91Ó°ÊÓ

Di-carbonyl compounds undergo intramolecular aldol condensation to form a five or six membered cyclic compound. Out of the two carbonyl groups, one which forms resonance stabilized enolate is protonated, and the other acts as an electrophilic center.

The deprotonation does not occur at the $\mathrm{Î\pm }$-carbonyl, it occurs in a way that the enolate is more resonance stabilized, as shown below:

Deprotonation and formation of an enolate

The nucleophilic attack occurs at the keto carbonyl group since it leads to the formation of a 6-membered ring. If the attack would have occurred at the ester carbonyl group, an unstable 3-membered ring would have been formed.

Nucleophilic attack and formation of 6-membered ring

In the final step, dehydration occurs via the E1cb mechanism, involving deprotonation and elimination of -OH group, as shown below:

Dehydration via E1cb mechanism