The present invention relates to the use of compounds of the general Formula (1): wherein in Formula (1), R is H, alkyl, cycloalkyl, aryl or heteroaryl; R.sup.1 is H, alkyl, cycloalkyl, aryl or heteroaryl; R.sup.2 is H, alkyl, cycloalkyl, aryl or heteroaryl; A.sup.1 and A.sup.2 each independently represent an optionally substituted C.sub.1-C.sub.20-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, NR.sup.4, CO, CO.sub.2, CS, SO or SO.sub.2 X is selected from the group consisting of S, O, N, NR.sup.4, SO or SO.sub.2.