200 mg (0.34 mmol) of 2-tert-butoxycarbonylamino-5-[3-(2-methoxy-phenyl)-1-(2-trimethylsilanyl-ethoxymethyl)-1H-pyrazolo[3,4-b]pyridin-5-yl]-benzoic acid were dissolved in 5 mL of glacial acetic acid. 500 μL of 70% perchloric acid was added and the mixture left at ambient temperature for 24 h. 3 mL of water and 300 μL of mercaptoacetic acid was added and the mixture heated to 80° C. for 20 h. The resulting mixture was cooled to room temperature and diluted with 50 mL of water. The resulting precipitate was filtered off and dried by suction to afford 101 mg (0.28 mmol, 82%) of 2-amino-5-[3-(2-methoxy-phenyl)-1H-pyrazolo[3,4-b]pyridin-5-yl]-benzoic acid as a red-brown solid. MS: m/z 361.1 (M+H+).
