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61.
An efficient, diastereoselective synthesis of substituted and unsubstituted 2,3,4,5‐tetrahydro‐1H‐1‐benzazepine‐5‐carboxylic esters has been developed based on the tandem reduction‐reductive amination reac tion. Catalytic hydrogenation of a series of 2‐(2‐nitrophenyl)‐5‐oxoalkanoic esters initiates a reaction sequence involving (1) reduction of the aromatic nitro group, (2) condensation of the N‐hydroxylamino (or amino) nitrogen with the side chain carbonyl, and (3) reduction of the seven‐membered cyclic imine. Cyclizations that produce 2‐alkyl‐2,3,4,5‐tetrahydro‐1H‐1‐benzazepine‐5‐carboxylic esters are diastereose lective for the product having the C2 alkyl and the C5 ester groups cis. In these reactions, the transannular ester group exerts a strong stereodirecting effect on the reduction of the cyclic imine intermediate, though not as strong as that observed in previous closures of 2‐alkyl‐1,2,3,4‐tetrahydroquinoline‐4‐carboxylic esters. This decrease in diastereoselectivity is attributed to (1) the greater distance between the ester and the imine double bond and (2) the increased conformational mobility of the larger ring, both of which diminish the stereodirecting effect of the ester. Finally, formation of the seven‐membered ring is sufficiently slow that reaction with the side chain ester group competes with heterocycle formation in several of the reactions.  相似文献   
62.
The O2–N2 and O2–Ar negative-ion chemical ionization mass spectra of aromatic amines show a series of unusual ions dominated by an addition appearing at [M + 14]. Other ions are observed at [M – 12], [M + 5], [M + 12], [M + 28] and [M + 30]. Ion formation was studied using a quadrupole instrument equipped with a conventional chemical ionization source and a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. These studies, which included the examination of ion chromatograms, measurement of positive-ion chemical ionization mass spectra, variation of ion source temperature and pressure and experiments with 18O2, indicate that the [M + 14] ion is formed by the electron-capture ionization of analytes altered by surfaceassisted reactions involving oxygen. This conversion is also observed under low-pressure conditions following source pretreatment with O2. Experiments with [15N]aniline, [2,3,4,5,6-2H5] aniline and [13C6]aniline show that the [M + 14] ion corresponds to [M + O ? 2H], resulting from conversion of the amino group to a nitroso group. Additional ions in the spectra of aromatic amines also result from surface-assisted oxidation reactions, including oxidation of the amino group to a nitro group, oxidation and cleavage of the aromatic ring and, at higher analyte concentrations, intermolecular oxidation reactions.  相似文献   
63.
The coordinatively unsaturated uranium(IV) complex U[N(C6H5)2]4 has been prepared via the stoichiometric reaction of diphenylamine with [(Me3Si)2N]2 H2. U[N(C6H5)2]4 coordinates Lewis bases such as Et2O, THF, pyridine or (EtO)3PO, based on electronic absorption spectroscopy and 1H NMR studies. Exchange between U[N(C6H5)2]4 and U[N(C6H5)2]4(L), where L is THF or pyridine, is rapid on the NMR time-scale between 307 and 323 K. Measurement of equilibrium constants for L = THF provides ΔH and ΔS values of −60 kJ mol−1 and −1.8 × 102 J K−1 mol−1, respectively. U[N(C6H5)2]4 coordinates and binds (EtO)3PO much more tightly (Keq = & > 104 M−1) than THF or pyridine with the exchange rate between U[N(C6H5)2]4 and U[N(C6H5)2]4[OP(OEt)3] being close to the NMR time-scale.  相似文献   
64.
A method is described for determining 10-5–10-4M fluoride in a variety of solutions potentiometrically with a fluoridc-specific electrode, by a standard addition method. Any change of ionic strength or the nature of the solution that might alter activity coefficients or junction potentials is minimized. The relationship between potential and fluoride concentration thus follows the Nernst equation, and the unknown concentration can be calculated. Experimental data are given for solutions of sodium choride, sodium nitrate, acidified sodium silicate and sodium hydroxide, lithium chloride, and phosphoric acid. Metal ions (e.g., Al3+, UO22+, Fe3+, Th4+) that interfere by forming complexes with fluoride can be precomplexed with phosphoric acid. The relative error is estimated at 10%, and the relative standard deviation is less than 5% over the concentration range 10-5–10-4M fluoride.  相似文献   
65.
The transformation of ammonium cyanate into urea, first studied over 170 years ago by W?hler and Liebig, has an important place in the history of chemistry. To understand the nature of this solid state reaction, knowledge of the crystal structure of ammonium cyanate is a prerequisite. Employing neutron powder diffraction, we demonstrate conclusively that, in the structure of ammonium cyanate, the NH(4)(+) cation forms N-H...N hydrogen bonds to four cyanate N atoms at alternate corners of a distorted cube, rather than our previously proposed alternative arrangement with N-H...O hydrogen bonds to cyanate O atoms at the other four corners.  相似文献   
66.
A tandem SN2‐Michael addition reaction has been developed for the synthesis of cis‐ and trans‐fused nitrogen and sulfur heterocycles from the cis and trans isomers of ethyl (±)‐(2E)‐3‐[2‐(iodomethyl)cyclo‐hexyl]‐2‐propenoate. Octahydro‐1H‐isoindole‐1‐acetic acid and octahydrobenzo[c]thiophene‐1‐acetic acid derivatives have been prepared and their stereochemistries elucidated using NMR and X‐ray crystallo‐graphic methods. Cyclization substrates for both the cis‐ and the trans‐fused rings are readily available in four steps from known compounds. Yields for the cyclization range from 80‐85% and stereochemical selec‐tivities with respect to the side chain vary from 12.5‐16:1 for the cis‐fused structures to 6‐7.5:1 for the trans‐fused structures. Steric interactions in the transition states for ring closure are proposed to rationalize the observed preferences.  相似文献   
67.
The reactions of a series of 5-alkyl-2-thiophenedithiocarboxylates with nickel(II) chloride afforded two types of complexes, blue nickel(II) complexes with two terminal dithiocarboxylate ligands, [Ni(S2CTR)2] and violet nickel(II) complexes with perthio- and dithiocarboxylate ligands, [Ni(S2CTR)(S3CTR)] (where T = 2,5-disubstituted thiophene, R = CnH2n+1, n = 4, 6, 8, 12, 16). The blue monomers are preferred for the shorter chains (C4 and C6) and the violet compounds form exclusively for the longer chains (C8, C12, and C16) in the alkylthiophene complexes. In addition to the above series, [Ni(S2CTCH3)2], was prepared in a one-pot reaction in THF and both the blue and violet products were isolated. It was possible to convert the blue complexes [Ni(S2CTR)2] (R = butyl, hexyl) into the corresponding violet complexes [Ni(S2CTR)(S3CTR)] after stirring in THF solutions for prolonged periods of time. Liquid-crystalline properties of these complexes were examined by DSC and POM. The violet complexes with C8 and C12 alkyl chains showed liquid-crystalline properties.  相似文献   
68.
Summary Comparable series of sulfur determinations by the silver absorbent method were performed on four test samples. One series was carried out with automatic ignition using a stationary electric sample burner during a constant time period for each analysis; the other series by igniting the sample with a Bunsen burner held in the hand until the last trace of sample disappeared.There was no obviously assured difference in the quality of the analytical figures obtained by the two methods of sample ignition. Statistical analysis of the data, however, disclosed that the automatic burner did actually give a slight improvement in precision over the hand burner although the ultimate limit of accuracy was the same in each case.
Zusammenfassung Mit vier reinen Verbindungen wurden Reihenbestimmungen des Schwefels durch Verbrennung und Absorption des Schwefeltrioxyds an Silberwolle so ausgeführt, daß eine Reihe von Verbrennungen mit dem von Hand geführten Bunsenbrenner immer einer entsprechenden Reihe von Bestimmungen gegenübergestellt werden konnte, bei der die Verbrennung mit einem ruhenden, automatisch regulierten elektrischen Heizkörper erfolgte.Die Resultate der automatischen Verbrennungen zeigten eine so geringe Verbesserung gegenüber jenen, die durch Verbrennung mit dem handregulierten Bunsenbrenner erhalten wurden, daß der augenscheinliche Unterschied lediglich dem Zufall zugeschrieben werden könnte. Eine statistische Untersuchung zeigte, daß die automatische Verbrennung tatsächlich eine geringfügige Verbesserung der Präzision gibt, ohne jedoch die Genauigkeit der arithmetischen Mittel der Versuchsreihen merkbar zu ändern.

Résumé On a effectué des déterminations de soufre en séries comparables par la méthode d'absorption par l'argent sur quatre échantillons types. L'une des séries comportait une combustion automatique à l'aide d'un brûleur électrique fixe pendant un temps constant pour chaque analyse. L'autre série comportait l'emploi d'un becBunsen conduit à la main pour la combustion de l'échantillon jusqu'à disparition de ses dernières traces.On n'a constaté aucune différence marquée dans la qualité des résultats analytiques obtenus par l'une ou l'autre méthode de combustion de l'échantillon. L'analyse statistique des résultats obtenus fait cependant apparaître une légère amélioration de la précision lorsque le brûleur automatique est substitué au brûleur à main bien que les écarts extrêmes obtenus soient les mêmes pour chacun des cas.


To Prof. Dr.Hans Lieb at his 70th birthday.  相似文献   
69.
R67 dihydrofolate reductase (DHFR) is a novel enzyme that confers resistance to the antibiotic trimethoprim. The crystal structure of R67 DHFR displays a toroidal structure with a central active-site pore. This homotetrameric protein exhibits 222 symmetry, with only a few residues from each chain contributing to the active site, so related sites must be used to bind both substrate (dihydrofolate) and cofactor (NADPH) in the productive R67 DHFR?NADPH?dihydrofolate complex. Whereas the site of folate binding has been partially resolved crystallographically, an interesting question remains: how can the highly symmetrical active site also bind and orient NADPH for catalysis? To model this ternary complex, we employed DOCK and SLIDE, two methods for docking flexible ligands into proteins using quite different algorithms. The bound pteridine ring of folate (Fol I) from the crystal structure of R67 DHFR was used as the basis for docking the nicotinamide-ribose-Pi (NMN) moiety of NADPH. NMN was positioned by both DOCK and SLIDE on the opposite side of the pore from Fol I, where it interacts with Fol I at the pore's center. Numerous residues serve dual roles in binding. For example, Gln 67 from both the B and D subunits has several contacts with the pteridine ring, while the same residue from the A and C subunits has several contacts with the nicotinamide ring. The residues involved in dual roles are generally amphipathic, allowing them to make both hydrophobic and hydrophilic contacts with the ligands. The result is a `hot spot' binding surface allowing the same residues to co-optimize the binding of two ligands, and orient them for catalysis.  相似文献   
70.
The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.  相似文献   
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