The present invention relates to synthesis precursors useful for the preparation of compounds of the formula I,wherein A, D, E, G, R10, R30, R40, R50 and R60 have the meanings indicated in the description, which in turn are valuable pharmaceutical active compounds. They are inhibitors of the protease cathepsin A, and are useful for the treatment of diseases such as atherosclerosis, heart failure, renal diseases, liver diseases or inflammatory diseases, for example.