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

An efficient, secondary amine-catalyzed cascade annulationof 2-O/N-propargylarylaldehydeswith 2,6-dialkylphenolswas established to access biologically relevant functionalized 2H-chromenes and 1,2-dihydroquinolines tethered with a syntheticallyuseful p-quinone methide scaffold in high yieldsunder microwave irradiation and conventional heating conditions. Themicrowave-assisted strategy was convenient, clean, rapid, and highyielding in which the reactions were completed in just 15 min, andthe yields obtained were up to 95%. This highly atom-economical dominoprocess constructed two new C-C double bonds and a six-memberedO/N-heterocyclic ring in a single synthetic operation. Its mechanismprocess was rationalized as involving sequential iminium ion formation,nucleophilic addition, and intramolecular annulation steps. Furthermore,the synthesized 2H-chromene derivatives were transformedinto valuable indeno[2,1-c]chromenes, 5H-indeno[2,1-c]quinolines, and oxireno[2,3-c]chromene via a palladium-catalyzed double C-H bondactivation process and epoxidation, respectively.