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-pyrones has been developed ring-opening transformations leading to various hydroxylated oxaheterocycles. The first step of the strategy includes the base-catalyzed epoxidation of 5-acyl-4-pyrones in the presence of hydrogen peroxide for the effective synthesis of pyrone epoxides in high yields. The epoxides bearing the COEt group are reactive molecules that can undergo both pyrone and oxirane ring-opening deformylation to produce hydroxylated 2-pyrones or 4-pyrones. The acid-promoted transformation led to 3-hydroxy-4-pyrones (24-76% yields), whereas the KCO-catalyzed ring-opening process of 2-carbethoxy-4-pyrone epoxides proceeded as an attack of alcohol at the C-3 position bearing the COEt group to give functionalized 6-acyl-5-hydroxy-2-pyrones (27-87% yields). The base-catalyzed reaction of 2-aryl-4-pyrone epoxides was followed by ring contraction and the dearoylation process to produce 3-hydroxyfuran-2-carbaldehydes in 42-80% yields. The transformation of 3-aroylchromone epoxides led to flavonols and 3-hydroxybenzofuran-2-carbaldehyde in the acidic and basic conditions, respectively. The prepared hydroxylated heterocycles demonstrated high reactivity for further transformations and low cytotoxicity and are promising fluorophores or UV filters.