In the preferred compound, R.sup.6 is a linear or branched alkyl radical having 1 to 6 carbon atoms, for example an ethyl group.(A5) is a dicarboxylic ester of the general formula R.sup.7OOC--R.sup.8--COOR.sup.7' and/or R.sup.7COO--R.sup.8--OOCR.sup.7'.