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991.
Heinz W. Gschwend 《Helvetica chimica acta》1973,56(5):1763-1775
The preparation of 2-(4-phenyl)butadienyl-piperidine 5 is described. An intramolecular Diels-Alder reaction of the intermediately formed fumaramide thereof produces stereoselectively the tricyclic lactam 6 . Its structure, as well as the configurational relationship of its 5 asymmetric centers, is corroborated on the basis of the NMR.-data. Cycloacylation of the thermodynamically stable precursors 13 and 20 leads to pentacyclic aza- or diaza-steroid type skeletons. Their structures ( 14, 16 , and 21 ) and in particular their relative configurations are elaborated. A few qualitative kinetic aspects of this intramolecular (4+2)-cycloaddition are presented. 相似文献
992.
The low temperature spectra of TcO
4
–
and ReO
4
–
both show two band systems with pronounced vibrational structures. The bands are identified as1
A
11
T
2 transitions. No other bands are observed with certainty. It seems likely that the KClO4 crystals contain KReO4 crystallites. They are therefore not pure mixed crystals. It is concluded that the virtual orbital (2e) used in the construction of the low lying states resembles an atomic nd orbital more and more when going from n=3, Mn to n=5, Re.
Nachwuchsstipendiat, Schweizerischer Nationalfonds. 相似文献
Zusammenfassung Die Tieftemperaturspektren von TcO 4 – und ReO 4 – zeigen beide zwei Bandensysteme mit ausgeprägten Schwingungsstrukturen. Die Banden werden als 1 A 11 T 2-Übergänge identifiziert. Keine anderen Banden werden mit Sicherheit beobachtet. Es scheint wahrscheinlich, daß die KClO4-Kristalle KReO4-Kristallite enthalten und deswegen keine reinen Mischkristalle sind. Es wird geschlossen, daß das virtuelle Orbital (2e), welches zur Konstruktion der tiefliegenden Zustände gebraucht wird, in der Reihe n=3, Mn bis n=5, Re immer weitgehender einem nd-Atomorbital ähnelt.
Nachwuchsstipendiat, Schweizerischer Nationalfonds. 相似文献
993.
The behaviour of the bright palladium electrode toward the redox systems Cr(2)O(7)(2-)/Fe(2+) and Ce(4+)/Fe(2+) (in potentiometric titration) has been investigated as a function of pretreatment of the electrode. Anodization of the electrode at potentials higher than 800 mV increases DeltaE at the equivalence point by 300-400 mV for the dichromate titration. The sharp change in potential is due to the reaction between PdO(2) and Fe(2+). The equivalence point corresponds to the beginning of the potential drop, rather than the inflexion point, especially for dilute solutions. If the electrode is ignited before use the surface oxide PdO is oxidized to PdO(2) by Cr(V), the potential increases during the titration and DeltaE is 200 mV bigger than when an untreated electrode is used. In the titration of Ce(4+) with Fe(2+) the DeltaE is largest with untreated electrodes, and if the anodized electrode is used, the titration curves clearly show the reaction between PdO(2) and Fe(2+). 相似文献
994.
The 3-acyloxypurine 8-substitution reaction involves elimination of the 3-acyloxy group and nucleophilic substitution at C-8 to yield 8-substituted xanthines or guanines. In aqueous solutions the reaction of 3-acetoxyxanthine proceeds slowly below pH 2, but is greatly accelerated with an increase of the pH from 3 to 7. It is proposed that the slow reaction involves heterolytic cleavage of the 3-acetoxy moiety from 3-acetoxyxanthine to yield a nitrenium ion at N-3 followed by intermolecular nucleophilic substitution of the incipient carbonium ion at the allylic C-8 position, also the most probable mechanism in polar aprotic solvents. The beginning of the fast reaction coincides with the beginning of ionization of the imidazole hydrogen of 3-acetoxyxanthine. It is proposed that this ionization induces a similar but more rapid departure of the 3-acetoxy group from the anion of 3-acetoxyxanthine to produce dehydroxanthine. The latter, upon protonation, yields the same reactive carbonium ion at C-8 that is formed in the slow reaction. Some reduction of 3-acetoxyxanthine to xanthine accompanies the fast reaction. That reduction has the characteristics of a free-radical mediated reaction. It is proposed that reduction results from a homolytic cleavage of the NO bond in the 3-acetoxyxanthine anion to produce a radical-anion, which abstracts hydrogen from water to yield xanthine. These reaction mechanisms and possible alternatives are evaluated. 相似文献
995.
6,7-Dimethoxy-2-naphthylethylamine, prepared by the diborane reduction of 6,7-dimethoxy-2-naphtlialeneacetamide, underwent a Pictet-Spengler cyclization to form 8,9-dimethoxy-1,2,3,4-tetrahydrobenz[h]isoquinoline. This compound is identical with that obtained by reduction of the corresponding dihydrobenzisoquinoline prepared from formamide cyclization. 6,7-Dialkoxy-2-naphthaleneacetic acids, the key intermediates for the preparation of these amides, were obtained from 6,7-dialkoxy-2-acetonaphthones by a modified Willgerodt reaction. 相似文献
996.
M. Fischer W. Knoche P. D. I. Fletcher B. H. Robinson N. C. White 《Colloid and polymer science》1980,258(6):733-742
Summary Complex formation between divalent metal ions and the hydrophilic murexide anion in the presence of anionic micelles has been employed to study aspects of micelle formation, the binding of divalent metal ions to micelles, and the kinetics of metal-complex formation in the presence of micelles.
With 8 figures and 1 table
Dedicated to Prof.G. Rehage on the occasion of his 60th birthday. 相似文献
Zusammenfassung Die Komplexbildung zweiwertiger Metallionen mit dem hydrophilen Indikator Murexid wurde in Gegenwart anionischer Mizellen untersucht. Die Indikatorreaktion wurde benutzt, um Aussagen über Mizellbildung, Bindung zweiwertiger Metallionen an Mizellen und die Kinetik der Metallkomplexbildung bei Anwesenheit von Mizellen zu erhalten.
With 8 figures and 1 table
Dedicated to Prof.G. Rehage on the occasion of his 60th birthday. 相似文献
997.
H. W. Kohlschütter und M. Daum 《Fresenius' Journal of Analytical Chemistry》1970,250(5):310-313
Zusammenfassung 2-, 3-, 4-Nitrophenole enthalten unterschiedliche Voraussetzungen für die Ausbildung intermolekularer Wasserstoffbrücken. Diese wirken sich bei der Säulen-Chromatographie an Silicagel mit dem Fließmittel Chloroform deutlich aus. Dazu werden Retentionskoeffizienten eingeführt und bestimmt. Mit dem Fließmittel Wasser sind die Einflüsse der Bildung von Wasserstoffbrücken verwischt. Die für Chloroform geltende Reihenfolge der Retentionskoeffizienten kehrt sich um. Als Funktionsanalyse für Silicagel wird der Oberflächenbedarf für die quantitative Trennung von 2-—Nitrophenol benutzt. Er ist unter Wasser größer als unter Chloroform. Die Einheit der Silicageloberfläche ist unter Wasser weniger trennwirksam als unter Chloroform.
Vortrag anläßlich der Tagung der GDCh-Fachgruppe Analytische Chemie vom 9.–11. April 1969 in Freiburg i. Br. 相似文献
Column chromatography of nitrophenols as function analysis for silica gel
The capacity to form intermolecular hydrogen-bridges is different for 2-, 3- and 4-nitrophenols. In chromatography on silica gel using chloroform as solvent these differences are very marked. For this reason retention coefficients are introduced and determined. When using water as solvent the effects of hydrogen-bridge formation are not visible. The sequence of retention coefficients applicable to chloroform is reverted. The surface requirements for the quantitative separation of 2-—nitrophenol on silica gel are used as function analysis. These requirements are greater under water than under chloroform. The separating effect of the silica gel unit is less under water than under chloroform.
Vortrag anläßlich der Tagung der GDCh-Fachgruppe Analytische Chemie vom 9.–11. April 1969 in Freiburg i. Br. 相似文献
998.
W. Czysz 《Fresenius' Journal of Analytical Chemistry》1967,226(4):382
Ohne Zusammenfassung 相似文献
999.
C. H. Brieskorn 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》1966,78(7):398-399
1000.
The extraction of the elements Zn, Cd, Hg, Ga, In, Tl, Sn, Pb, As, Sb and Bi into the solvents cyclohexanone, cyclohexanol, and cyclohexane from hydrochloric acid solutions, has been studied. Cyclohexanone is a more efficient but less selective extracting agent than cyclohexanol while cyclohexane has little extracting power. Solvent extraction of hydrochloric acid solutions of materials of geochemical interest into cyclohexanone has been used to enrich trace elements prior to spectrochemical analysis. 相似文献