Dipeptides (I) derived from an alpha -amino acid and a cyclopentane- beta -amino acid are new. Dipeptides of formula (I) and their salts derived from an alpha -amino acid and a cyclopentane- beta -amino acid are new. R<1>, R<2> = H; or R<1> + R<2> = form CH2; R<3> = CH3 or -CH(CH3)CH2CH3; R<4>, R<5> = H; or R<3> + R<4> = form -(CH2)3-. An Independent claim is also included for the preparation of compounds (I).