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    TOTAL SYNTHESIS OF STRYCHNOS AND ASPIDOSPERMATAN ALKALOIDS

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    Genre
    Thesis/Dissertation
    Date
    2017
    Author
    Kokkonda, Praveen
    Advisor
    Andrade, Rodrigo B.
    Committee member
    Sieburth, Scott McNeill
    Wengryniuk, Sarah E.
    Cannon, Kevin C.
    Department
    Chemistry
    Subject
    Chemistry, Organic
    Permanent link to this record
    http://hdl.handle.net/20.500.12613/1649
    
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    DOI
    http://dx.doi.org/10.34944/dspace/1631
    Abstract
    The Strychnos class of indole alkaloids contain a pyrrolo[2,3-d]carbazole ABCE tetracyclic framework. The second-generation ABCE tetracycle approach was employed in the total synthesis of (±)-20-epi-lochneridine and progress toward total synthesis of (±)-alstolucine B. The second-generation approach featured Mitsunobu activation of the hydroxyethyl group in a gramine intermediate followed by intramolecular aza-Baylis-Hillman reaction. The substrate for hydroboration was redesigned to (±)-18-desmethyl akuammicine (1,1-disubstituted double bond), since the hydroboration of trisubstituted alkenes afforded tertiary alcohol via Markovnikov addition. The key steps were n-Bu3SnH mediated cyclization reaction to accomplish D-ring, tert-butyl hypochlorite indoline oxidation, and anti-Markovnikov hydroboration to introduce a primary alcohol. The total syntheses of Strychnos-Strychnos type bis-indole alkaloids (−)-leucoridine A and C were accomplished from the biomimetic dimerization of (−)-dihydrovalparicine. En route to (−)-dihydrovialparicine, known alkaloids (+)-geissoschizoline and (−)-dehydrogeissoschizoline were also prepared from commercially available N-tosyl indole 3-carboxaldehyde. Key steps consisted of an in situ dimerization of (−)-dihydrovalparicine from (−)-1, 2-dehydrogeissoschizoline with trifluoroacetic acid in the presence of 4 Å molecular sieves. Acid mediated ring-opening of the indolenine in (−)-leucoridine A to afford (−)-leucoridine C. DFT calculations were employed to elucidate the mechanism of dimerization, which suggested that a stepwise aza-Michael/spirocyclization sequence was preferred over the alternate hetero Diels-Alder cycloaddition reaction. A novel domino Michael/Mannich [4+2] annulation method was applied for concise total synthesis of Aspidospermatan alkaloids (+)-20-epi-condyfoline and progress toward the total synthesis of (+)-condyfoline. The additional key steps consisted of a LiHMDS mediated cyclization to form D-ring, dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) mediated spirocyclization to form pentacyclic thioether and indoline oxidation with MnO2.
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