91Ó°ÊÓ

It is an intramolecular Claisen condensation of a compound when two ester groups undergo intramolecular reaction giving a five-membered ring product.Dieckmann condensation converts 1,6-diester to five membered ring $\mathrm{β}$keto ester.

The mechanism is shown below:

Proposed mechanism

In the first step two ester containing compounds fused to form 1,6-diester. A base$\left({\mathbf{CH}}_3{\mathbf{O}}^{-}\right)$removes proton fromcarbon of diester forming an enolate ion in the fourth step. Then an intramolecular condensation reaction adds enolate ions to the carbonyl carbon of the estergroup forming a five membered ring. Thebond reforms by eliminating${\mathbf{OCH}}_3$group and leaving a proton as${\mathbf{CH}}_3\mathbf{OH}$. In the last step of the reaction mechanism, keto group gets protonated and forms five membered $\mathrm{β}$keto ester.