Claisen Condensation

Aldehydes undergo the Aldol reaction to form β-hydroxyaldehydes.

Esters on the otherhand undergo Claisen condensation to form β-ketoesters.

In the Claisen condensation, an ester enolate forms in equilibrium with the ester.  Alkoxide bases are commonly used to avoid any hydrolysis that may occur if OH^- were used.  The ester enolate undergoes acyl substitution with an ester that has not been deprotonated.

Similar to the Aldol reaction Crossed Claisen's are possible giving four β-ketoesters products.

Crossed Claisen are more efficient when directed (i.e. one of the esters cannot form an enolate...no α-protons).  You can also play the slow addition game as in the Aldol condensation to maximize efficiency/yield.

With diesters you can also do intramolecular Claisen Condensation (Dieckman Cyclization).