Pechmann condensation

The Pechmann condensation is a synthesis of coumarins, starting from a phenol and a carboxylic acid or ester containing a β-carbonyl group.

The mechanism involves an esterification/transesterification followed by attack of the activated carbonyl ortho to the oxygen to generate the new ring.

To synthesize coumarin derivatives, β-ketoesters can be condensed with phenols under acidic conditions.

This provides a useful route to umbelliferone derivatives: For coumarins unsubstituted at the 4-position, the method requires the use of formylacetic acid or ester.

In the example shown, umbelliferone itself is produced, albeit in low yield: The mechanism of the reaction has been studied in details with theoretical treatment.