Doctor of Philosophy (PhD)
The development of new chemical transformations is of paramount importance in organic chemistry. In fact the study and creation of new reactions often uncovers new chemical reactivities and creates a new area of chemistry. The purpose of this dissertation is to present the development of new synthetic reactions that were uncovered through previous discoveries. Additionally the underlying theme of this work relies on the intermediacy of cationic reactive intermediates such as silyloxyallyl cations and epoxonium ions. Chapters 1 and 2 present the reactivity of novel silyloxyallyl cations. These cationic species are generated via the mild ionization of a-hydroxy silylenol ethers with Py•TfOH and are intercepted by various nucleophiles with exquisite regiochemical control. The role of solvent effects and in situ generated water were examined as these factors aided in the generation and stabilization of these unique intermediates.
Chapter 3 examines the role of triphosgene-amine base mixtures for the chlorination of simple alcohols, diols, and ketones. Methods for the synthesis of vicinal dichlorides is presented in Chapter 4, as well as our recent contribution utilizing triphosgene-pyridine for the chlorination of unactivated epoxides. This study showcased the utility of triphosgene-pyridine mixtures via the chlorination of numerous aliphatic epoxides with commonly utilized oxygen atom protecting groups. Additionally mechanistic analyses revealed the presence of several transitional intermediates that aided in the understanding of how triphosgene is able to chlorinate epoxides, and alcohols. Also the stereochemical outcome of this process was examined revealing a stereospecific process, similar to our previous investigations.
Lastly Chapter 5 provides a brief review of bicyclic epoxonium ions and methods for their generation, as well as their utility in the synthesis of interesting heterocycles. Also Chapter 5 illustrates our attempts at applying the information we gained from our previous projects utilizing triphosgene and pyridinium acids for the chemoselective cyclization of epoxides to novel chlorinated pyran and pyranoside heterocycles. Ultimately through this study we were able to uncover the unprecedented chemoselective activation of an epoxyketone with triphosgene, as well as discover a novel counterion based selectivity during the ionization of epoxyketones.
Cleveland, Alexander Houston, "New Synthetic Transformations Utilizing Silyloxyallyl Cations and Epoxonium Ions as Reactive Intermediates" (2020). LSU Doctoral Dissertations. 5249.