Ethyl benzoate, mixed Claisen

Ethyl benzoate, mixed Claisen condensation reaction of, 890-891... 

Mixed Claisen condensations (Section 21.3) Diethyl carbonate, diethyl oxalate, ethyl formate, and benzoate esters cannot form ester enolates but can act as acylating agents toward other ester enolates. 

The mixed Claisen condensation of two different esters is similar to the mixed aldol condensation of two different aldehydes or ketones (Section 23.5). Mixed Claisen reactions are successful only when one of the two ester components has no a 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 electrophilic acceptor components in reactions with other ester anions to give mixed /3-keto ester products. 

The Claisen reaction is a carbonyl condensation that occurs between two ester components and gives a /3-keto ester product. Mixed Claisen condensations between two different esters are successful only when one of the two partners has no acidic a hydrogens (ethyl benzoate and ethyl formate, for instance) and thus can function only as the acceptor partner. Intramolecular Claisen condensations, called Dieckmann cyclization reactions, provide excellent syntheses of five- and six-membered cyclic /3-keto esters starting from 1,6- and 1,7-diesters. 

Analogous to the mixed aldol condensation (Sec. 9.18), mixed Claisen condensations are possible. Predict the structure of the product obtained when a mixture of ethyl benzoate and ethyl acetate is heated with sodium ethoxide in ethanol. 

Mixed Claisen condensations start with two different esters. Like crossed aldol condensations (Section 18-6), they are typically unselective and furnish product mixtures. However, a selective mixed condensation is possible when one of the reacting partners has no a-hydrogens, as in ethyl benzoate. 

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