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Exocyclic alkene synthesis via stereoselective radical cyclizations

University of British Columbia

The stereoselectivity of radical cyclizations of stabilized radicals to alkynyl esters was investigated. In most cases, reactions mediated by tri(A7-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 a to the exocyclic double bond. For example, reaction of 37 with tris(trimethylsilyl)silane in refluxing benzene produced nearly equal amounts of 52 and 53, whereas 72 was selective for the Z-exocyclic alkene 76. The formation of exocyclic alkenes via cyclization of secondary alkyl radicals to both (£)- and (Z)-bromo, iodo, and tri(n-butyl)stannyl-a,B-unsaturated esters was investigated. In many cases reactions were highly stereoselective. For example, the vinyl bromide 125 cyclized to afford predominantly the (E)-exocyclic alkene 123. Unlike its analogous vinyl bromide, the vinyl iodide 155 cyclized to afford predominantly the (Z)-exocyclic alkene 124. [Chemical model]

Thesis/Dissertation

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2009-04-20

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http://hdl.handle.net/2429/7416

Chemistry

University of British Columbia

UBCV

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