At 0°C, 10.0 g (0.046 mmol) of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid was dissolved in 200 ml of N,N-dimethylformamide and 5.0 g (0.032 mmol) of potassium permanganate (KMnO4) was slowly added over 30 min. The reaction mixture was stirred at room temperature for 100 h. The progress of the reaction was monitored by thin layer chromatography (TLC, unfolding agent ratio CCl3:CH3OH = 5:1) to confirm complete consumption of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid. Subsequently, the reaction mixture was concentrated under vacuum. The residue was washed several times with distilled water to give 8.1 g (83% yield) of isoquinoline-3-carboxylic acid as a yellow powder. The product was analyzed by ESI-MS (m/e) showing [M + H]+ peak of 174; 1H NMR (300 MHz, DMSO) δ/ppm = 10.81 (s, 1H), 9.52 (s, 1H), 8.43 (s, 1H), 7.52 (m, 4H); 13C NMR (75 MHz, DMSO) δ/ppm = 164.9, 152.3, 143.5, 136.2, 131.3, 129.7, 127.2, 125.8.
