Diethyl hexanedioate (diethyl adipate) is treated with $\mathrm{NaOEt}$ in ethanol (Dieckmann condensation), and the cyclic product is then heated with aqueous acid. The final product is

The enolate at C-2 of one ester attacks the C-6 ester carbonyl; counting C-2 to C-6 gives a five-membered ring, so ethyl 2-oxocyclopentane-1-carboxylate forms. Acid hydrolysis converts the ester to a beta-keto acid, which decarboxylates on heating. The product is cyclopentanone.