The number of substituents in substituted alkyl is generally 1 or 2.Any oxygen-interrupted alkyl appearing in the abovementioned formulae is preferably interrupted by 1 or 2 oxygen atoms in ether function.Any substituted phenyl appearing in the abovementioned formulae may have as substituents for example C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.4 -alkoxy, halogen, in particular chlorine or bromine,