A process for preparing 3-[(S)-7-bromo-2-(2-oxo-propylamino)-5-pyridin-2-yl-3H-1,4,-benzodiazepin-3-yl]propionic acid methyl ester at a high conversion rate with good reproducibility by oxidizing 3-[(S)-7-bromo-2-(2-hydroxy-propylamino)-5-pyridin-2-yl-3H-benzo[e][1,4]diazepin-3-yl]propionic acid methyl ester in the presence of an oxidation catalyst is provided by defining the ammonium ion content of 3-[(S)-7-bromo-2-(2-hydroxy-propylamino)-5-pyridin-2-yl-3H-benzo[e][1,4]diazepin-3-yl]propionic acid methyl ester.
