Cyclization of ortho-hydroxycinnamates to coumarins under mild conditions: A nucleophilic organocatalysis approach

(E)-Alkyl ortho-hydroxycinnamates cyclize to coumarins at elevated temperatures of 140-250 °C. We find that the use of tri-nbutylphosphane (20 mol %) as a nucleophilic organocatalyst in MeOH solution allows cyclization to take place under much milder conditions (60-70 °C). Several coumarins were prepared, starting from ortho-hydroxyarylaldehydes, by Wittig reaction with Ph ₃P=CHCO ₂Me to (E)-methyl ortho-hydroxycinnamates, followed by the phosphane catalyzed cyclization.