Synthesis of 5,6-Dihydrodibenzo[b,h][1,6]naphthyridines via Copper Bromide Catalyzed Intramolecular [4 + 2] Hetero-Diels-Alder Reactions.

A highly efficient synthesis of 5,6-dihydrodibenzo[b,h][1,6]naphthyridines was achieved by reaction between 2-(N-propargylamino)benzaldehydes and arylamines in the presence of CuBr2. The in situ generated electron-deficient heterodienes bearing a tethered alkyne partner underwent an intramolecular inverse electron-demand hetero-Diels-Alder reaction followed by air oxidation to furnish the products in high yields. This reaction tolerated a large number of substituents to afford diverse products under mild conditions. This strategy was also successfully extended to the synthesis of 12,13-dihydro-6H-benzo[h]chromeno[3,4-b][1,6]naphthyridin-6-ones starting from 3-amino-2H-chromen-2-one, again in high yields.