Process for the preparation of 1-[3'-{mercapto}-(2')-{methyl}-propionyl]pyrrolidine-(2S)-carboxylic acid or its salts

The invention relates to a process for the preparation of 1-[3'-{mercapto}-(2'S)-{methyl}-propionyl]-pyrrolidine-(2S)-carboxylic acid of the formula or its salts by reacting compounds of the general formula in which X stands for a halogen atom, or their salts with thiourea and subsequent hydrolysis, in which the reaction of the compounds of the general formula III or their salts with the thiourea is carried out in organic dipolar aprotic solvents and the resulting isothiuronium compound of the formula if appropriate in the form of its internal salt, or resulting acid addition salts thereof with hydrohalic acids, are hydrolysed, if appropriate after their isolation, and the resulting compound of the formula I is optionally converted in a manner known per se into acid addition salts and/or the resulting acid addition salts of the compound of the formula I are converted into the free 1-[3'-{mercapto}-(2'S)-{methyl}-propionyl]-pyrrolidine-(2S)-carboxylic acid of the formula I and/or into other acid addition salts.