MN(III)-BASED OXIDATIVE FREE-RADICAL CYCLIZATIONS OF UNSATURATED-KETONES

Mn(III)-based oxidative free-radical cyclization of unsaturated ketone s is a versatile synthetic procedure with broad applicability. For exa mple, oxidation of cyclopentanone la with 2 equiv of Mn(OAc)(3) . H2O and 1 equiv of Cu(OAc)(2) . H2O in AcOH at 80 degrees C for 1.5 h affo rds 75% of bicyclo-[3.2.1]oct-3-en-8-one 8a and 15% of bicyclo[3.2.1]o ct-2-en-8-one 9a. Bridged bicyclic ketones that cannot enolize further are isolated in good yield. Monocyclic beta,gamma-unsaturated ketones that can enolize are oxidized further to give gamma-acetoxy enones. T he formation of bicyclo[3.3.1]non-2-en-9-one (57a) in 52% yield from 2 -allylcyclohexanone (56a) suggests that kinetically controlled enoliza tion is the rate-determining step in a-keto radical formation. A wide variety of examples delineating the scope, limitations, and stereosele ctivity of this reaction are presented.