Alkoxy radical cyclizations onto silyl enol ethers relative to alkene cyclization, hydrogen atom transfer, and fragmentation reactions

Montserrat Rueda-Becerril, Joe C.T. Leung, Christine R. Dunbar, Glenn M. Sammis

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20 Scopus citations

Abstract

This study examines the chemoselectivity of alkoxy radical cyclizations onto silyl enol ethers compared to competing cyclizations, 1,5-hydrogen atom transfers (1,5-HATs), and β-fragmentations. Cyclization onto silyl enol ethers in a S-exo mode is greatly preferred over cyclization onto a terminal alkene. The selectivity decreases when any alkyl substitution is present on the competing alkene radical acceptor. Alkoxy radical 5-exo cyclizations displayed excellent chemoselectivity over competing β-fragmentations. Alkoxy radical S-exo cyclizations onto silyl enol ether also outcompeted 1,5-HATs, even for activated benzylic hydrogen atoms. In tetrahydropyran synthesis, where 1,5-HAT has plagued alkoxy radical cyclization methodologies, 6-exo cyclizations were the dominant mode of reactivity. β-Fragmentation still remains a challenge for tetrahydropyran synthesis when an aryl group is present in the β position.

Original languageEnglish (US)
Pages (from-to)7720-7729
Number of pages10
JournalJournal of Organic Chemistry
Volume76
Issue number19
DOIs
StatePublished - Oct 7 2011

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