Alkoxy Radical Cyclizations onto Silyl Enol Ethers Relative to Alkene Cyclization, Hydrogen Atom Transfer, and Fragmentation Reactions

作者：Rueda Becerril Montserrat; Leung Joe C T; Dunbar Christine R; Sammis Glenn M^*

来源：Journal of Organic Chemistry, 2011, 76(19): 7720-7729.

DOI：10.1021/jo200992m

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 beta-fragmentations. Cyclization onto silyl enol ethers in a 5-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 beta-fragmentations. Alkoxy radical 5-exo cyclizations onto silyl enol ether also outcompeted I,5-HATs, even for activated benzylic hydrogen atoms. In tetrahydropyran synthesis, where I,5-HAT has plagued alkoxy radical cyclization methodologies, 6-exo cyclizations were the dominant mode of reactivity. beta-Fragmentation still remains a challenge for tetrahydropyran synthesis when an aryl group is present in the beta position.

• 出版日期2011-10-7