A synthesis of dibenzo[b,h][1,5]naphthyridin-7(12H)-ones: the Pictet–Spengler reaction vs a rearrangement of 4-phenyl[1,3]oxazolo[4,5-c]quinolines

Two approaches were proposed to the synthesis of dibenzo[b,h][1,5]naphthyridin-7(12H)-ones. The one-step method based on the Pictet–Spengler reaction of 3-amino-2-phenylquinolin-4(1H)-one with aromatic aldehydes requires heating in a strong acidic media, which leads to significant limitations on the possible products. The second approach is based on the conversion of 3-amino-2-phenylquinolin-4(1H)-one to 4-phenyl[1,3]oxazolo[4,5-c]quinolines followed by rearrangement under the action of AlCl3. A significant advantage of the two-step synthesis is the possibility of obtaining a wider range of dibenzo[b,h][1,5]naphthyridin-7(12H)-ones, including those not available using the Pictet–Spengler reaction.