Microwave Assisted, 4-dimethylaminopyridine (DMAP) Mediated, Onepot, Three-component, Regio- and Diastereoselective Synthesis of Trans- 2,3-dihydrofuro[3,2-c]coumarins

The regio- and diastereoselective base catalyzed one-pot, multicomponent reaction of 4-hydroxycoumarin, aldehyde, and 2-bromo-1-phenylethanone giving trans 2,3-dihydrofuro[3,2-c]coumarins is greatly accelerated by a combination of 4-(N,N-dimethylamino)pyridine (DMAP) and microwave heating. The classical thermal reaction takes a long reaction time and uses environment unfriendly bases, pyridine, NaOH. The microwave reaction is carried out in Biotage Synthesizer at different temperatures under built-in pressure. Among the organic and inorganic bases used, DMAP is the best base. Further, replacement of pyridine/NaOH by DMAP makes the reaction greener. Microwave irradiation (140°C, 4 bar) accelerates the reaction greatly as compared to its thermal counterpart. The effect of microwaves could be correlated with the dielectric loss of the polar solvents. 1-propanol and ethanol that have high dielectric loss are the best solvents under microwave irradiation. In the reaction DMAP functions as a nucleophile, a base, and a good leaving group. The method is highly regioselective, diastereoselective and gives good yield of the product.