Amino acid ester prodrugs and the use thereof

A compound of the formula (I)

• in which
• RA is hydrogen or methyl
• and
• RPD is a group of the formula
• in which
• # means the point of linkage to the respective O atom,
• L1 is a bond or —CH2—,
• L2 is straight-chain (C3-C6)-alkanediyl, which may be substituted up to two times, identically or differently by (C1-C4)-alkyl, hydroxyl and/or (C1-C4)-alkoxy,
• R1 and R2 are identical or different and independently of one another are hydrogen or (C1-C4)-alkyl which may be substituted by hydroxyl, (C1-C4)-alkoxy, amino, mono-(C1-C4)-alkylamino or di-(C1-C4)-alkylamino
• or
• R1 and R2 are linked to one another and, together with the nitrogen atom to which they are attached, form a 5- or 6-membered saturated heterocycle which may comprise a further ring heteroatom from the series consisting of N and O, and may be substituted one or two times, identically or differently, by (C1-C4)-alkyl, amino, hydroxyl and/or (C1-C4)-alkoxy,
• R3 is hydrogen or the side group of a natural α-amino acid or its homologs or isomers,
• or
• R3 is linked to R1 and both, together with the atoms to which they are attached, form a 5- or 6-membered saturated heterocycle which may be substituted once or twice by identical or different (C1-C4)-alkyl, amino, hydroxyl and/or (C1-C4)-alkoxy substituents,
• R4 is hydrogen or methyl
• or
• R3 and R4 are linked to one another and, together with the carbon atom to which they are attached, form a 3- to 6-membered saturated carbocycle,
• and
• R5 and R6 are identical or different and independently of one another are hydrogen or (C1-C4)-alkyl which may be substituted by hydroxyl, (C1-C4)-alkoxy, amino, mono-(C1-C4)-alkylamino or di-(C1-C4)-alkylamino,
• and the salts thereof.