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

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