Phosphorus-containing Lewis base catalyzed highly regioselective cyclization of isatin derived electron-deficient alkenes with but-3-yn-2-one

In this paper, we reported that phosphorus-containing Lewis bases catalyzed [4+2] cyclization of N-protected isatin derived electron-deficient alkenes with but-3-yn-2-one could proceed smoothly to give the corresponding dihydropyrano[2,3-b]indoles in good to excellent yields under mild conditions. The substrate scope has been carefully examined and the plausible reaction mechanism has been also proposed.