Facile intramolecular Diels–Alder reaction of 4-(o-propargyloxy)styrylcoumarins leading to highly fluorescent coumarin-containing polycyclic compounds

The 4-(o-propargyloxy)styrylcoumarins are prepared by the condensation of O-propargylated salicylaldehyde with substituted coumarin-4-acetic acids. The intramolecular Diels–Alder reaction of 4-(o-propargyloxy)styrylcoumarins, without any catalyst gives fused-ring coumarins. The reaction in boiling nitrobenzene leads to aromatization of the initial Diels–Alder adduct and these aromatized products are highly fluorescent.