Exocyclic alkene synthesis via stereoselective radical cyclizations

The stereoselectivity of radical cyclizations of stabilized radicals to alkynyl esters was investigated. In most cases, reactions mediated by tri(n-butyl)tin hydride afforded predominantly E-exocyclic alkenes, whereas reactions mediated by tris(trimethylsilyl)silane afforded predominantly Z-exocyclic alkenes. In the case of cyclizations to the tetrahydropyran derivatives, the stereoselectivity could be improved by increased substitution ;The formation of exocyclic alkenes via cyclization of secondary alkyl radicals to both (E)- and (Z)-bromo, iodo, and tri(n-butyl)stannyl-...