Perkin described a synthesis of coumarin by heating the sodium salt of salicylaldehyde with acetic anhydride. Further study of this reaction led to the development of a new general method for preparing cinnamic acid and its analogs, which became known as Perkin reaction.
The Perkin reaction is beleived to proceed by forming an enolate anion from the acid anhydride which subsequently reacts with the aldehyde to form an alkoxide. The ensuring intramolecular acylation affords beta acyloxy derivative which undergoes acylation and elimination to form the cinnamic acid ester. Hydrolysis of ester affords the cinnamic acid.