Studies in the Total Synthesis of the Gelsemium Alkaloids PDF Download

Author: Phillip Conrad Knutson
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ISBN:
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Languages : en
Pages : 1108

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Book Description
The Gelsemium alkaloids represent a diverse class of natural products that have displayed interesting biological and medicinal properties. Because of their pharmacological interest as well as inherent synthetic challenges, chemists have been tackling the construction of these molecules for decades. Many of the Gelsemium alkaloids share a characteristic oxabicyclic core that is both densely packed and relatively unfunctionalized. We noticed that this core could be generated rapidly through a tandem cycloisomerization/Cope rearrangement. Through our execution of this methodology we were able to accomplish the total syntheses of Gelsenicine and Gelsedine. These total syntheses represent an entry into a divergent total synthesis program. Through the synthesis of a common core through the tandem cycloisomerization/Cope rearrangement, we envision access to other members of the Gelsemium alkaloid family, such as Gelsemoxonine. We have also demonstrated the enantioselective total synthesis of Gelsenicine through an enantioselective gold-catalyzed cycloisomerization. This was accomplished by using chiral ligands such as phosphoramidites and bisphosphines. We found that bisphosphine ligands bearing electron rich and bulky aryl substituents on phosphorous worked well to give high levels of asymmetric induction in the cycloisomerization product. Lastly, a convenient Cadiot8́2Chodkiewicz protocol that facilitates the use of low molecular weight alkyne coupling partners is described. The method entails an in situ elimination from a dibromooleƠ̐1n precursor and immediate subjection to copper-catalyzed conditions, circumventing the hazards of volatile brominated alkynes. The scope of this method is described, and the internal 1,3-diyne products are preliminarily evaluated in ruthenium-catalyzed azide-alkyne cycloadditions.