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).