Crossed Claisen Condensation:
Step 1: The alkoxide ion deprotanates the enolizable ester reversibly.
Step 2 and 3: Enolate ion 1 undergoes a nucleophilic acyl substitution preferentially with the non-enolizable ester, which has the sterically less hindered and, therefore, more accessible carbonyl carbon, giving a β-ketoester.
Step 4: The alkoxide ion deprotonates the β-ketoester irreversibly.
Step 5: The acid protonates enolate ion 2
see also Dieckmann condensation