Synthesis of ethyl 4-(isoxazol-4-yl)-2,4-dioxobutanoates from ethyl 5-aroyl-4-pyrone-2-carboxylates and hydroxylamine

Ethyl 5-aroyl-4-pyrone-2-carboxylates react with hydroxylamine in ethanol at –20°C for 30 days to produce ethyl 4-(5-arylisoxazol-4-yl)-2,4-dioxobutanoates (yields 20–67%). The similar reaction of diethyl 4-pyrone-2,5-dicarboxylate gives ethyl 4-oxo-4H-pyrano[3,4-d]isoxazole-6-carboxylate in 37% yield.