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b) 11.3 g (50.0 mmol) of 3-(2-aminophenyl)-1H-benzimidazol-2-one was added to 50 g of polyphosphoric acid and the reaction was stirred under nitrogen protection at 220 °C for 3 hours. Upon completion of the reaction, the mixture was poured into water and the solid product was collected by filtration and washed sequentially with water and methanol. The resulting product was suspended in 200 mL of tetrahydrofuran, 50 mL of 30% sodium hydroxide solution was added and stirred for 30 minutes. The organic phase was separated, dried with magnesium sulfate, and the solvent was removed by distillation under reduced pressure to give 9.26 g of the target compound benzimidazo-benzimidazole (yield: 89%). The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 7.88 (d, J = 7.7 Hz, 2H), 7.39 (d, J = 8.0 Hz, 2H), 7.12-7.16 (m, 2H), 6.97-7.01 (m, 2H).

[1] Patent: US2012/241681, 2012, A1. Location in patent: Page/Page column 256
[2] Patent: WO2012/130709, 2012, A1. Location in patent: Page/Page column 192
[3] Patent: WO2013/68376, 2013, A1. Location in patent: Page/Page column 57
[4] Bulletin des Societes Chimiques Belges, 1987, vol. 96, # 10, p. 787 - 792