Overview:

The general form of a Claisen condensation is as follows:

The first step involves adding a strong base to an ester to generate an enolate at the a carbon (note that the enolate has an additional resonance structure).

The enolate can then add to another ester molecule by attacking the carbonyl to make the tetrahedral intermediate. The carbonyl reforms with loss of the alcoxy group to make the b-keto ester.

Example of a Claisen condensation:

This is an example of an intramolecular Claisen reaction, called a Dieckmann condensation.