PartI. Synthesiss 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 vinyls
Abstract (Summary)Part One: The Barton-McCombie reaction has been widely acknowledged for the efficient deoxygenation of 2o 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 ?-position to give alkyl radicals. Although the sp^3 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.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/2004