Lewis Acid-Catalyzed 1,4-Addition and Annulation of 4-Hydroxy-coumarins with -Hydroxyphenyl Propargyl Amines: Entry to Regio-Selective Synthesis of Furano[3,2-]coumarins and Pyrano[3,2-]coumarins.

A facile and regioselective Lewis acid-catalyzed cascade annulation of -hydroxyphenyl propargyl amines with 4-hydroxycoumarin to afford furano[3,2-]coumarin and pyrano[3,2-]coumarin derivatives is reported. The reaction presumably proceeds by the conjugate addition of 4-hydroxycoumarin to the in situ-generated alkynyl -quinone methide and is followed by intramolecular 5- and 6- annulation to form furano[3,2-]coumarins and pyrano[3,2-]coumarins, respectively. The prepared -hydroxyl substituted pyrano[3,2-]coumarins could be readily transformed into the corresponding coumarin-derived dioxabicycles by acid-mediated cyclization.