A facile and novel synthesis of 5‐phenylimidazo[4,5‐ c ][1,8]naphthyridin‐4(5 H )‐ones

A convenient and regioselective synthesis of a new heterocycle, 5‐phenyl‐1 H or 3 H ‐imidazo[4,5‐c][1,8]‐naphthyridin‐4(5 H )‐one 1‐a or 1‐b , is described. Methyl 2‐anilinonicotinate 15 was transformed into the valuable intermediate, N ‐phenyl‐3‐azaisatoic anhydride 4 using trichloromethyl chloroformate (TCF). Treatment of 4 with the anion of ethyl nitroacetate gave 4‐hydroxy‐3‐nitro‐1‐phenyl‐1,8‐naphthyridin‐4(5 H )‐one 3 . Compound 3 was chlorinated, aminated, reduced, and cyclized to afford 5‐phenylimidazo[4,5‐ c ][1,8]naphthyridin‐4(5 H )‐one 1 . Regioselective substitution at the 1 or 3‐position in the imidazole moiety of 1 was achieved by minor changes of the above scheme.