Catalytic Asymmetric Nazarov Cyclizations & Methods for the Synthesis of Fluorenes and Advanced Geodesic Polyaromatic Hydrocarbons

Dickinson, Cody Francis
Tius, Marcus
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PART I: CATALYTIC ASYMMETRIC NAZAROV CYCLIZATIONS The synthesis of all-carbon quaternary stereocenters remains a significant challenge in organic synthesis. Even more challenging is the synthesis of vicinal all-carbon quaternary stereocenters. While there are a small number of methodologies that have attempted to address both of these problems, there are very few reports on the catalytic asymmetric synthesis of the latter type. The Nazarov cyclization offers a unique opportunity to address this problem, in that the orbital symmetry-controlled process can be catalyzed in a number of different ways because of the diversity of the acyclic precursors. This thesis will describe a new catalytic asymmetric synthesis of compounds containing vicinal all-carbon quaternary stereocenters through a Nazarov cyclization. In the first chapter the Nazarov cyclization is introduced and briefly reviewed. A brief review on catalytic asymmetric variants of this reaction is given. In the second chapter an (R)-BINOL-derived N-triflyl phosphoramide-catalyzed Nazarov cyclization of geometrically pure dienones to α-hydroxycyclopentenones is described. This follow up work describes an expanded substrate scope to all-alkyl C1,C2-tetrasubstituted cyclopentenones containing vicinal quaternary centers and to C1,C5-trisubstituted cyclopentenones containing vicinal quaternary and tertiary centers. The mode of asymmetric induction is discussed.In the third chapter a novel Lewis acid-Brønsted base-catalyzed cyclization of α-diketoesters to α-hydroxycyclopentenones is described. The chiral-at-rhodium catalyst for this metal enolate Nazarov cyclization is unique and demonstrates high catalytic activity and enantioselectivity. The substrate scope of all-aliphatic α-hydroxycyclopentenones bearing either vicinal quaternary or vicinal quaternary and tertiary stereocenters is described. PART II: METHODS FOR THE SYNTHESIS OF FLUORENES AND ADVANCED GEODESIC POLYAROMATIC HYDROCARBONS Polyaromatic hydrocarbons (PAH) have a long and rich history in organic synthesis. At the forefront today is the synthesis of highly strained geodesic compounds, for example, buckminsterfullerene, C60 (buckyball). Interest in non-planar PAHs has grown because of their unique UV-vis, electronic, and structural properties. The use of these compounds in asymmetric catalysis has not been described to date, possibly because their synthesis is impractical on scale. As such, our group has an interest in developing new methodologies to address these shortcomings. We have focused on the fluorene motif as an initial starting point in the synthesis of more advanced PAHs, since many others have identified it in geodesic hydrocarbons. In the first chapter a new methodology for the preparation of fluorenes and dibenzo[g,p]chrysenes through an oxidative cascade process using copper(II) bromide is described. A novel dibenzo[g,p]chrysene containing an ether linkage is reported. In the second chapter a synthesis of a geodesic, C30 semibuckminsterfullerene “buckybowl” is described. The synthetic route used to access this compound is versatile and modular. The scalable and simple synthesis of a strategic precursor 1,8-difluoro-9H-fluorene is also described.
Organic chemistry, asymmetric catalysis, geodesic polyaromatic hydrocarbon, Nazarov cyclization, vicinal quaternary stereocenters
681 pages
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