Methyl 4-amino-1,3-thiazole-5-carboxylate → 4-Hydrazinothiazole-5-carboxylic acid methyl ester

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To a solution of 4-aminothiazole-5-carboxylic acid methyl ester (15.3 g) in concentrated hydrochloric acid (90 ml) there was added dropwise an aqueous solution (10 ml) containing sodium nitrite (7.32 g) at 0-10° C. The mixture was then stirred at 0° C. for 30 minutes. To this mixture there was added dropwise a concentrated hydrochloric acid solution (100 ml) containing stannous chloride (73.2 g) at 0-10° C., and the mixture was stirred at the same temperature for 2 hours. The mixture was filtered, and the filtrate was carefully added to a suspension of potassium carbonate and celite in ethyl acetate (3 liters) with stirring, with regular addition of potassium carbonate to prevent solution acidic. After adding the filtered substance to a solution of this mixture in ethyl acetate, it was rendered basic with a 5N aqueous sodium hydroxide solution. The mixture was allowed to stand, and then most of the supernatant (organic layer A) was separated off. The remaining suspension was filtered through celite and the filtrate was separated into organic layer B and aqueous layer A. Ethyl acetate (500 ml) and anhydrous magnesium sulfate were added to the filtered substance, and the mixture was stirred and then filtered. Aqueous layer A was re-extracted with the resulting filtrate. Washing of the filtered substance and re-extraction of aqueous layer A were repeated 4 times in the same manner. Organic layer A and organic layer B were combined with the obtained organic layer, and the mixture was dried over anhydrous magnesium sulfate. The desiccating agent was filtered off and the filtrate was concentrated under reduced pressure. The residue was purified by silica gel column chromatography (ethyl acetate-methanol) to give the title compound (11.6 g) as a light yellow solid.