Preparation method of favipiravir

The invention provides a method for preparing favipiravir, which comprises the following steps of: preparing 2, 2-dihalogenated malonic acid diester from malonic acid diester and a halogen reagent through substitution reaction, then performing amidation reaction on 2-aminoacetamide obtained by dissociating 2-aminoacetamide hydrochloride to obtain N-aminocarbonyl methyl-2, 2-dihalogenated malonic acid monoester monoamide, and finally obtaining the favipiravir through the amidation reaction of the N-aminocarbonyl methyl-2, 2-dihalogenated malonic acid monoester monoamide and the amidation reaction of the N-aminocarbonyl methyl-2, 2-dihalogenated malonic acid diester monoamide. The method comprises the following steps: taking 4-chloro-4H-[1, 3] dioxin [4, 5-b] pyrazine-2, 4-diketone as a raw material, carrying out cyclization reaction under the action of alkali to obtain 3, 6-dihydroxypyrazine-2-formic acid, then carrying out cyclization halogenation reaction with a cyclization halogenation phosgenation reagent