Pechmann condensation
Named reaction for synthesis of coumarins
The Pechmann condensation is a synthesis of coumarins, starting from a phenol and a carboxylic acid or ester containing a β-carbonyl group. The condensation is performed under acidic conditions. The mechanism involves an esterification/transesterification followed by attack of the activated carbonyl ortho to the oxygen to generate the new ring. The final step is a dehydration, as seen following an aldol condensation. It was discovered by the German chemist Hans von Pechmann
.
Mechanism
To synthesize coumarin derivatives, β-ketoesters can be condensed with phenols under acidic conditions. In this case, a transesterification reaction occurs first with the formation of the phenol ester. This is then followed by ring closure, similar to Friedel-Crafts alkylation.
Examples
With simple phenols, the conditions are harsh, although yields may still be good.
With highly activated phenols such as resorcinol, the reaction can be performed under much milder conditions.
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