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Klaus-Peter Zeller

Publications and source records attributed to Klaus-Peter Zeller.

10 recordsLinked to original sources
PubMed2005

The dimethyldioxirane-mediated oxidation of phenylethyne.

The product pattern found for the dimethyldioxirane-mediated oxidation of phenylethyne strongly depends on the reaction conditions. Dimethyldioxirane generated in situ from caroate (HSO(5)(-)) and acetone in acetonitrile-water furnishes phenylacetic acid as the main product. With solutions of dimethyldioxirane in acetone, mandelic acid and phenylacetic acid are mainly formed. The relative abundances of the two acids depend on the residual water present in the dimethyldioxirane-acetone solution. Application of thoroughly dried solutions of the reagent effects increased formation of mandelic acid. When phenylethyne is oxidized by dimethyldioxirane transferred into tetrachloromethane, to minimize traces of water even further, oligomeric mandelic acid is obtained. The results are rationalized by the initial formation of phenyloxirene, which is known to equilibrate with phenylformylcarbene and benzoylcarbene. Subsequent Wolff rearrangement produces intermediate phenylketene, which can be trapped by water as phenylacetic acid or suffer from further oxidation to the alpha-lactone of mandelic acid. The alpha-lactone can either react with water to yield mandelic acid or, under anhydrous conditions, to yield oligomeric mandelic acid. In addition to mandelic acid and phenylacetic acid phenylglyoxylic acid, benzoic acid and benzaldehyde are observed as reaction products. The formation of phenylglyoxylic acid by transfer of two oxygen atoms to the unrearranged carbon skeleton of phenylethyne followed by oxygen insertion into the aldehydic C-H bond of the intermediately formed phenylglyoxal is discussed. In a second pathway this acid is formed by partial oxidation of mandelic acid. Benzaldehyde and benzoic acid are explained as products of the oxidative degradation of the alpha-lactone by dimethyldioxirane. Under in situ conditions benzoic acid is also formed by caroate initiated oxidative decarboxylation of phenylglyoxylic acid and/or intermediate phenylglyoxal.

Journal Article↗
PubMed2003

The photochemical Wolff rearrangement of 3-diazo-1,1,1-trifluoro-2-oxopropane revisited.

Ethyl 2-diazo-4,4,4-trifluoroacetoacetate (1a) and 3-diazo-1,1,1-trifluoro-2-oxopropane (1b) exhibit a deviating behavior in solution photolysis (hydrogen abstraction for 1a; Wolff rearrangement for 1b) [(a) F. Weygand, W. Schwenke and H. J. Bestmann, Angew. Chem., 1958, 70, 506; (b) F. Weygand, H. Dworschak, K. Koch and S. Konstas, Angew. Chem., 1961, 73, 409]. As shown by 13C-labelling of 1b this difference is not caused by rearrangement of the primarily formed alpha-oxocarbene to an isomeric alpha-oxocarbene presenting a hydrogen atom as a migrating substituent for the Wolff rearrangement. It is discussed that the singlet alpha-oxocarbene generated from 1a rapidly undergoes spin equilibration followed by hydrogen abstraction of the triplet alpha-oxocarbene. In contrast, due to a larger singlet-triplet splitting in the singlet alpha-oxocarbene generated from 1b, the intramolecular Wolff rearrangement on the singlet surface can efficiently compete with the singlet-triplet interconversion.

Journal Article↗
PubMed2003

[Methyl-D3]2hypericin as internal standard for quantification in human plasma.

The multistep synthesis and negative ion-ESI fragmentation pattern of [methyl-D(3)](2)hypericin (1-D(6)) is described. The application of 1-d(6) as internal standard for the quantification of hypericin (1) in the ng mL(-1) range in human plasma by isotope-dilution LC-MS is demonstrated. The hypericin-containing plasma samples are spiked with 1-D(6), deproteinized and extracted with ethyl acetate. The extracts are injected into a HPLC-ESI-ion-trap system and the mass-separated negative ions from 1 and 1-D(6) are analysed. From their intensities linear standard curves over the concentration range from 1 to 10 ng mL(-1) are obtained. Accuracy, precision and recovery are discussed.

Anthracenes↗
PubMed2003

Terpenes from Inula verbascifolia.

The aerial parts of Inula verbascifolia afforded two new xanthanes and a new germacranolide derivative, together with the known compounds inusoniolide, 4-O-dihydroinusoniolide and 9beta-hydroxyparthenolide. The structures were determined by spectral methods (IR, HRMS,1H NMR, 13C NMR, DEPT, 1H-1H COSY, HMQC and HMBC).

Chromatography↗
PubMed2003

The chemical constituents of Capparis spinosa of Jordanian origin.

Investigation of Capparis spinosa of Jordanian origin lead to isolation of two new compounds beta-sitosterylglucoside-6'-octadecanoate (1) and 3-methyl-2-butenyl-beta-glucoside (2). Linked Scan MS measurements were used to propose a mass fragmentation pattern for the alkaloid Cadabicine isolated here for the second time from nature.

Capparis↗
PubMed2002

Three pyrone glucosidic derivatives from Conyza albida.

Three new pyrone glucosidic derivatives, together with the known pyromeconic acid glucoside, three acytelenes and two eudesmanes, were obtained from the aerial parts of Conyza albida. The structures were elucidated by high field NMR spectroscopy.

Asteraceae↗
PubMed2002

The oxidative biotransformation of losoxantrone (CI-941).

The oxidative biotransformation of the anticancer drug 7-hydroxy-2-[2-[(2-hydroxyethyl)amino]ethyl]-5-[[2-[(2-hydroxyethyl)amino]ethyl]amino]anthra[1,9-cd]pyrazol-6(2H)-one dihydrochloride (losoxantrone, CI-941) after incubation of primary cultures of rat hepatocytes has been investigated. The structures of twelve losoxantrone metabolites have been elucidated by means of high-performance liquid chromatography-mass spectometry, tandem mass spectrometry, and two-dimensional NMR. In these mammalian hepatocytes, the CI-941 biotransformation includes a monohydroxylation of the phenolic substructure of the CI-941-chromophore via cytochrome P450 catalysis, resulting in metabolites having an ortho- and para-hydroquinonoid substructure, respectively. The identification of a glutathione conjugate as a follow-up metabolite confirms the oxidative activation of the ortho-hydroxylated losoxantrone metabolite. The oxidative activation establishes the ability of CI-941 to form covalent bonds to intracellular nucleophilic targets. Furthermore, the CI-941 metabolism was shown to be extremely suppressed in rat hepatocytes incubated with metyrapone. In contrast to these results, human tumor HepG2 cells did not show any CI-941 biotransformation after incubation.

Animals↗
PubMedDate not supplied

Flavonoids and terpenoids from the resinous exudates of Madia species (Asteraceae, Helenieae).

The resinous material accumulated on aerial parts of Madia species is shown to consist mainly of diterpenes, containing a series of flavonoid aglycones. A6- and/or 8-O-substitution is characteristic for many of these flavonoids. Three known rare diterpenes were found and the structure elucidation of a diterpene with a new carbon skeleton, named madiaol, is reported.

Asteraceae↗