Superoxide Anion Radical (O2-anionradical) Mediated Base Catalyzed Autooxidation of α-Keto Enols

Article abstract of DOI:10.1021/jo00281a031

Eight 4,4-disubstituted 2-hydroxycyclohexa-2,5-dien-1-ones were prepared by the base-catalysed autooxidation (BCA) of the corresponding 4,4- or 5,5-disubstituted cyclohex-2-en-1-ones.Upon reaction with superoxide anion radical (O2-anionradical, generated from KO2/18-crown-6) in inert nonpolar aprotic media at room temperature, α-keto enols 3a-g undergo initial deprotonation of the enol hydrogen followed by BCA at C3 of the resulting enolate.Aqueous acid workup of the reaction mixture yields lactols 4, while methyl iodide quenching generates methoxy lactones 5.Lactols 4 can be readily converted to their acetoxy analogues 8, opened to aldehydo methyl esters 6, or reduced to the related lactones 7.The latter suggests a convenient one-pot synthesis of 2,3-unsaturated δ-valerolactones from the corresponding cyclohex-2-en-1-ones. 4,4-Diphenyl enol 3h, by contrast, resists BCA (whether mediated by O2-anionradical or t-C4H9O-anion) to the corresponding lactol yielding instead a variety of oxidative cleavage products 13-18. 2-Hydroxyspiro<4.5>dec-1-en-3-one (21) also underwent O2-anionradical-mediated BCA, yielding diacids 22 and 26 as well as lactol 30.The synthetic applications of these results are also discussed.