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Abstract

Pericyclic reactions are, arguably, one of the most important classes of reactions in organic synthesis. Their ability to provide access to complex and sterically hindered structures in a regio- and stereo-controlled manner in a highly atom economical fashion has played a crucial role in the total synthesis of innumerable complex natural products over the past several decades. Despite the significant interest from the synthetic community, multiple aspects of pericyclic reactions still remain unexplored. In this dissertation, we focused our research efforts on two different classes of pericyclic reactions, the Diels-Alder reaction and Claisen rearrangement, and their application towards the total synthesis of the natural products Hinckdentine and Melonine. The first part of this dissertation (Chapter 2) describes the development of a novel class of doubly activated dienes – oxazolidinebutadienes – for the Diels-Alder reactions. Previously, doubly-activated dienes, specifically Danishefsky’s diene, Rawal’s diene and Brassard’s diene, have found enormous popularity in the synthetic community due to their high reactivity towards a range of dienophiles, including heterodienophiles. Another reason for their popularity is the ability to easily convert the resulting Diels-Alder cycloadducts into substituted cyclohexenones, dihydropyranones and phenols, thus significantly expanding the scope and application of the Diels-Alder reaction. While these dienes provide direct access to key building blocks of many complex molecules, it produces only 4,4-disubstituted cyclohexenones, limiting the applications of these useful 4 components. We envisioned that the development of other activation patterns on dienes could expand the utility of the Diels-Alder reaction in organic synthesis and provide a useful tool to construct the core fragments of multiple natural products. In this vein, we developed a novel class of doubly activated 1-amino-1-oxo-1,3-butadienes, which proved to be highly reactive dienes in the Diels-Alder reaction with a variety of common dienophiles, giving rise to corresponding cyclohexenes in high yields and with high endo-selectivity. Hydrolysis of the resulting cyclohexenes gave rise to 6,6-disubstituted cyclohexenones, a valuable intermediate for the total synthesis of various natural products. Brief kinetic studies revealed that our newly developed diene is ~100 times more reactive than Danishefsky’s diene. The second part of this dissertation (Chapter 3) is focused on the development of a 3,3-sigmatropic rearrangement as an efficient tool to access 2,2-disubstituted indolines, the core motive of multiple bioactive natural products. The downside of the approaches to the 2,2-disubstituted indolines, that have been demonstrated to date, is a limited substrate scope - in many cases minor modifications in the structures of the compounds dramatically change the reaction outcome. We envisioned that the Claisen rearrangement of the indole alcohols, which can be easily accessed from the corresponding ketones and aldehydes, can provide a general route to access various substituted indolines. Our research indicated that, among other sigmatropic rearrangements, only the Meerwein-Eschenmoser-Claisen rearrangement was able to provide access to the desired indolines in a nearly quantitative yield, starting from the corresponding indole alcohols. Use of enantioenriched alcohols, which can be accessed from the corresponding ketones utilizing the Corey-Bakshi-Shibata asymmetric reduction protocol, as substrates for rearrangement resulted in a complete transfer of chirality, providing the corresponding 2,2-disubstituted indolines in an enantioenriched fashion. The third part of this dissertation (Chapters 4-5) describes the application of our newly developed indolic Claisen rearrangement towards the total synthesis of two distinctive indoline-based alkaloids, Hinckdentine A and Melonine. Successful application of our methodology provided an efficient access to the core structures of these natural products, which were further elaborated to complete the formal synthesis of these complex molecules.

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