Imino Diels-Alder reactions: Efficient synthesis of pyrano- and furanoquinolines catalyzed by 4-nitrophthalic acid

4-Nitrophthalic acid was found to be an effective catalyst for the imino Diels-Alder reaction of N-benzylideneanilines with 3,4-dihydro-2H-pyran and 2,3-dihydrofuran to afford pyrano- and furanoquinolines in good yields. It was also found that aryl amines react smoothly with 3,4-dihydro-2H-pyran and 2,3-dihydrofuran under the same condition to afford the corresponding pyrano- and furanoquinolines in high yields. This catalyst is inexpensive, easily available, water soluble, and stable to aqueous reaction conditions.