Propose mechanisms for the two Dieckmann condensations just shown.

Internal “Claisen condensation or Claisen cyclization’’ of a diester to form a ring with the elimination of an alkoxide ion is known as Dieckmann condensation or a Dieckmann cyclization.

Diethyl adipate reacts with base ethoxide ion, which abstract alpha-hydrogen from it, forming resonance stabilizedenolate.

The enolate acts as a nucleophile and attacks the second carbonyl carbon of the same diethyl adipate molecule to form the 5-membered ring. Finally, the removal of ethoxide ion form the desired product ethyl 2-oxocyclohexanecarboxlate.

Formation of ethyl 2-oxocyclohexanecarboxlate

Dimethyl heptanedioate reacts with base methoxide ion, which abstract alpha-hydrogen from it, forming a resonance stabilized enolate.

The enolate attacks the second carbonyl carbon of the same molecule Dimethyl heptanedioate to form the 5-membered ring. Finally, the removal of methoxide ion forms the desired product ethyl 2-oxocyclohexanecarboxlate.

Formation of ethyl 2-oxocyclohexanecarboxlate