Browsing by Subject "Alkynes"
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Item 1,3-Dipolar Cycloaddition of Electron-rich Alkynes and Optically Active Allenes in Asymmetric Catalysis(2009-09-04) Qi, Xiangbing; Ready, Joseph M.This dissertation includes two parts. The first part focuses on two 1,3-dipolar cycloadditions of electron-rich alkynes. Chapter one describes a copper-promoted cycloaddition reaction of acetylides with diazocarbonyl compounds. This novel cycloaddition offers a direct and efficient approach to the synthesis of pyrazoles. The method is a rare example of an inverse-electron-demand cycloaddition, it represents a conceptually novel approach to this important class of heterocycles. Chapter two investigates a cycloaddition reaction between donor-acceptor cyclopropanes and silyl ynol ethers. Lewis acid promoted ring-opening of donor-acceptor cyclopropanes generates a 1,3-zwitterion; cycloaddition with a silyl ynol ether leads to a general synthesis of cyclopentenones. Substitution is tolerated on the ynol and on all positions of the cyclopropane to give tri-, tetra-, and penta-substituted cyclopentenones in high yield. methyl(methoxyl)aluminumchloride, which is generated from dimethylaluminumchloride by oxidation, appears strong reactivity towards ring-opening of donor-acceptorcyclopropanes and cycloaddition with silyl ynol ethers. This infrequently used species might be extended to other classes of cycloadditions or Lewis acid promoted reactions. The second part describes a new asymmetric catalysis design using optically active allenes as backbone and the application in an asymmetric meso-epoxides opening reaction. Allenes are inherently chiral and can be prepared in optically pure form. They have not been incorporated into ligands or catalysts for asymmetric reactions. Since allenes project functionality differently than either tetrahedral carbon or chiral biaryls, they may create complementary chiral environments. Chapter three demonstrates that optically active, C2-symmetric allene-containing bisphosphine oxides can catalyze the addition of silicon tetrachloride to meso-epoxides with high enantioselectivity. The fact that high asymmetric induction is observed suggests that allenes may represent a new platform for the development other classes of organic catalysts or ligands for asymmetris reactions.Item [3+2]-Cycloadditions of Azomethine Imines and Ynolates and Progress Towards a Radical Three-Component Cross-Coupling Reaction(2018-07-16) Winterton, Sarah Elizabeth; Chen, Chuo; Tambar, Uttam; De Brabander, Jef K.; Ready, Joseph M.This manuscript consists of three chapters. The first chapter describes a novel [3+2]-cycloaddition of azomethine imines and ynolates to construct bicyclic pyrazolidinones in high yields and diastereoselectivities. This methodology is the first example in which the azomethine imine acts as a chiral auxiliary to control the cycloaddition. Optically active azomethine imines yield bicyclic pyrazolidinones in high yields, and conditions for removal of the azomethine imine chiral auxiliary have been defined to yield optically active monocyclic pyrazolidinones in high yields and enantioselectivities. The second chapter consists of progress towards a novel [2+2]-annulation/fragmentation to construct cyclohexanones. Among all the methods to synthesize highly substituted cyclohexanols/cyclohexanones, there is no general, one-step enantioselective, intermolecular method to synthesize a highly substituted cyclohexanol/cyclohexanone with a quaternary center at C4. Therefore, we developed a synthesis of cyclohexanones via a formal [2+2]-annulation/fragmentation between a cyclobutanone containing an electron-withdrawing group and an electrophile. The second chapter describes attempts to render the reaction asymmetric, which proved to be difficult. Future ideas to improve yields and to render the reaction asymmetric are also described. The last chapter involves progress towards a novel three-component, enantioselective radical cross-coupling reaction to synthesize complex products in a single step. Current literature contains only scattered examples of three-component radical cross-coupling reactions, all of which have a severely limited substrate scope (e.g., only alkynes, one nucleophile, contains fluorine). Therefore, we envisioned a three-component radical cross-coupling reaction with a broad substrate scope encompassing multiple radical initiators and olefin acceptors. This method could provide access to products that are currently challenging to synthesize. Progress towards this three-component enantioselective reaction is presented in chapter three, as well as future ideas to improve enantioselectivity and yields.Item Preparation of Substituted Enol Derivatives from Terminal Alkynes and Progress Toward the Total Synthesis of Nigricanoside A(2011-02-01) DeBergh, John Robbins; Ready, Joseph M.This manuscript consists of two chapters. The first chapter describes the preparation of stereodefined enol derivates of alpha-branched aldehydes from terminal alkynes. Specifically, alkenyl alanes, derived from the methylalumination of alkynes, are shown to be efficiently oxygenated with peroxyzinc species. The resulting metallo-enolate may be trapped with benzoic anhydride, acetic anhydride, and TESOTf to generate E-trisubstituted enol esters and silanes. Traditional approaches to these types of olefins involve enolization of aldehydes; these methods are often inefficient and generally afford mixtures of olefin stereoisomers. Therefore, the methodology presented Chapter One represents a conceptually novel and useful strategy. The development and scope of the methylalumination-oxygenation reaction is discussed along with applications of the enol derivatives in the context of asymmetric and natural product synthesis. Finally, the alkenyl alane intermediates are shown to be efficiently aminated to afford ene-hydrazine products. The second chapter involves the progress towards the asymmetric synthesis and structural assignment of nigricanoside A, a potent antimitotic glycolipid isolated from marine green algae. A convergent synthetic route is presented along with an analysis the natural product’s relative and absolute stereochemistry. Various diastereomers of orthogonally protected subunits of nigricanoside A were prepared through vinyl-metal additions to alpha-hydroxy aldehydes. The chapter includes attempts to join the subunits through etherification reactions as well as descriptions of future strategies to effect etherification.