Pd-Catalyzed One-Pot Coupling-Annulation Synthesis of Pyrano[4,3-b]quinolines

Significance

Described is a Pd-catalyzed, Cu-free synthesis of 1,3-disubstituted pyrano[4,3-b]quinolines by a Sonogashira coupling-cyclization reaction of 2-chloro-3-formylquinolines. After some optimization studies, the reaction was found to efficiently proceed under Pd(OAc)₂ catalysis at 80 ˚C in MeCN. The presence of Ph₃P was additionally found to facilitate the cyclization reaction and decrease the reaction times (without Ph₃P: 16 h, 70% yield; with PPh₃: 2.5 h, 80% yield). Good yields of products were achieved but the substrate scope was very poorly studied.