the nucleophilic reagent IIIb is carried out under conditions similar to those described above for the reaction of IIa and IIIa.With regard to the intended use of the compounds Ia and Ib, the following substituents are suitable:R.sup.1 is halogen such as fluorine, chlorine and bromine, in particular chlorine;R.sup.2 is alkoxycarbonylalkenyl such as 2-methoxycarbonyl-2-propenyl, 2-ethoxycarbonyl-2-