[N+:1]([C:4]1[CH:9]=[CH:8][CH:7]=[CH:6][C:5]=1[C:10]1[NH:11][C:12]2[C:18]([CH:19]=[O:20])=[CH:17][CH:16]=[CH:15][C:13]=2[N:14]=1)([O-:3])=[O:2].[CH2:21](O)[CH2:22][OH:23].O.C1(C)C=CC(S(O)(=O)=O)=CC=1>C1C=CC=CC=1>[CH2:22]1[O:23][CH:19]([C:18]2[C:12]3[NH:11][C:10]([C:5]4[CH:6]=[CH:7][CH:8]=[CH:9][C:4]=4[N+:1]([O-:3])=[O:2])=[N:14][C:13]=3[CH:15]=[CH:16][CH:17]=2)[O:20][CH2:21]12-(2-Nitrophenyl)-benzimidazole-4-carboxaldehyde 4 (6.27 g, 23.5 mmol) was dissolved in benzene (150 mL), and mixed with ethylene glycol (6.5 mL, 118 mmol) and p-toluenesulfonic acid monohydrate (0.450 g, 2.35 mmol). The mixture was heated to reflux overnight and the water was removed by azeotropic distillation. The resulting solution was allowed to cool to room temperature, and was then neutralized with saturated NaHCO3, washed with water and brine and dried over Na2SO4. The compound was purified by column chromatography on silica gel using hexane/ethyl acetate (1:2) as the eluent to give 2-(2-nitrophenyl)-benzimidazole-4-carboxaldehyde ethylene acetal (4.552 g, 62.2%) as a yellow solid. Rf=0.24 (hexane/ethyl acetate=1:2); 1H NMR (200 MHz, CDCl3) δ 4.11 (m, 2H), 4.20 (m, 2H), 6.15 (s, 1H), 7.30 (m, 2H), 7.57-7.83 (m, 3H), 7.93 (d, J1=7.8 Hz, J2=1.3 Hz, 1H), 8.13 (d, J1=7.7 Hz, J2=1.5 Hz, 1H), 10.48 (broad s, 1H). HRMS: Calcd. for C16H13N3O4 311.0906, found 311.0915.
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