posted on 1996-11-01, 00:00authored byBridget McCarthy Cole, Luning Han, Barry B. Snider
Mn(III)-based oxidative free-radical cyclization of
unsaturated ketones is a versatile synthetic
procedure with broad applicability. For example, oxidation of
cyclopentanone 1a with 2 equiv of
Mn(OAc)3·2H2O and 1 equiv of
Cu(OAc)2·H2O in AcOH at 80 °C for 1.5
h affords 75% of bicyclo[3.2.1]oct-3-en-8-one 8a and 15% of
bicyclo[3.2.1]oct-2-en-8-one 9a. Bridged
bicyclic ketones that
cannot enolize further are isolated in good yield. Monocyclic
β,γ-unsaturated ketones that can
enolize are oxidized further to give γ-acetoxy enones. The
formation of bicyclo[3.3.1]non-2-en-9-one (57a) in 52% yield from 2-allylcyclohexanone
(56a) suggests that kinetically controlled
enolization is the rate-determining step in α-keto radical formation.
A wide variety of examples
delineating the scope, limitations, and stereoselectivity of this
reaction are presented.