The general procedure for the synthesis of isoquinoline-5-carboxylic acid from 5-cyanoisoquinoline was as follows: isoquinoline-5-carbonitrile (6 g, 38.62 mmol) prepared in step (1) was mixed with distilled water (90 mL) and stirred until homogeneous. Concentrated hydrochloric acid solution (15 mL) was slowly added to the reaction system, followed by heating the reaction mixture to 100 °C and continuous stirring for 6 hours. Upon completion of the reaction, the mixture was cooled to room temperature and the pH was adjusted to 5~6 by dropwise addition of N,N-diisopropylethylamine (DIPEA, ca. 40 mL).The resulting suspension was continued to be stirred at room temperature for 3 hr, followed by diafiltration. The collected solid product was dried in an oven at 50 °C to give final isoquinoline-5-carboxylic acid (5.6 g, 84% yield). Mass spectrum (ESI+, m/z): 174 [M+H]+.
