The acylation of benzoxazolin-2-one (I) with propionic anhydride (II) by means of P2O5 and MsOH gives 6-propionylbenzoxazolin-2-one (III), which is treated with chloroacetyl chloride to yield 7-propionyl-3,4-dihydro-2H-1,4-benzoxazin-3-one (IV). The Mannich condensation of (IV) with CH2O and NH3 followed by methylation with methyl iodide affords the trimethylammonium compound (V), which by reaction with KCN, followed by hydrolysis with HCl gives 3-(3-oxo-3,4-dihydro-2H-1,4-benzoxazin-7-ylcarbonyl)butyric acid (VI). Finally this compound is cyclized with hydrazine to furnish the target pyridazinone.