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141.
Wolfgang Marterer Horst Prinzbach Grety Rihs Jakob Wirz Jaques Lecoultre Edgar Heilbronner 《Helvetica chimica acta》1988,71(8):1937-1965
The propensity of the N?N bond to undergo photo [2 + 2] cycloadditions has been further explored. In the specifically designed 1,5-azo/enes 1–3 , no [2 + 2] cycloaddition has been observed upon either direct or sensitized excitation with light of various wave lengths at temperatures down to 77 K, in line with expectations based on X-ray ( 1 : d = 2.71 Å, ω = 129°) and PE measurements ( 1 : I1 = 8.00, I2 = 9.05 eV; 2 ; I1 = 8.00, I2 = 9.25eV). The steric/stereoelectronic demands for the participation of the N?N bond in pericyclic reactions are clearly more stringent than those for the C?C bond. 相似文献
142.
Pd(dba)2[dba = dibenzylideneacetone]/PPh3-or Pd(PPh3)4-catalyzed cyclizations of acetoxy-dienes 2 → 3 and 10 → 11 gave 1-vinyl-2-methylidene-subsituted cyclopentances and cyclohexanes in high yield, consistent with a palladium-ene/β-elimination mechanism ( D → E → F , Scheme 2). The efficient and highly stereoselective cyclizations 7 → 7 and 8 → 9 illustrate intramolecular allylpalladium insertions into 1,2-dialkyl-, trialkyl-, trialkyl-, and cyclic alkenes followed by elimination of the exocyclic β–H giving 1,2-divinylcyclopentanes. These new olefin insertions proceed faster in AcOH (compared to THF) and occur preferentially cis relative to the Pd ( 13 → 14 → 15 ). 相似文献
143.
Wolfgang Oppolzer Mark Kurth Daniel Reichlin Frank Moffatt 《Tetrahedron letters》1981,22(27):2545-2548
The chiral induction in the Diels-Alder addition → , assessed reliably by 19F-NMR-spectroscopy of the endo-esters , varied between 47 - 93% in favor of the 2-(R)-adducts depending on the auxiliary chiral group and the Lewis-acid catalyst. 相似文献
144.
A graphite rod electrothermal atomizer has been used for the AAS determination of traces of gold in hydrochloric and in hydrobromic acid solutions, and also after extraction into HBr-saturated methyl isobutyl ketone. Photographic film samples were decomposed first by enzyme action then by nitric acid/peroxide oxidation, and the gold was extracted into MIBK. For 10-μl aliquots of solution the 3s limits of detection were 3 × 10?10g for aqueous solutions, 7 × 10?10g for MIBK, and 7 × 10?9 g/cm2 for film. 相似文献
145.
Wolfgang Reischl Harald Bernhard Christoph Kratky Erich Zbiral 《Monatshefte für Chemie / Chemical Monthly》1985,116(6-7):831-839
Epoxidation of vitamin D3 with benzonitrile ¦H2O2¦ KHCO3 (Payne's reagent) yields exclusively the 5,6-mono-oxirane2. Further epoxidation with the same reagent gives the 5,6-7,8-bisepoxide3 a. Its stereochemistry and therefore the stereochemistry of2 was established by single crystal X-ray analysis of thep-Brbenzoate3b. Thereby for the first time a directive effect of a homoallylic hydroxyl group during thePayne-epoxidation was established.
Herrn Prof.Mihailovic (Universität Belgrad) mit den besten Wünschen zum 60. Geburtstag gewidmet. 相似文献
146.
The enantiomerically pure (+)-3-methoxy-1, 3, 5 (10)-estratrien-11, 17-dione 11 (with trans-anti-trans configuration) was synthesized in a highly stereocontrolled fashion from (±)-t-butyl 4-methoxy-1-benzocyclobutene carboxylate (8) and the (+)-carboxylic acid 6 , obtained from 4 in two steps, followed by one crystallization of the (+)-ephedrine salt. The key step 10→11 (Scheme 2) involves a thermal intramolecular cycloaddition reaction. 相似文献
147.
Wolfgang Ostwald 《Colloid and polymer science》1927,43(3):210-214
Zusammenfassung Es werden zwei Viskositätsanomalien an Gelatine- und Stärkesolen zu erklären versucht, die bei Messungen mit dem Couette gefunden, aber nicht mit Kapülarviskosimetern bisher reproduziert worden sind. Die Erklärung läuft in einem Fall (Gelatinesol) auf die Anahme einer Adsorption des Kolloids am Innenzylinder heraus, die eine Erhöhung der Wandrauhigkeit im hydrodynamischen Sinne und damit eine scheinbare Steigerung der Viskosität zur Folge hat. Die Erklärung des zweiten Falles (Stärkesol) schließt an eine von E. Hatschek für einen Teil der Anomalie bereits gegebene Deutung an, vervollständigt sie aber in einem wesentlichen Punkt. Sie erklärt die Anomalie durch die im Couette während der Messungen selbst erfolgende Zerstörung der Solstruktur bis zum praktischen Verlust der sogen. Strukturviskosität und durch die dann eintretende Strukturturbulenz. Das Stärkesol verhält sich damit ganz analog dem von E. Hatschek und R. S. Jane neuerdings untersuchten Ammoniumoleatsol.Die beiden Anomalien erscheinen bewirkt durch die spezielle Konstruktion des Couette-Apparates. Ihre Nichtreproduzierbarkeit mit Kapillarviskosimetern steht nicht im Widerspruch zu der These des Verfassers, derzufolge alle wesentlichen viskosimetrischen Erscheinungen an kolloiden Systemen mit beiden Apparaturen in gleicher Form gefunden werden.Es wird auf die von der Prandtl'schen Schule studierten Erscheinungen der Wirbelbildung in Grenzschichten und auf die Möglichkeit hingewiesen, daß solche Vorgänge auch im Couette, speziell bei Solen, auftreten und zu Anomalien führen können. 相似文献
148.
This article describes a new complementary peptide separation and purification concept that makes use of a novel mixed-mode reversed-phase/weak anion-exchange (RP/WAX) type stationary phase. The RP/WAX is based on N-(10-undecenoyl)-3-aminoquinuclidine selector, which is covalently immobilized on thiol-modified silica particles (5 microm, 100 A pore diameter) by radical addition reaction. Remaining thiol groups are capped by radical addition with 1-hexene. This newly developed separation material contains two distinct binding domains in a single chromatographic interactive ligand: a lipophilic alkyl chain for hydrophobic interactions with lipophilic moieties of the solute, such as in the reversed-phase chromatography, and a cationic site for anion-exchange chromatography with oppositely charged solutes, which also enables repulsive ionic interactions with positively charged functional groups, leading to ion-exclusion phenomena. The beneficial effect that may result from the combination of the two chromatographic modes is exemplified by the application of this new separation material for the chromatographic separation of the N- and C-terminally protected tetrapeptide N-acetyl-Ile-Glu-Gly-Arg-p-nitroanilide from its side products. Mobile phase variables have been thoroughly investigated to optimize the separation and to get a deeper insight into the retention and separation mechanism, which turned out to be more complex than any of the individual chromatography modes alone. A significant anion-exchange retention contribution at optimal pH of 4.5 was found only for acetate but not for formate as counter-ion. In loadability studies using acetate, peptide masses up to 200 mg could be injected onto an analytical 250 mm x 4 mm i.d. RP/WAX column (5 microm) still without touching bands of major impurity and target peptide peaks. The corresponding loadability tests with formate allowed the injection of only 25% of this amount. The analysis of the purified peptide by capillary high-performance liquid chromatography (HPLC)-UV and HPLC-ESI-MS employing RP-18 columns revealed that the known major impurities have all been removed by a single chromatographic step employing the RP/WAX stationary phase. The better selectivity and enhanced sample loading capacity in comparison to RP-HPLC resulted in an improved productivity of the new purification protocol. For example, the yield of pure peptide per chromatographic run on RP/WAX phase was by a factor of about 15 higher compared to the standard gradient elution RP-purification protocol. 相似文献
149.
Boca R Gembický M Herchel R Haase W Jäger L Wagner C Ehrenberg H Fuess H 《Inorganic chemistry》2003,42(22):6965-6967
Triethylenetetramine (L(4)) was used as a tetradentate blocking ligand that, after complexation with Ni(II), leaves two sites ready for ligation with tricyanomethanide. The formed binuclear complex [L(4)Ni(NCC(CN)CN)(2)NiL(4)](ClO(4))(2) exhibits a ferromagnetic coupling with J/hc = +0.15 cm(-1) and g(Ni) = 2.126; below 16 K, a ferromagnetic ordering is evidenced by ac magnetic susceptibility (both in-phase and out-of-phase), magnetization, field-cooled magnetization, and zero-field-cooled magnetization measurements. 相似文献
150.
Glenn Gillies Daniel Dönnecke Wolfgang Imhof 《Monatshefte für Chemie / Chemical Monthly》2007,138(7):683-686
Summary. The reaction of the unsaturated imine methyl(3-phenylallylidene)amine with ethylene and carbon monoxide in the presence of
catalytical amounts of Ru3(CO)12 leads to the formation of two heterocyclic products. One of the products is a chiral γ-lactam, the other one a 2,3-disubstituted
pyrrole derivative, in which only the carbon atom from carbon monoxide is incorporated. The selectivity in the formation of
the products may be controlled by the choice of solvent. In general, in nonpolar solvents the formation of the lactam is preferred
whereas the use of more polar solvents enhances the yield of the pyrrole. For most of the solvents used there is a linear
dependence of the product ratio on the relative permittivity of the corresponding solvent. Typically, polar aprotic solvents
do not follow this rule. 相似文献