One-pot Selective Synthesis of 2,3-Dihydro-4H-Furo[3,2-c] coumarins by Palladium-Catalyzed Silver-Assisted Propargylation/Intramolecular 5-exo-dig Cyclization

A new catalytic system using bis(triphenylphosphine) palladium (II) dichloride and Ag2CO3 was developed for the one-pot selective synthesis of dihydrofuro[3,2-c] coumarin 3 from 4-hydroxycoumarin (1) and propargylic carbonate 2 under the mild condition. Our catalytic system afforded distinct regioselectivity compared to reported methodologies using palladium catalysts, 2 and active methylene compounds. In addition, furo [3,2-c] coumarin 4 was derived from 3 with Et3N. The proposed reaction mechanism involved 4-hydroxycoumarin (1) reacting with the eta(1)-(propargyl) palladium complex to generate a terminal alkyne, and subsequent intramolecular 5-exo-dig cyclization selectively synthesized 3 in one-pot. The newly developed methodology is beneficial in the construction of a novel compound library related to furocoumarins with distinct regioselective preference.