The general procedure for the synthesis of 7-nitro-1H-benzo[d][1,3]oxazine-2,4-dione from tritiated gas and 2-amino-4-nitrobenzoic acid was as follows: to a 500 mL single-necked round-bottomed flask equipped with a soccer ball shaped PTFE stirring bar (16 mm × 37 mm) was added 2-amino-4-nitrobenzoic acid (10.0 g, 46.3 mmol, 1.0 eq. ), followed by tetrahydrofuran (230 mL, 0.2 M) and solid triphosgene (13.7 g, 46.3 mmol, 1.0 eq.) to form a suspension. The reaction flask was placed in a device equipped with a metal heating jacket with a 24/40 Liebig condenser attached to the neck of the flask. The suspension was stirred at 500 rpm and the temperature of the heating jacket was set to 70°C. After about 30 minutes, the suspension became homogeneous and subsequently precipitated a white solid. The non-homogeneous reaction mixture was allowed to stand for 12 hours and then cooled to room temperature (25°C). The reaction slurry was poured into a 600 mL beaker equipped with an overhead mechanical stirrer (PTFE 75 mm paddle) and 250 mL of deionized water was added. Under vigorous stirring, the mixture first became homogeneous and then precipitated as a pale white solid. The solids were collected by vacuum filtration through a Brinell funnel (7.6 cm diameter, lined with Whatman 1 filter paper, 70 mm) and purged with air for 5 min. The resulting solids were transferred to a 250 mL conical flask, 50 mL of methanol was added, and the slurry was stirred using a cylindrical rod for 10 min, after which the solids were again collected by vacuum filtration. The filter cake was dried under vacuum (0.1 mmHg, 25 °C) for 12 h. The final product was 7-nitro-1H-benzo[d][1,3]oxazine-2,4-dione as a white powder in 90% yield.
