Transformation of Reactive Isochromenylium Intermediates to Stable Salts and Their Cascade Reactions with Olefins

Transformation of reactive isochromenylium intermediates to the corresponding storable and stable reagents has been achieved, and a number of isochromerylium tetrafluoroborates (ICTBs, 1) have been conveniently prepared and characterized. Direct metal-free treatment of isochromenylium tetrafluoroborate la with olefins afforded a variety of polycyclic frameworks 4 via mild cascade reactions. Starting from the prefunctionalized o-alkynylbenzaldehydes, a one-pot metal-free procedure of intramolecular cascade annulation to 2,3-dihydrophenanthren-4(1H)-one derivatives was also developed.