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According to the procedure of A. T. James, et al., J. Chem. Soc., 1515 (1950), a mixture of 2-hydroxy-5-nitro-1H-benzimidazole (10.0 g, 0.0558 mol) and phosphoryl chloride (100 mL) was heated under reflux for 3 hours. The mixture was cooled, concentrated, and ice water was added. The resulting precipitate was collected by filtration. NMR analysis showed the material to be a 1:1 mixture of starting material and product. (Note: Longer heating does not improve the conversion.) Purification by flash chromatography (material preadsorbed onto silica; eluent 40% EtOAc/hexane) gave 4.4 g (40%) of the off-white solid product m.p. 215°-217° C.

1. temperaturewas heated
2. temperatureunder reflux for 3 hours
3. temperatureThe mixture was cooled
4. concentrationconcentrated
5. additionice water was added
6. filtrationThe resulting precipitate was collected by filtration
7. additiona 1:1 mixture of starting material and product
8. temperature(Note: Longer heating
9. custom) Purification by flash chromatography (material preadsorbed onto silica; eluent 40% EtOAc/hexane)