Compound of formula (I): wherein:  represents an optionally substituted, 5-membered, nitrogen-containing heterocycle, R1 represents hydrogen, alkyl, acyl, prolyl, alanyl, histidylprolyl or phenylalanylprolyl, Ak represents an alkylene or heteroalkylene chain, X represents a single bond or optionally substituted phenylene, R2 represents optionally substituted alkyl, (C3-C10)cycloalkyl, or —NR2aR2b wherein R2a and R2b, which may be the same or different, each represent hydrogen or alkyl or, together with the nitrogen atom carrying them, form a nitrogen-containing heterocycle, Y represents R3 represents hydrogen or a group selected from alkyl, (C3-C7) cycloalkyl and aryl, R4, R5 and R6, which may be the same or different, each represent hydrogen or alkyl, or R4 and R6, or R5 and R6, together with the atoms carrying them, form an imidazolidine or dihydrobenzimidazole ring, its optical isomers and addition salts thereof with a pharmaceutically acceptable acid. Medicinal products containing the same which are useful as inhibitors of dipeptidyl-peptidase IV.