O-carboxyphenylacetic acid (20.0 g, 111 mmol) was mixed with finely ground urea (7.33 g, 122 mmol) and the reaction was heated at 175-185 °C for 2 hours. After completion of the reaction, the mixture was cooled and purified by recrystallization from methanol to give 1,3-[2H,4H]-isoquinolinedione (12.0 g, 67% yield) as a white solid with a melting point of 220-222 °C (literature value 3: 236-238 °C). The structure of the product was confirmed by 1H NMR and 13C NMR: 1H NMR ((CD3)2SO) δ 4.09 (3H, s, CH2), 7.44 (1H, d, J = 7.6 Hz, 5-H), 7.51 (1H, t, J = 7.6 Hz, 7-H), 7.70 (1H, td, J = 7.6, 1.2 Hz, 6-H). 8.07 (1H, dd, J = 7.8, 1.1 Hz, 8-H), 11.36 (1H, s, NH); 13C NMR ((CD3)2SO) (HSQC/HMBC) δ 35.92 (4-C), 124.95 (8a-C), 127.16 (7-C), 127.41 (8-C), 127.87 (5-C) , 133.47 (6-C), 136.66 (4a-C), 165.34 (1-C), 170.99 (3-C).
