3.8.1. Preparation of Quinoxaline-2,3(1H,4H)-dione (1)

A mixture of o-phenylenediamine (5.0 g, 46.3 mmol) in 100 mL 4N HCl and oxalic acid (4.17 g, 46.3 mmol) was heated under reflux for 4 h. Then the reaction mixture was cooled to room temperature and the resulting solid was filtered and washed with ethanol to give the required compound 1 as a white solid according to the reported method [33]. Yield 91.5%, m.p. > 300 °C