The von Pechmann reaction, commonly called the Pechmann condensation, forms a coumarin by acid-catalyzed condensation of a phenol with a β-ketoester or β-keto acid derivative. Acid activation enables transesterification or direct electrophilic acylation of the phenolic oxygen, followed by intramolecular electrophilic aromatic substitution at an ortho position, dehydration, and formation of the fused α-pyrone ring ^[1]. The β-keto component supplies the carbonyl-containing three-carbon unit, while the phenol supplies the benzene ring and ring oxygen of the coumarin. Substituent placement follows the orientation of the phenolic ring and the constitution of the β-keto reagent ^[2]. The transformation is distinct from the Perkin and Knoevenagel routes to coumarins because it constructs the heterocycle directly from a phenol and a β-carbonyl carboxylic derivative under acidic conditions. Formation of the fused 2H-chromen-2-one framework is its defining structural outcome.