Compounds of the formula I ##STR1## wherein R.sub.1 is C.sub.1 -C.sub.18 alkyl, R.sub.2 is hydrogen or C.sub.1 -C.sub.18 alkyl, and A is a substituted primary or secondary aliphatic or alicyclic, aromatic or araliphatic amine, in which the substituents are identical or different, a heterocyclic amine or a hydrazine derivative.
Compounds of the formula I ##STR1## wherein R.sub.1 is C.sub.1 -C.sub.18 alkyl, R.sub.2 is hydrogen or C.sub.1 -C.sub.18 alkyl, and A is a substituted primary or secondary aliphatic or alicyclic, aromatic or araliphatic amine, in which the substituents are identical or different, a heterocyclic amine or a hydrazine derivative.