wherein X denotes a halogen atom, especially chlorine or bromine, or a ##STR46## group, wherein X1 represents an aliphatic, cycloaliphatic or aromatic hydrocarbon radical, especially an alkyl radical with 1 to 6 carbon atoms or the phenyl radical, and D has the indicated meaning, are reacted in a strongly acid medium, in a molar ratio of at least 1:2, with organic sulphides of the formula III ##ST