The Pechmann reaction is the most widely applied
method for synthesising coumarins as it involves the
condensation of phenols with -ketonic esters in the
presence of a variety of acidic condensing agents and
gives good yields of 4-substituted coumarins.7,8 Several
acid catalysts have been used in the Pechmann reaction
including sulfuric acid,5a phosphorus pentoxide,9 aluminium
chloride,10 trifluoroacetic acid11 and many
more.7 However, these catalysts have to be used in
excess; for example, sulfuric acid in 10–12 equiv.,8
trifluoroacetic acid in three to 4 equiv.,11 and phosphorous
pentoxide is required in a five-fold excess. Further,
the disposal of acid waste leads to environmental
pollution.