The use of half-concentrated or concentrated organic or inorganic acids is preferred here, in particular hydrochloric acid (HCl), e.g. 6N hydrochloric acid, optionally in an aqueous or an organic solvent, such as diethyl ether, concentrated hydrobromic acid (HBr), hydrogen bromide in glacial acetic acid (HBr/HOAc), e.g. a 33% hydrogen bromide solution in glacial acetic acid, methanesulfonic acid,