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

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.