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Studies on deacetoxy/deacetylcephalosporin C synthasePereira, Inês Antunes Cardoso January 1993 (has links)
This thesis describes an investigation of the mechanism of the bifunctional, a-ketoglutarate dependent dioxygenase, deacetoxy/deacetylcephalosporin C synthase (DAOC/DACS), which catalyses the ring-expansion of penicillin N to deacetoxycephalosporin C (DAOC) and the hydroxylation of this to deacetylcephalosporin C (DAC). The conversion of the unnatural substrate 3-exomethylene cephalosporin C by DAOC/DACS has been investigated in detail. A new metabolite was isolated from incubations of the deuterated [4-<sup>2</sup>H]-3-exomethylene cephalosporin C, and was identified as the 3β-spiroepoxide cepham, (2Ṟ,3Ṟ,6Ṟ,7Ṟ)-l-aza-[2-<sup>2</sup>H]-3-spiroepoxy-7-[(5Ṟ)-5-amino- 5-carboxypentanamido]-8-oxo-5-thiabicyclo[4.2.0]octane-2-carboxylic acid. The results obtained indicate that this metabolite is a shunt product whose formation is enhanced by the operation of a deuterium kinetic isotope effect on an enzyme-bound intermediate. It has also been found that this 3β-spiroepoxide cepham is further converted by DAOC/DACS to 3-formyl cephalosporoate products. The mechanism of oxygenation of DAOC/DACS was investigated through <sup>18</sup>O-labelling studies. Incubations of [2-<sup>13</sup>C,3-<sup>2</sup>H]penicillin N and [4-<sup>2</sup>H]-3-exomethylene cephalosporin C with DAOC/DACS were carried out under <sup>18</sup>O<sub>2</sub> or in H<sub>2</sub><sup>18</sup>O. Incorporation of <sup>18</sup>O-label into the products [3-<sup>13</sup>C]DAC, [3-<sup>13</sup>C,4-²H]-3β-hydroxycepham and 3β-spiroepoxide cepham was observed from both sources. The results suggest that intermediates capable of oxygen-exchange are formed during the enzymatic reactions. Two substrate analogues, the 5-epipenicillin N and the 2β-difluoromethyl penicillin N, have been synthesised in order to probe the substrate specificity of DAOC/DACS with respect to the ring-expansion activity. The 5-epipenicillin N was not accepted as a substrate by DAOC/DACS, and the observations made indicate that it was unstable under the incubation conditions. No product was either observed from incubations of the 2β-difluoromethyl penicillin N with DAOC/DACS, although bioassay tests suggested a cephem product had been formed in very small amounts. Finally, the results of a substrate specificity comparison between the soluble recombinant enzymes deacetoxy/deacetylcephalosporin C synthase (DAOC/DACS) from Cephalosporium acremonium and deacetoxycephalosporin C synthase (DAOCS) from Streptomyces clavuligerus are described.
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Ruthenium-catalyzed azide-alkyne cycloaddition, and cyclometallation of 2-vinylpyridine with MCl[subscript 2](PPh[subscript 3])[subscript 3] and MHCl(PPh[subscript 3])[subscript 3] (M=Ru, Os) /Zhang, Li. January 2008 (has links)
Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references. Also available in electronic version.
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The kinetics of poly(acrylonitrile) homopolymer and co-polymer cyclization /Beltz, Linda Ann, January 1992 (has links)
Thesis (Ph. D.)--University of Washington, 1992. / Vita. Includes bibliographical references (leaves [126]-128).
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Intramolecular ring opening reactions of aziridines by [pi]-nucleophiles /Pulipaka, Aravinda B. January 2008 (has links)
Thesis (Ph.D.)--Ohio University, March, 2008. / Abstract only has been uploaded to OhioLINK. Includes bibliographical references (leaves 115-125)
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Methodological and synthetic studies of [4+3] cycloaddition reactions /Carter, Kevin William, January 1997 (has links)
Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 362-370). Also available on the Internet.
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Methodological and synthetic studies of [4+3] cycloaddition reactionsCarter, Kevin William, January 1997 (has links)
Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 362-370). Also available on the Internet.
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Studies on the use of (triphenylphosphine)copper(I) hydride hexamer in the tandem reduction-intramolecular aldol cyclisation reaction /Szeto, Chun-pong. January 2000 (has links)
Thesis (M. Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 106-108).
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Synthesis of precursors of a highly pyramidalized alkene and ab initio calculations on methylenecyclopropane, cyclopropene, and 1,3-diradicals /Johnson, William T. G. January 1999 (has links)
Thesis (Ph. D.)--University of Washington, 1999. / Vita. Includes bibliographical references (leaves 84-88).
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Asymmetric epoxidation of olefins and cyclization reactions catalyzed by amines /Ho, Chun-yu. January 2005 (has links)
Thesis (Ph. D.)--University of Hong Kong, 2005.
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A bridgehead organolithium reagent, the Retro-Nazarov reaction, and 4+3 cycloadditions with a nicotinic acid derivative /Kirchhoefer, Patrick L., January 2004 (has links)
Thesis (Ph.D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 263-272). Also available on the Internet.
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