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Mn(III)-Based Oxidative Free-Radical Cyclizations of Unsaturated Ketones

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journal contribution
posted on 01.11.1996, 00:00 by Bridget 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.