Enolate anions cross Claisen condensations

When carboxylic esters containing an a hydrogen are treated with a strong base, such as sodium ethoxide, a condensation occurs to give a p-keto ester via an ester enolate anion. ° This reaction is called the Claisen condensation. When it is carried out with a mixture of two different esters, each of which possesses an a hydrogen (this reaction is called a mixed Claisen or a crossed Claisen condensation), a mixture of all four products is generally obtained and the reaction is seldom useful synthetically. However, if only one of the esters has an... 

In a crossed Claisen condensation (a Claisen condensation between two different esters, each with its own a-hydrogens), a mixture of four j8-ketoesters is possible therefore, crossed Claisen condensations of this type are generally not synthetically useful. Such condensations are useful, however, if appreciable differences in reactivity exist between the two esters, as, for example, when one of the esters has no a-hydrogens and can function only as an enolate anion acceptor. These esters have no a-hydrogens ... 

A crossed Claisen condensation is most effective when one of the carbonyl compounds can only function as an enolate anion acceptor. (15.3)... 

Crossed Claisen condensations are useful only when an appreciable difference exists in the reactivity between the two esters. Such is the case when an ester that has no a-hydrogens can function only as an enolate anion acceptor ... 

Preformed enolate anions using LDA can be used to carry out a wide variety of crossed enolate reactions, including aldol reactions, Claisen condensations, Michael additions, alkylations, and acylations. 

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