The present invention relates in part to a process for the preparation of a proline derivative of formula I wherein, R1 is C1-7-alkyl or wherein R4 is selected from the group consisting of C1-7-alkyl, halogen-C1-7-alkyl and phenyl optionally substituted by halogen; R2 is halogen or halogen-C1-7-alkyl; and R3 is selected from the group consisting of hydrogen, halogen, halogen-C1-7-alkyl, C1-7-alkoxy, halogen-C1-7-alkoxy and a 5- or 6-membered heterocyclic ring containing one or two nitrogen atoms, said ring being optionally substituted by C1-7-alkyl or halogen. The proline derivatives of the formula I are preferential inhibitors of the cysteine protease Cathepsin S and are therefore useful to treat metabolic diseases like diabetes, atherosclerosis, abdominal aortic aneurysm, peripheral arterial disease and diabetic nephropathy.