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Claisen Condensation

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The base catalysed condensation of two moles of an ester to give a $\beta$-keto ester. Claisen condensations are more favorable with thioesters. This reaction is of great importance in the biosynthesis of fatty acids and polyketides. 

Claisen condensation is a reaction forming carbon-carbon bonds that take place between two or one ester and carbonyl compounds, this happens in the with a strong base which results in a $\beta$-keto ester or a $\beta$-diketone. This reaction is named after Rainer Ludwig Claisen, who worked on this reaction and published in 1887. 

Esters contain a-hydrogens, they can undergo condensation reaction similar to aldol reaction, this reaction is known as Claisen condensation. In this reaction, one ester acts as a nucleophile while a second ester acts as a the electrophile, which reacts to form a carbon-carbon bond resulting in the formation of $\beta$-keto ester.

Claisen Condensation

Mechanism

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The mechanism of the claisen condensation is similar to that of the aldol condensation and involves the nucleophilic addition of an ester enolate ion to the carbonyl group of a second ester molecule. The only difference between the aldol condensation of an aldehyde or ketone and the Claisen condensation of an ester involves the fate of the initially formed intermediate. The alkoxide intermediate in the aldol reaction is protonated to give an alcohol product exactly the behavior previously seen for aldehydes and ketones. The alkoxide intermediate in the claisen reaction however exples an alkoxide leaving group to yield a nucleophilic acyl substitution product exactly the behavior seen for esters.

In the classic condensation there is a self-condensation between two molecules of a compound containing an enolizable ester.
Classic Condensation

The steps of Claisen condensataion mechanism are as follows.

1. Enolate formation
Enolate Formation
2. Nucleophilic attack

Nucleophilic Attack

3. Removal of leaving group
Removal of Leaving Group

Mixed Claisen Condensation

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The mixed claisen condensation of two different esters is similar to the mixed aldol condensation of two different aldehydes or ketones. Mixed claisen reactions are successful only when one of the two ester components has no $\alpha$ hydrogens and thus can't form an enolate ion. For example, ethyl benzoate and ethyl formate can't form enolate ions and thus can't serve as donors. They can however act as the electrophile acceptor components in reactions with other ester anions to give mixed $\beta$-keto ester products.

Mixed Claisen Condensation

Dieckmann Condensation

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Dieckmann condensation reaction is where there is the formation of a cyclic β-keto ester by the reaction of a molecule with two ester groups intermolecularly.Here, the ring formed must not be strained, usually a 5 or 6-membered chain ring is formed. This is an intermolecular chemical reaction of diesters with base to give b-ketoesters. The reaction is named after a German scientist Walter Dieckmann it is equivalent to the intermolecular reaction in the Claisen condensation.
Dieckmann Condensation

Stobbe Condensation

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The Stobbe condensation is an important C-C bond forming reaction. However, when a Stobbe condensation reaction of cyclohexanone and diethyl succinate is carried out under usual conditions such as heating under reflux in BOH in the presence of BuOK a mixture of cyclohexylidenesuccinic acid and cyclohexenylsuccinic acid is produced. Interestingly Stobbe condensation reactions proceed efficiently and selectively in the absence of solvent.

This reaction selectively can be accomplished only under solvent free conditions since the Stobbe reaction in the solid state.

Stobbe Condensation