Rapid Assembly of Functionalized 2-Chromenes and 1,2-Dihydroquinolines via Microwave-Assisted Secondary Amine-Catalyzed Cascade Annulation of 2-/-Propargylarylaldehydes with 2,6-Dialkylphenols.

An efficient, secondary amine-catalyzed cascade annulation of 2-/-propargylarylaldehydes with 2,6-dialkylphenols was established to access biologically relevant functionalized 2-chromenes and 1,2-dihydroquinolines tethered with a synthetically useful -quinone methide scaffold in high yields under microwave irradiation and conventional heating conditions. The microwave-assisted strategy was convenient, clean, rapid, and high yielding in which the reactions were completed in just 15 min, and the yields obtained were up to 95%. This highly atom-economical domino process constructed two new C-C double bonds and a six-membered O/N-heterocyclic ring in a single synthetic operation. Its mechanism process was rationalized as involving sequential iminium ion formation, nucleophilic addition, and intramolecular annulation steps. Furthermore, the synthesized 2-chromene derivatives were transformed into valuable indeno[2,1-]chromenes, 5-indeno[2,1-]quinolines, and oxireno[2,3-]chromene via a palladium-catalyzed double C-H bond activation process and epoxidation, respectively.