Phosphorus oxychloride → 4-Chloroimidazo(2,1-f)(1,2,4)triazine

A mixture of ethyl 1-amino-1H-imidazole-2-carboxylate (C-4) (800 mg, 5.16 mmol, 1.0 eq) and formamidine acetate (2.68 g, 25.78 mmol, 5.0 eq) in EtOH (100 mL) is stirred at reflux overnight. The resulting mixture is cooled to RT. The solid is collected by filtration, rinsed with EtOH (3×2 mL) and petroleum ether (2 mL×3), and then dried in vacuo to afford the product, imidazo[1,2-f][1,2,4]triazin-4-ol (C-5). Imidazo[1,2-f][1,2,4]triazin-4-ol (C-5) (400 mg, 2.94 mmol, 1.0 eq) is dissolved in POCl3 (10 mL, 109.2 mmol, 37.1 eq) and the resulting mixture is stirred at reflux for 2 h. The mixture is concentrated in vacuo to remove POCl3. The residue is poured into ice water (30 mL) and neutralized with saturated aqueous NaHCO3 solution to adjust the pH to 6-7 while keeping the temperature below 5° C. The mixture is extracted with ethyl acetate (30 mL×4). The combined organic layers are washed with brine, dried over Na2SO4 and filtered. The filtrate is concentrated in vacuo and the residue is purified by flash column chromatography on silica gel (16% ethyl acetate in petro ether) to afford the product, 4-chloroimidazo[1,2-f][1,2,4]triazine (C-6).