A series of pyridazino[3,4-d][1,3]oxazin-5-one derivatives were designed and synthesized through a versatile method. Reaction of chloro-ester 1 with sodium azide has afforded azido-ester 2 which reacted with triphenylphosphine to produce compound 3; amino-ester 4 was formed upon treatment of compound 3 with acetic acid 80%; this amino-ester 4 was then hydrolyzed to the amino-acid 5 which is considered the starting material. The approach involves treatment of pyridazinamino-acid 5 with acetic anhydride, benzoyl chloride, triethylorthoformate, ethyl chloroformate, diethyl oxalate, phthalic anhydride as well as succinic anhydride to yield pyridazino[3,4-d][1,3]oxazin-5-one derivatives 6, 8, 9, 10, 11, 12 and 13. The new synthesized compounds were confirmed by their infrared, mass spectrum and 1H-NMR.