alic acid and succinic acid.The compounds of formula I can be obtained by methods which are known per se, e.g. as follows: ##STR3##In formulae II and III above, R.sub.1 to R.sub.4 are as defined for formula I and X in formula III is a halogen atom, especially a fluorine or chlorine atom.The process is carried out at a reaction temperature between -50.degree. and +130.degree.