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    Photocycloaddition of Conjugated Enynes and its Application in Natural Products Synthesis

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    Genre
    Thesis/Dissertation
    Date
    2016
    Author
    Khatri, Buddha Bahadur
    Advisor
    Sieburth, Scott McNeill
    Committee member
    Davis, Franklin A.
    Andrade, Rodrigo B.
    Fox, Joseph M.
    Department
    Chemistry
    Subject
    Chemistry
    Permanent link to this record
    http://hdl.handle.net/20.500.12613/1608
    
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    DOI
    http://dx.doi.org/10.34944/dspace/1590
    Abstract
    Aromatic 2-pyridones and 2-pyrones are known to undergo a facile [4+4] photocycloaddition with themselves and other conjugated molecules. The unifying objective of this work is to explore the potential of conjugated enynes as a photocycloaddition partner with a variety of unsaturated systems and establish it as a path for the synthesis of valuable natural products and molecular scaffolds. With a 1,3-enyne as a reactant, the immediate photocycloadduct, a highly strained cyclic allene, was stabilized and functionalized to give advanced intermediates in natural product synthesis. Participation of substituted benzenes in higher order photocycloaddition with 2-pyridones was also discovered. Intramolecular enyne-pyridone [4+4] photocycloaddition led to the formation of cyclic, strained allenic products that undergo rapid dimerization through various modes leading to complex mixtures. Such allene-allene dimerization was suppressed by introducing steric shielding. With reactive tethered functional group present, the intermediate allene can undergo a secondary cycloaddition with either the proximal or distal double bond, depending on the tether length. In the absence of such functional groups, the allene can isomerize to a 1,3-diene. Oxidation of the allene led to a cyclopropanone, a bicyclo-[5.1.0]-octane, which was transformed into pseudoguaiane-like 7-5 ring systems. Participation of enynes in the photocycloaddition of other unsaturated systems was also investigated. Anthracene, naphthalene derivatives and 2-pyrones were all found to undergo quantitative photocycloaddition with an enyne. During this investigation, a low temperature Cope rearrangement was also discovered. In addition, meta-substituted benzenes were reluctant partners in [4+4] photocycloaddition with enynes but underwent efficient [4+4] photocycloaddition with 2-pyridones. The synthetic utility of enyne photocycloaddition was applied in the synthesis of the cyclooctanoid containing natural product, (+)-dactylol. The intramolecular enyne – 2-pyrone [4+4] photocycloaddition-isomerization gave a lactone bridged-cyclooctanoid product which serves as an advanced intermediate for sesquiterpene synthesis.
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