The inventions relates to a compound häving general formula l, wherein: X represents CH2, C=O, C=S or CHOH, X represents CH2, C=O, C=S or CHOH, R1, represents an amino acid which is optionally substituted by one or more halogen atoms, preferably fluorine, or by one or more CF3 groups and n= 0.1 or 2, or X represents...