Oxidative Ring-Opening Transformation of 5-Acyl-4-pyrones as an Approach for the Tunable Synthesis of Hydroxylated Pyrones and Furans

A selective and tunable approach for oxidation of 4-pyroneshasbeen developed via ring-opening transformations leadingto various hydroxylated oxaheterocycles. The first step of the strategyincludes the base-catalyzed epoxidation of 5-acyl-4-pyrones in thepresence of hydrogen peroxide for the effective synthesis of pyroneepoxides in high yields. The epoxides bearing the CO(2)Etgroup are reactive molecules that can undergo both pyrone and oxiranering-opening via deformylation to produce hydroxylated2-pyrones or 4-pyrones. The acid-promoted transformation led to 3-hydroxy-4-pyrones(24-76% yields), whereas the K2CO3-catalyzedring-opening process of 2-carbethoxy-4-pyrone epoxides proceeded asan attack of alcohol at the C-3 position bearing the CO2Et group to give functionalized 6-acyl-5-hydroxy-2-pyrones (27-87%yields). The base-catalyzed reaction of 2-aryl-4-pyrone epoxides wasfollowed by ring contraction and the dearoylation process to produce3-hydroxyfuran-2-carbaldehydes in 42-80% yields. The transformationof 3-aroylchromone epoxides led to flavonols and 3-hydroxybenzofuran-2-carbaldehydein the acidic and basic conditions, respectively. The prepared hydroxylatedheterocycles demonstrated high reactivity for further transformationsand low cytotoxicity and are promising fluorophores or UV filters.