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91.
Ernst Bayer Gerd Wahl und Heinz Günther Witsch 《Fresenius' Journal of Analytical Chemistry》1961,181(1):384-390
Zusammenfassung Es werden zwei neue, sehr selektive Trennflüssigkeiten, Äthylenglykol-bis(propionitril)-äther und Glycerin-tri(propionitril)-äther, angegeben. Die Trennflüssigkeiten sind so selektiv wie ,-Bis(propionitril)-äther, erlauben aber höhere Arbeitstemperaturen von 140° bzw. 170°C. An Säulen kleinen Querschnittes wird die präparative Trennung größerer Mengen unter Ausnutzung der Selektivität dieser Trennflüssigkeiten aufgezeigt.Die Trennwirksamkeit und die Belastbarkeit von Säulen verschiedenen Durchmessers zwischen 1–10 cm wird angegeben. Die Trennwirksamkeit liegt in der gleichen Größenordnung wie die der analytischen Säulen. Die Belastbarkeit steigt proportional dem Querschnitt an.I. Mitteilung: E. Bayer u. H. G. Witsch, diese Z. 170, 278 (1959). 相似文献
92.
Tarek A.?SalamaEmail author Bernd?Lackner Heinz?Falk 《Monatshefte für Chemie / Chemical Monthly》2003,134(8):1113-1119
Summary. An efficient synthesis of tri-O-methylemodin aldehyde was achieved via bromination of tri-O-methylemodin utilizing N-bromosuccinimide yielding the monobromo and dibromo derivatives. Sommelet reaction of the monobromomethyl derivative as well as hydrolysis of the dibromomethyl analog with aqueous silver nitrate afforded the protected aldehyde in good yield. Accordingly, both bromo derivatives can be used even when they are obtained as a mixture of the bromination reaction, which could not be controlled easily to yield the bromo products selectively. From the aldehyde the tri-O-methylemodin nitrile was prepared in a one-pot reaction using hydroxylamine-O-sulfonic acid.Received February 18, 2003; accepted March 3, 2003
Published online June 2, 2003 相似文献
93.
Broussy S Bernardes-Génisson V Gornitzka H Bernadou J Meunier B 《Organic & biomolecular chemistry》2005,3(4):666-669
An ortho-metallation-electrophilic substitution sequence was employed as a key step to build the 4-benzoylpyridine framework. It was found that 4-benzoylpyridine-3-carboxamide and an N-pyridyl alkylated derivative both exist in a unique cyclized hemiamidal structure, not in the usually expected keto-amide open form. These structures represent fragment models of the Isoniazid-NAD adducts involved in the mechanism of action of the antituberculous drug Isoniazid. 相似文献
94.
Summary Stepwise electrophilic bromination of hypericin in pyridine as the solvent yields regioselectively 2,5-dibromohypericin, 2,5,9-tribromohypericin, and 2,5,9,12-tetrabromohypericin. The compounds were characterized by means of NMR-, UV-Vis-, and fluorescence-spectra, andpK
a
andpK*
a
measurements. These properties were compared with those of hypericin on the one hand and of the recently isolated gymnochromes on the other hand.
Zur Bromierung des Hypericins: Die Gymnochrom-Chromophore
Zusammenfassung Schrittweise elektrophile Bromierung von Hypericin in Pyridin als Lösungsmittel liefert regioselektiv 2,5-Dibromhypericin, 2,5,9-Tribromhypericin und 2,5,9,12-Tetrabromhypericin. Diese Verbindungen wurden durch NMR-, UV-Vis- und Fluoreszenz-Spektrometrie sowie durchpK a - undpk* a -Messungen charakterisiert. Diese Eigenschaften wurden mit jenen des Hypericins und der kürzlich isolierten Gymnochrome verglichen.相似文献
95.
Spiro 3-Oxazolines from the 1,3-Dipolar Cycloaddition of Benzonitrilio-2-propanide and 1,4-Quinones On irradiation with light of wavelength 290–350 nm, 2,2-dimethyl-3-phenyl-2H-azirine (1b) reacts with 1,4-naphthoquinone to give the 1H-benzo [f]isoindol-4,9-dione (11) (Scheme 3) via cycloaddition of the benzonitrilio-2-propanide (2b) onto the quinone C, C-double bond. With 2-methyl- and 2,3-dimethyl-1,4-naphthoquinone, the nitrile ylide 2b undergoes cycloaddition preferentially onto the C, O-double bond of the quinone, leading to spiro-oxazolines 12 and 14 (Scheme 4). Steric as well as electronic effects can be discussed to explain the observed site selectivity of the cycloaddition. With the 1,4-benzoquinones 15a, 15b, 15d and 15f , nitrile ylide 2b undergoes the 1,3-dipolar cycloaddition exclusively onto the C, O-double bond. The corresponding spiro-oxazolines have been isolated in 17–32% yield. This contrasts with the previously reported results with benzonitrilipo-phenylmethanide (2a) , which undergoes cycloaddition to the C, C-double bond of 1,4-benzoquinones (cf. [1]). This difference in the site selectivity of the 1,3-dipolar cycloaddition can be explained with Houk's concept of LUMO-polarization, that is, the stronger nucleophilic dipol 2b polarizes the LUMO of a α,β-unsaturated carbonyl compound more efficient than the less nucleophilic 2a. This leads to a preference of the cycloaddition to the C, O-double bond in the case of 2b. With 2,3-dimethyl- (15c) and 2,3,5,6-tetramethyl-1,4-benzoquinone (15e) , nitrile ylide 2b undergoes C, O- as well as C, C-cycloaddition (Schemes 7 and 8). 相似文献
96.
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-50 The 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. 相似文献
97.
Alexander Orahovats Barry Jackson Heinz Heimgartner Hans Schmid 《Helvetica chimica acta》1973,56(6):2007-2011
Irradiation of 2-azido-3-phenyl-propene ( 5 ) in pentane or benzene solution with a high pressure lamp (pyrex filter) yields 3-benzyl-2H-azirin ( 6 ), which on further irradiation behind quartz or vycor in the presence of trifluoroacetic acid methylester or carbon dioxide yields 4-benzyl-5-methoxy-5-trifluoromethyl-3-oxazolin ( 8 ) and 4-benzyl-3-oxazolin-5-one ( 9 ), respectively (scheme 2). A small amount of 3-phenylacetonitrile is also formed. 相似文献
98.
Static properties of an isotropic magnet are calculated in the whole critical region including the magnetization curve. The method proposed is a resummation of renormalized perturbation theory without use of recursion relations. This is possible because only special diagrams or subdiagrams show infrared divergencies at the magnetization curve due to Goldstone modes. The arguments given are heavily based on Ward identities. The resulting perturbation theory is well behaved in the total critical region and exhibits explicitely the form of the Goldstone mode singularities at the magnetization curve. The equation of state is calculated including two-loop contribution. Resulting effective exponents are then correct in order in the whole critical region. In various limits agreement with known results is found. A one-loop calculation of the correlation functions is also given. 相似文献
99.
The redox coupleb-nor-biladiene-ac (2)/b-nor-bilatriene-abc (3) exhibits an oxidative half wave potential of 800 mV in acetonitrile solution. It is demonstrated that it can be used as an extremely efficient electron carrier through bulk membranes. Whereas2 is not suitable as a ligand in carrier mediated cation transport,3 is a medium suited carrier for Cu(II) or Zn(II) ions, however, it is the most efficient one found so far for transport of Hg(II) ions. A 1:1 zinc chelate of3 is isolated and characterized. it is deduced to represent a polymer consisting of about 73 monomer units. 相似文献
100.
Günther Kraft Dieter Lindenberger Heinz Beck 《Fresenius' Journal of Analytical Chemistry》1976,282(2):119-121
Zusammenfassung Die Intensität des flammenabsorptionsspektrometrischen Cr-Signals hängt in einer sehr komplexen Weise von der Wertigkeit des Chroms und der Flammentemperatur ab. Ferner ist ein ausgeprägter Kalium-Einfluß vorhanden. Alle diese Schwierigkeiten können am besten dadurch eliminiert werden, daß in der Lachgasflamme gearbeitet und die Analysenlösung mit einer höheren K-Konzentration gepuffert wird.
Effect of valency on the determination of chromium by flame absorption spectrometry
The intensity of the chromium signal in AAS depends in a very complex manner strongly on the valency of the Cr and the flame temperature. It is furthermore influenced by potassium, if present. The best way to overcome all these interferences is to use the nitrous oxide flame and to buffer the solution with a higher concentration of potassium.相似文献