Methyl 3-aryl-3-hydroxy-2-methylene propionates react with phenols, 2-naphtol, 1,4-dihydroxybenzene, 2,3-dihydroxynaphtalene and 2,7-dihydroxynaphtalene in the presence of silica gel-supported ZnBr2 or FeCl3 to give 3-arylidene-3,4-dihydro coumarins or 3-methylene-3,4-dihydrocoumarins.
The reaction of coumarin-3-carboxylic acid with benzylic C(sp3)-H bond was established in this paper. A series of 3-benzylcoumarin derivatives were efficiently isolated in moderated to high yield using Cu(OAc)2 as catalyst and TBPB as oxidant.