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Title page for ETD etd-07202007-012537


Type of Document Dissertation
Author Tung, James Chenkuo
URN etd-07202007-012537
Title Towards the synthesis of the cornexistins
Degree Doctor of Philosophy
Department Chemistry and Biochemistry
Advisory Committee
Advisor Name Title
Marvin J. Miller Committee Member
Olaf Wiest Committee Member
Richard Taylor Committee Member
Xavier Creary Committee Member
Keywords
  • nine membered ring
  • cornexistin
  • ring closing
Date of Defense 2007-07-13
Availability unrestricted
Abstract
Cornexistin and hydroxycornexistin are natural products derived from the fungus

Paecilomyces variotti. Both cornexistin and hydroxycornexistin are highly potent

herbicides that have the unique quality of being harmless to corn plants. Because of this

quality, both molecules have attracted research interest from agrochemical companies.

Their interesting nine-membered ring structure contains a maleic anhydride and a key

exocyclic ethylidene; this structure also indicates membership in the nonadride family.

We targeted this interesting structure for total synthesis for both its fascinating biological

activity and its highly functionalized structure.

Cornexistin was found to be susceptible to retro-aldol opening and reclosure of its

nine-membered ring by workers at Dow Agrosciences. We targeted a retro-aldol

precursor through a series of aldol reactions. Utilizing a Johnson-Claisen rearrangement

and an enantioselective Evans-type aldol to generate the starting fragments, a series of

reactions was performed to find the appropriate conditions for a key bond-forming ester

enolate aldol. Oxidation of the product yielded a beta-ketoester that contained the complete

carbon skeleton of cornexistin.

Previous progress towards cornexistin included the use of a Diels-Alder reaction

to generate a [6.5] bicyclic system that was oxidatively cleaved to deliver a cyclononene

diketone. During the course of analyzing the relative stereochemistry of the Diels-Alder

reaction, analog synthesis and X-ray crystallography was used to unambiguously

determine the structure of the cycloadduct. Through ozonolysis, regioselective titanium-mediated olefination and further functional group manipulation, highly complex nine-membered

ring structures were generated. Finally, ring-closing metathesis and Tamao- Fleming oxidation produced the key ethylidene moiety of both cornexistin and hydroxycornexistin.

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