Dicarbonyl compounds condense via the intramolecular aldol in basic media. In this reaction, cycles of five or six members are obtained.
Thus, 2,6-heptanedione condenses with methoxide in methanol to form 3-methylcyclohex-2-enone.



The reaction mechanism proceeds through the following stages:

Stage 1. Formation of the enolate.



Step 2. Intramolecular nucleophilic addition

Step 3. Protonation of the aldol base



Step 4. Dehydration of the aldol