Propanone is stirred with but-3-en-2-one ($\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CO}-\mathrm{CH}{}_3$) in the presence of aqueous $\mathrm{NaOH}$ and the mixture is heated. A Michael addition is followed by an intramolecular aldol condensation. The final cyclic product is

The propanone enolate adds to the beta carbon ($\mathrm{CH}{}_2=$ end) of the enone, giving $\mathrm{CH}{}_3\mathrm{COCH}{}_2\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{COCH}{}_3$ (heptane-2,6-dione). A terminal $\mathrm{CH}{}_3$ enolate then attacks the carbonyl at the other end, closing a six-membered ring (C-1 to C-6 of the chain). Dehydration gives the conjugated enone with the methyl on the beta carbon, i.e. 3-methylcyclohex-2-en-1-one.