Compound selected from those of formula (I): wherein: represents an optionally substituted 5-membered nitrogen-containing heterocycle, Ak represents an alkylene chain, X represents a single bond or phenylene, R1 and R2, which may be identical or different, each represent hydrogen or alkyl, R3 represents alkyl, nitro or cyano, Y represents NR4 or CHNO2, R4 represents hydrogen or alkyl, its tautomers when they exist, its optical isomers, and addition salts thereof with a pharmaceutically acceptable acid, with the exclusion of compounds wherein, simultaneously, represents an unsubstituted 5-membered nitrogen-containing heterocycle, Ak represents —(CH2)3—, X represents a single bond, Y represents NH and R3 represents nitro. Medicinal products containing the same which are useful as dipeptidyl peptidase IV inhibitors.