Pechmann condensation

The Pechmann reaction is named after the German chemist Hans Freiherr von Pechmann and describes the synthesis of coumarin derivatives.

Reaction mechanism

For the synthesis of coumarin derivatives can condense under acidic conditions, β -keto esters with phenols.

Initially enters a transesterification to form the phenolic ester. Subsequently, the ring closure is similar to a Friedel-Crafts alkylation is carried out.

Simple phenols must be used at elevated temperatures in order to obtain a reasonable yield of 80 to 85%.

With active phenols such as resorcinol, the reaction can be carried out at room temperature. The yield is about 85%

Variants

For coumarins that are unsaturated in the δ - position, the Halbaldehyd of malonic acid is required. This is, however, unstable and not available for purchase. Therefore, it is synthesized by in situ from malic acid and sulfuric acid above 100 ° C to condense immediately after the birth in a Pechmann reaction. However, the yield is low.