Cascade Alkynyl Prins Cyclization and Aza-Michael Reaction: En Route to Regioselective Pyrano- and Isochromenoquinoline Scaffolds

A metal-free, Lewis acid approach for the regioselective synthesis of dihydropyranoquinoline scaffolds has been unveiled. The methodology employs a cascade alkynyl Prins-aza-Michael reaction sequence to deliver the products in good to excellent yields. The strategy features mild reaction conditions, broad substrate scope, and high functional group tolerance. The protocol has been further extended to include isochromenoquinoline derivatives. The utility of the reaction lies in the synthesis of highly fused polycyclic N,O-heterocycles via intramolecular Heck coupling. Additionally, a Rh(III)-catalyzed annulation results in the formation of highly fluorescent pentacyclic ammonium salts in excellent yields. Photophysical studies reveal that these pentacyclic ammonium salts exhibit strong emission in the green region (500-550 nm).