Cascade Alkynyl\nPrins Cyclization and Aza-Michael\nReaction: En Route to Regioselective Pyrano- and Isochromenoquinoline\nScaffolds

A metal-free, Lewis acid approach for the regioselective\nsynthesis\nof dihydropyranoquinoline scaffolds has been unveiled. The methodology\nemploys a cascade alkynyl Prins–aza-Michael reaction sequence\nto deliver the products in good to excellent yields. The strategy\nfeatures mild reaction conditions, broad substrate scope, and high\nfunctional group tolerance. The protocol has been further extended\nto include isochromenoquinoline derivatives. The utility of the reaction\nlies in the synthesis of highly fused polycyclic N,O-heterocycles\nvia intramolecular Heck coupling. Additionally, a Rh(III)-catalyzed\nannulation results in the formation of highly fluorescent pentacyclic\nammonium salts in excellent yields. Photophysical studies reveal that\nthese pentacyclic ammonium salts exhibit strong emission in the green\nregion (500–550 nm).