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Thermal and Photochemical Approaches to the Synthesis of vic-Tricarbonyl Compounds from 4-Hydroxyquinolin-2(1H)-ones

vic-Tricarbonyl compounds are important both as interesting synthetic intermediates and as biologically active compounds.1–6 As a result, the investigation of synthetic methodology of vicinal triketones remains of current research interest. However, the preparation of victricarbonyl compounds from 1,3-dicarbonyl compounds usually requires their initial transformation to an active precursor such as the keto-phosphorane or enamine, which is then readily oxidized to the tricarbonyls either thermally (e.g. by ozone) or photochemically.7–12 Hypervalent iodine (V and III) compounds have also been used as alternative oxidants but the explosive character is a drawback for hypervalent iodine (V) and the employment of the ozone as the co-oxidant restricts practical application of hypervalent iodine (III).13–14 Two decades ago, Wasserman et al. described the dye-sensitized photooxidation of cyclic enolic 1,3-dicarbonyl compound with singlet oxygen. However, for heterocyclic 1,3-dicarbonyl compounds, such as 4-hydroxycoumarin, none of the corresponding trione was obtained under this dye-sensitized reaction condition.15 Shortly thereafter, Wamhoff et al. reported the synthesis of quinolinetriones from the singlet oxygen photolysis of dihalo4-hydroxyquinolin-2(1H)-one.16 With the goal of expanding the scope of an efficient route to vic-tricarbonyl compounds, we now report the convenient synthetic methods of vicheterocyclic tricarbonyl compounds directly from enolic dicarbonyl compounds through photochemical and thermal approaches without the co-oxidant. In the context of our research interest in the photochemistry of quinoline and isoquinoine derivatives,17–18 the synthesis of vic-tricarbonyls was achieved from the reaction of 4-hydroxyquinolin-2(1H)-ones with singlet oxygen using methylene blue