The Pechmann Condensation allows the synthesis of coumarins by reaction of phenols with β-keto esters.
Mechanism of the Pechmann Condensation
The reaction is conducted with a strong Brønstedt acid such as methanesulfonic acid or a Lewis acid such as AlCl3. The acid catalyses transesterification as well as keto-enol tautomerisation:
A Michael Addition leads to the formation of the coumarin skeleton. This addition is followed by rearomatisation:
Subsequent acid-induced elimination of water gives the product: