The invention relates to a process for the preparation of compounds of the general formula (I) where X are halogen atoms, the rings labeled A are linearly or angularly fused in position 1,2; 2,3 or 3,4, and in position 8,9; 9,10 or 10,11, to a 5- or 6-membered heterocyclic ring containing in the ring one nitrogen atom and one oxygen atom, or containing in the ring two nitrogen atoms, comprising an oxidation of a 3,6-bisarylamino-2,5-dihalobenzoquinone of the formula (II) in an aprotic-polar organic solvent in the presence of an organic sulfonic acid.