The general procedure for the synthesis of ethyl 4-amino-1,2,5-oxadiazole-3-carboxylate from 4-amino-1,2,5-oxadiazole-3-carboxylic acid was as follows: to a 2 mL solution of 4-amino-1,2,5-oxadiazole-3-carboxylic acid (0.25 g, 1.9 mmol) in ethanol (EtOH) was added drop-wise, under ice bath cooling, thionyl chloride (SOCl2. 0.2 mL). The reaction mixture was heated to reflux for 4 h and subsequently concentrated. The resulting oil was diluted with 50 mL of water (H2O) and extracted with ether (3 x 100 mL). The organic layers were combined, dried with anhydrous magnesium sulfate (MgSO4) and concentrated to give a white solid product (0.15 g, 50% yield). The product was characterized by 1H NMR (DMSO-d6, 500 MHz): δ 6.39 (s, 2H), 4.39 (q, 2H), 1.35 (t, 3H). Mass spectrometry (FIA MS) showed the molecular ion peak m/z 158.1 ([M + H]+).
