The present invention relates to molecules of formula (I) where R1 = -SO3H, -PO3H, -PO2(OH)2, -OPO2H2, -NHSO3H, -S (N=H)Me, SH, SR o guanidyl; R = C1-4 alkyl, phenyl or 5 or 6 membered aromatic nitrogen heterocycles; n = 1, 2, 3, 4 o 5; X = C=0, C(OH)H, C(OAIk)H, C=S, CH2; Alk = C1-6 alkyl linear, branched or cyclic, optionally hydroxylated or polyhydroxylated; their preparation and use as analgesics and in the treatment of pain induced by chemotherapies.