SolutionsIntramolecular aldol condensation

Octane-2,7-dione: the C3 (or C6) enolate attacks the other carbonyl and forms a five-membered ring; after dehydration 1-(2-methylcyclopent-1-en-1-yl)ethanone is obtained. The enolate of a terminal methyl would give a seven-membered ring, less favourable. Nonane-2,8-dione: the C3 enolate attacks C8 and forms a six-membered ring: 1-(2-methylcyclohex-1-en-1-yl)ethanone.

Disconnecting the C2=C3 double bond (the one formed by the aldol condensation) leads to hexane-2,5-dione. This 1,4-diketone can be obtained, for example, by alkylation of ethyl acetoacetate with chloroacetone followed by hydrolysis and decarboxylation (acetoacetic synthesis), or by hydrolysis of 2,5-dimethylfuran. Cyclization is carried out with hot aqueous NaOH.