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Stereocontrolled radical cyclization reactions to yield exocyclic alkenes Lowinger, Timothy Bruno


The intramolecular reactions of a number of (E)- and (Z)-3-tri(n-butyl)stannyl-2-alkenoates with ω-alkyl radicals generated from alkyl iodides, epoxides, and aldehydes were investigated. These reactions were found to generate exocyclic alkenes in a stereoselective, and in some cases stereospecific, manner via a consecutive radical addition-fragmentation mechanism. In most cases, the geometry of the resulting exocyclic alkene was determined by the geometry of the starting vinylstannane. For example, reaction of the (E)-vinylstannane 75 with bis(cyclopentadienyl)titanium(III) chloride affords the (E)-exocyclic alkene 82 exclusively, whereas the analogous reaction of the (Z)-vinylstannane 76 affords the (Z)-exocyclic alkene 83 as the exclusive product. The formation of exocyclic alkenes via intramolecular addition reactions of secondary alkyl radicals to α,β-alkynyl esters was also investigated. The stereoselectivity of these reactions was found to be highly dependent on the reaction conditions. For example, reaction of 146 with tri(n-butyl)tin hydride in refluxing benzene affords predominantly the (E)-exocyclic alkene 142. In contrast, reaction of 146 with tris(trimethylsilyl)silane at low temperature affords predominantly the (Z)-exocyclic alkene 144. [ Formulas omitted ]

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