Synthesis and reactivity of 4-hydroxy-5-methyl-2-(2-oxo-2<Emphasis Type="Italic">H</Emphasis>-chromen-3-yl)-6<Emphasis Type="Italic">H</Emphasis>-1,3-oxazin-6-ones

4-Hydroxy-6H-1,3-oxazin-6-ones with a coumarin fragment in position 2 of the oxazine ring were synthesized. Their hydrolysis, alcoholysis, and hydrazinolysis afforded a number of new coumarin derivatives containing malonamic acid and 1,2,4-triazole residues. The low stability of the title compounds toward oxygencentered nucleophiles was interpreted by quantum chemical calculations.