Part I. Synthesis of N-heterocyclic Furanosides and Pyranosides Via 5- Or 6-exo-trig-radical Cyclization. Part II. (a) Palladium Catalyzed Silystannylative Cyclization of Diynes and Allenynes. (b) Regioselective Diels-Alder Reaction of Vinylsilane and Its Application to Papulacandin D Core Structure PDF Download

Author: Jong Uk Rhee
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Category : Catalysis
Languages : en
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Book Description
Abstract: Part One: The Barton-McCombie reaction has been widely acknowledged for the efficient deoxygenation of 2° alcohols through radical process. For the chain process, the radical source initially generated by the catalytic amount of AIBN attacks the thiocarbnyl sulfur to produce a radical intermediate that is cleaved at the beta-position to give alkyl radicals. Although the sp3 carbon centered radicals have been used commonly as precursors of 6-exo-trig ring closure, the radical intermediate in the thiourethane-mediated deoxygenation of alcohol has never been considered as a precursor of radical cyclization. However, the radical intermediate can participate in an exo-hex-5-enyl or exo-hept-6-enyl type intramolecular cyclization when a suitable radical acceptor is appropriately placed. Moreover, structurally rare examples of N-furanosides and N-pyranosides have been synthesized efficiently from carbohydrate-derived imidazoyl and triazoyl thioates by utilizing the new Barton's radical-mediated methodology. Depending on the radical acceptors, glycosides with either C2-carbon or C2-amino substituents are formed. The C2 stereogenic center of the N-pyranosides formed via exo-hept-6-enyl type radical cyclization can also be controlled by the stereochemistry of the radical acceptor. While (E)-olefin acceptors make allo/altro mixture, (Z)-olefin acceptors give stereoselectively altro-isomers. Part Two: Palladium catalyzed tandem bifunctionalization/cyclization of enynes, diynes, bis(allenes), alleneynes, allene aldehydes, and allene ketones with R3Sn-SiR'3 is a useful tactic for the synthesis of synthetically interesting heterocyclic and/or carbocyclic compounds. Although the reaction has a broad tolerance to functional groups, operationally simple procedure, and high catalytic turnover and regioselcetivity, the applications of the silylstannylated carbocyclization to further transformation are rare. The palladium catalyzed silylstannylation/cyclization of 1, 6-diynes gives two atropisomeric isomers with axial chirality. The silylstannyl dienes themselves do not take part in Diels-Alder reactions, but the removal of Sn has been achieved efficiently under acidic condition and the resulting silyldienes have restored this reactivity. In this study a number of fused carbocyclic or heterocyclic compounds were made by the Diels-Alder reaction and the regioselectivity was controlled by steric effects. This regioselective Diels-Alder route has been used for the synthesis of an intermediate for a Papulacandin D core synthesis. Asymmetric version of silylstannylation/carbocyclization of alleneynes has been screened with a various chiral phosphine ligands.