2-substituted-3-carbonyl butanoate

Draw two molecules of ethyl butanoate. Convert one of the molecules to an enolate anion and show it adding to the carbonyl carbon of the other. Because Claisen condensations occur with nucleophilic acyl substitution, the —OR group of the carbonyl being attacked is eliminated from the final product. It often helps to number the atoms in the enolate anion and the ester being attacked. 

Aqueous acid workup of 92 gives the alcohol, 93. With malonic ester derivatives, loss of water to form 94 occurs very easily, with dilute acid or with gentle heating because the C=C unit is conjugated to two carbonyl groups, facilitating dehydration. Although it is possible to isolate 83, it is more usually difficult. The enolate anion of malonate esters also reacts with ketones and may be condensed with other esters in acyl substitution reactions. When 90 is treated with NaOEt in ethanol and then with ethyl butanoate, the final product after mild hydrolysis is a keto-diester, 95. 

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