Reaction of singlet oxygen with enolic tautomers of 1-aryl-2-methyl 1,3-diketones

Abstract

The 1-(2′,4′,6′-trialkylphenyl)-2-methyl 1,3-diketones 6 exist in the enol form in solution, and on reaction with singlet oxygen in acetonitrile give products arising from hydrogen abstraction from both the enolic hydroxy and the 2-methyl groups by the singlet oxygen; namely, the 2-hydroperoxy 1,3-diketones 7, the 2-methylene 1,3-diketones 8 and the epoxy ketones 9. The 2-hydroperoxy 1,3-diketones 7 readily undergo deoxygenation by triphenylphosphine to give the 2-hydroxy 1,3-diketones 12. In the reaction of 6 in methanol or ethanol, the initially formed enedione 8 reacts with the solvent to give the corresponding Michael adducts 10 and 11.