Base-free NIS promoted electrophilic cyclization of alkynes: an efficient synthesis of iodo substituted pyrano[4,3-b]quinolines

A simple and mild procedure for the synthesis of iodo substituted 1H-pyrano[4,3-b]quinolines has been achieved using NIS reagent in the absence of base from 2-alkynylquinoline-3-carboxaldehydes via intramolecular electrophilic cyclization onto alkynes in good to excellent yields in a short duration of time. The reactions proceeded smoothly in a normal solvent in aerobic atmosphere at room temperature. The presence of substituent at either quinoline or alkyne moieties did not show effect on reaction rate of cyclization. The palladium-catalyzed transformations of iodo group to C–C bond are also discussed.