structure, and the increments in going from the one to the other, are compared with the values for the corresponding conformers of performic and formic acids. As the ring size increases from four in trans‐formic acid, to five in cis–cis‐performic acid and to six in the cCc conformer of β‐hydroxyacrolein there is a successive increase in the mechanical strength of the hydrogen‐bridging unit. The energy changes for the chain → ring conversion do not follow this progression: performic acid is out of order. But, since a force constant is a localized bond property, whereas the energy changes are determined not only by interactions specific to the hydrogendonor and hydrogen‐acceptor groups but also by interactions involving more distant parts of the molecule, the force constants for the bonds directly concerned in the formation of the hydrogen bridge provide a less ambiguous basis for comparing the strength of the intramolecular hydrogen bonding.
Planar and nonplanar vibrations of cytosine
Authors: Putnam B. F. , Van Zandt L. L. 
Published: December 2004

AbstractThe formalism developed in the preceding article is used in a normal coordinate analysis of the pyrimidine base cytosine. The results of both planar and nonplanar vibrations are reported. The model yields 66 frequencies with an average error of about 6.5 cm−1 (ca. 0.5%). The vibrational modes are compared with experimental data and discussed in terms of potential energy distributions and Cartesian displacements. Two isolated low‐frequency (near 200 cm−1) out‐of‐plane modes are predicted in the vicinity of where two such modes are believed to occur. In addition, the model has taken into account the observed coalescence of the torsional and wagging modes of the amino group upon deuteration of the amino group. Recent data from sulfur‐substituted cytosine (2‐thiocytosine) were useful in making assignments.
Koopmans' theorem ionization potentials of methyl and silyl halides, chalcogenides, amines, and phosphines: An ab InitioSCF study
Authors: Glidewell Christopher , Thomson Colin 
Published: December 2004

AbstractKoopmans' theorem ionization potentials have been calculated for a series of hydrides, methyls, and silyls HnX, (CH3)nX, and (SiH3)nX (X = F, Cl, n = 1; X = O, S, n = 2; X = N, P, n = 3), together with some mixed species (MH3)nXH3‐n (X = N, P; M = C, Si) using ab initio SCF methods. The calculated values give excellent agreement with experimental values without the inclusion of d functions. For the chlorides, HCl, CH3Cl, and SiH3Cl, the values vary rather little over a wide range of basis sets, and are unaffected by the inclusion of d functions.
A unified treatment of valence and bond order from density operators
Authors: Jug Karl 
Published: December 2004

AbstractA generalization of the quantum chemical definition of valence of atoms in molecules is suggested. Valence is considered as expectation value of diatomic parts of density operators. It appears as a sum of contributions from occupied orbitals of all atomic pairs that contain the reference atom. This definition is applicable on self‐consistent‐field (SCF) and configuration interaction (CI) level in any atomic orbitals (AO) basis. Its usefulness is demonstrated in an application to special molecules. Photoelectron spectroscopy and reactivity is discussed in this context.
Molecular associations: Values of the expansion parameters for new classes of atoms
Authors: Sordo J. A. , Fraga S. 
Published: December 2004

AbstractExpansion parameters, for use in a 1/R expansion developed at this laboratory for the study of molecular interactions, have been determined for new classes of atoms. Test calculations with these new parameters have yielded very satisfactory results for the glycine zwitterion, in dimer form and solvated, confirming the goodness of the parameters and the appropriateness of the formulation.
Computational modeling of the mechanisms of the free radical‐chain reaction of alkanes with oxygen. The oxidation of isobutane, n‐butane, and isopentane
Authors: Allara D. L. , Edelson D. , Irwin K. C. 
Published: December 2004

AbstractA general computational method for obtaining complete solutions of time‐dependent kinetic equations has been developed and applied to free radical‐initiated reactions of alkanes with oxygen. The method has been applied to the low‐temperature, peroxide‐initiated oxidations of isobutane and isopentane. Using available independently measured and estimated values for the rate constants and activation parameters for each of the 20 proposed reaction steps for the oxidation of isobutane, the rates and products have been calculated for both the liquid phase and gas phase in the range of 100°–155°C. The calculated rates and products of oxidation agree with published experimental values. The oxidation of isopentane was examined by a 32‐reaction model. The rate constants were estimated using values for the appropriate rate steps in the oxidation of n‐butane and isobutane. The calculation of the oxidation rate and products agree with our experiments.
Free‐radical chain dechlorination of chloroethanes in liquid triethylsilane
Authors: Aloni R. , Rajbenbach L. A. , Horowitz A. 
Published: December 2004

AbstractThe kinetics and mechanism of the free‐radical chain dechlorination of C2Cl6, C2Cl5H, and sym‐C2Cl4H2 in Et3SiH wereinvestigated over a wide temperature range. The propagation step of the dechlorination of chloroethanes (C2Clx H6−x) proceeds by the following reactions:
equation image
Analysis of the temperature dependence of product formation gave the Arrhenius expressions for k4/k3 which in turn were utilized for the estimation of the absolute Arrhenius parameters for hydrogen abstraction from Et3SiH. Our results show that the values of Eabs are lower in Et3SiH than in c‐C6H12 by about 2–3 kcal/mol, while the A factors are almost equal. In competitive studies k2 was determined versus Br abstraction from n‐C5H11Br. The relative Arrhenius parameters determined by this method show that variations in both A factors and activationenergies are responsible for the reactivity trends observed in the Cl transfer reactions of Et3Si radicals.
über die Herstellung von Legierungen nach dem Tränkverfahren
Authors: Anders Heinz 
Published: December 2004

AbstractDie Entwicklung der Fertigungsverfahren fü Preßteile aus Sintermetallen und die Verbesserung ihrer Gebrauchs‐eigenschaften hat besonders im Ausland beachtliche Fortschritte erzielt und den pulvermetallurgischen Erzeugnissen neue Anwendungsgebiete erschlossen. Eine wesentliche Neuerung in der Herstellung von Maschinenpreßteilen bedeutet die Anwendung des Tränkverfahrens, bei dem zunächst ein poriges Sintereisen‐ oder Sinterstahlpreßteil angefertigt wird, des‐sen Poren man mit einem geeigneten Tränkmetall ausfüllt.Der Bericht behandelt die in den letzten Jahren erzielten Ergebnisse und soll an‐regen, die Vorteile der Pulvermetallurgie und die des Tränkverfahrens in noch stärkerem Maße als bisher in Anwendung zu bringen, zumal derartig hergestellte Werkstoffe korrosionstechnisch von aus‐gesprochen guter Beständigkeit sind.
Intrinsically photosensitive polyimides: Approaches to fluorinated homologs
Authors: Moore J. A. , Gamble Donald R. 
Published: December 2004

AbstractCyclobutane tetracarboxylic acid dianhydrides, which were prepared via photodimerization of maleic anhydride or methylmaleic anhydride, gave high molecular weight film‐forming polyimides with oxydianiline. The resulting polyimides were found to be photosensitive, presumably because of retrocyclization of the cyclobutane ring. Despite the presence of the cyclobutane ring, the polyimides show good thermal stability. In an effort to enhance the thermal stability and lower the dielectric constant of these polyimides, an approach was made to prepare the fluorinated analogs of the cyclobutane dianhydride monomers via the preparation and photodimerization of fluorinated maleic anhydride derivatives. Thus, both fluoromaleic and difluoromaleic anhydrides were photolyzed to give the target monomers. The proposed application of these monomers, however, has yielded disappointing results due to complications in their synthesis and attempted polymerization.
Zur Lichtabsorption der polycyclischen Komplexverbindungen. 2. Kuprikomplexe der Aldiminreihe
Authors: v. Kiss Á. , Bácskai G. , Csokán P. 
Published: December 2004

AbstractEs wurden die Extinktionskurven von mehreren polycycli‐schen innerkomplexen Kuprisalze und deren Ausgangsstoffe bei Zimmertemperatur zwischen 700 und 200 mμ genau aus‐gemessen.
Die Darstellung optisch aktiver Aminoacylmercaptane durch Racematspaltung der D, L‐Verbindungen mit Dibenzoyl‐D‐weinsäure
Authors: Losse Günter , Müller Gerhard 
Published: December 2004

AbstractD,L‐Valinthiophenylester und D, L‐Phenylalanin‐p‐nitrothiophenylester wurden mit Dibenzoyl‐D‐weinsäure über ihre Hydrogentartrate in die optischen Antipoden gespalten und damit aus racemischem Material für Peptidsynthesen direkt verwendbare optisch aktive Bausteine isoliert.