The general procedure for the synthesis of 4-bromo-1(2H)-phthalazinone from phthalodicarbohydrazide was as follows: 2,3-dihydro-1,4-diazanaphthalenedione (12.5 g, 78 mmol) was suspended in 1,2-dichloroethane (DCE, 200 ml) and phosphorus pentabromide (50.0 g, 116 mmol) was added. The reaction mixture was heated to reflux for 24 hours. Subsequently, phosphorus pentabromide (20.0 g, 70 mmol) was added and heating to reflux was continued for 24 hours. Upon completion of the reaction, the mixture was cooled to room temperature and slowly poured into ice water. The precipitate was collected by filtration and washed with water to give a crude mixture of mono- and dibrominated products (22.8 g). The crude product was suspended in acetic acid (HOAc, 230.0 mL) and the reaction was heated to 120 °C for 2 hours. At the end of the reaction, it was cooled to room temperature, poured into ice water and filtered to obtain a precipitate. The solid was washed with water and dried to give the final 4-bromo-1(2H)-phthalazinone (10.4 g, 60% yield) as a white solid. The product was characterized by 1H-NMR (400 MHz, D6-DMSO): δ 12.95 (1H, s), 8.25 (1H, dd), 8.03 (1H, ddd), 7.96-7.90 (2H, m); the mass spectrum (ESI+) showed the molecular ion peaks (M + H)+ of 225 and 227.
