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I've given a strong hint by changing the title of your question!
Methoxide as a base can abstract a proton from an acidic $\alpha$-position of an ester, that is from $\ce{-C\color\red{H}_2COOMe}$.
The resulting carbanion/enolate can add to the other ester group of your starting material.
The cyclic intermediate eventually loses methoxide to form ...
This Dieckmann condensation is the intramolecular variant of a Claisen condensation.
I suggest to do it on paper!
More
VOTE
2026-08-06
I've given a strong hint by changing the title of your question!
Methoxide as a base can abstract a proton from an acidic $\alpha$-position of an ester, that is from $\ce{-C\color\red{H}_2COOMe}$.
The resulting carbanion/enolate can add to the other ester group of your starting material.
The cyclic intermediate eventually loses methoxide to form ...
This Dieckmann condensation is the intramolecular variant of a Claisen condensation.
I suggest to do it on paper!
I've given a strong hint by changing the title of your question!
Methoxide as a base can abstract a proton from an acidic $\alpha$-position of an ester, that is from $\ce{-C\color\red{H}_2COOMe}$.
The resulting carbanion/enolate can add to the other ester group of your starting material.
The cyclic intermediate eventually loses methoxide to form ...
This Dieckmann condensation is the intramolecular variant of a Claisen condensation.
I suggest to do it on paper!
More
VOTE
VOTE
VOTE