General procedure for the synthesis of 2-amino-6-bromo-1,8-naphthyridine from 2,6-diaminopyridine and 2-bromomalonaldehyde: 2,6-diaminopyridine (6.0 g, 55 mmol) and 2-bromo-1,3-malonaldehyde (8.0 g, 53 mmol) were dissolved in 85% orthophosphoric acid (160 mL) and heated and refluxed for 6 hours. Upon completion of the reaction, the reaction endpoint was confirmed by thin layer chromatography (TLC) monitoring. After cooling the reaction mixture to room temperature, it was neutralized with ammonia (NH4OH) solution to give a dark brown solid. After filtration and drying of this solid, it was dissolved in methanol (MeOH) with the aid of sonication. Purification by column chromatography afforded the light yellow solid product 2-amino-6-bromo-1,8-naphthyridine (3.85 g, 30.3% yield), which had an Rf value of 0.39 (unfolding reagent ratio CH2Cl2:MeOH = 10:1) and a melting point of 160-162°C (literature value 4: 210-212°C). The 1H NMR (500 MHz, DMSO-d6) data of the product were as follows: δ 7.82 (d, J = 2.6 Hz, 1H), 7.46 (d, J = 2.6 Hz, 1H), 7.04 (d, J = 8.8 Hz, 1H), 6.14 (s, 2H), 5.99 (d, J = 8.8 Hz, 1H).
