The general procedure for the synthesis of 5-aminopyrazine-2-carboxylic acid from 5-chloropyrazine-2-carboxylic acid was as follows: first, the substitution reaction was carried out by substituting 5-chloropyrazine-2-carboxylic acid (317 mg, 2 mmol) with 25% (m/m) aqueous ammonia solution (3 mL) in a 10 mL microwave pressurized vial. The reaction conditions were set to 100 °C, 30 min, 80 W power output, and under stirring. This reaction was repeated 20 times to obtain sufficient amount of starting acid. Upon completion of the reaction, the vial contents were transferred to a petri dish, heated on a water bath with intermittent stirring until a dry solid (i.e., the ammonium salt form of the product) was obtained. To obtain the free acid form, the ammonium salt was dissolved in water and adjusted dropwise to pH 4 by addition of 10% hydrochloric acid.Subsequently, the mixture was cooled at room temperature for 5 min and then placed in a refrigerator for 15 min. The free acid crystals formed were collected by diafiltration and dried overnight. Next, the dried 5-aminopyrazine-2-carboxylic acid (278 mg, 2 mmol) was mixed with 3 mL of anhydrous propanol and 2 drops of concentrated sulfuric acid and placed in a microwave pressurized vial for esterification. The esterification conditions were 100 °C, 1 h and 80 W power output. The reaction process was monitored by TLC (unfolding agent: hexane/ethyl acetate, 1:3). Finally, the ester product was purified by fast chromatography with an elution gradient of 40% to 100% ethyl acetate in hexane solution.
