The present invention relates to the use of compounds of the general Formula (I): wherein in Formula (I), R is H, alkyl, cycloalkyl, aryl or heteroaryl; R1 is H, alkyl, cycloalkyl, aryl or heteroaryl; R2 is H, alkyl, cycloalkyl, aryl or heteroaryl; A1 and A2 each independently represent an optionally substituted C1-C20-alkyl group which may contain one or more group(s) Z, or a monocyclic or polycyclic optionally substituted aromatic or non-aromatic ring system which may contain one or more group(s) X, and in case of a polycyclic ring system, said system contains at least one aromatic ring; Z is selected from the group consisting of S, O, N, NR4, CO, CO2, CS, SO or SO2 X is selected from the group consisting of S, O, N, NR4, SO or SO2;