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Claisen condensation is a chemical reaction in organic chemistry that allows the formation of β-keto esters through the condensation of two ester groups. This reaction was first described by Louis Claisen in 1887 and is named after him. The Claisen condensation involves the formation of a new carbon-carbon bond without the loss of any atoms. It is a two-step reaction with the initial formation of a dianion, where one of the ester groups is deprotonated, then attacked by the second ester group.

In this reaction, one of the alkoxides is converted to a dianion through the action of a strong base. The dianion then attacks the alkyl group of the other ester, leading to the formation of a new carbon-carbon bond and resulting in the elimination of an alkoxide ion. This process creates a new β-keto ester which is released from the reaction mixture and can be isolated.

Claisen condenses react with each other in the presence of a base catalyst such as a metal hydroxide. The reaction is commonly used in the synthesis of a wide range of compounds such as steroids, prostaglandins, and conjugated fatty acids. It allows chemists to prepare compounds which contain a β-keto ester group.