wherein X denotes halogen, especially chlorine or bromine, hydroxyl or a group ##STR1## wherein X1 represents an aliphatic cycloaliphatic or aromatic hydrocarbon radical, especially alkyl having 1 to 6 carbon atoms, or phenyl, and D has the indicated meaning, in a strongly acid medium, in a molar ratio of at least 1:2, with an organic sulphide containing 5 to 8 ring members, of the formula ##STR2#