To a solution of 0.464 g (0.719 mmol) of 60 in 12 mL of CH2Cl2, was added dropwise 8.4 mL of 1M BCl3 at -78° C. The reaction mixture was stirred at -78° C. for 2 h and then at -40° C. for 2 h. MeOH (5 mL) was added and stirring was continued at -40° C. for 10 min. The reaction mixture was diluted with EtOAc (75 mL). The EtOAc solution was washed with cold H2O (50 mL), sat. NaHCO3 solution (50 mL), sat. NaCl solution (50 mL), dried (Na2SO4), and evaporated. The residue was coevaporated with MeOH (3 x) and then suspended in a small amount of CHCl3 for a few hours. The solid product was filtered to give 0.209 g of 61. This sample was contaminated by a small amount of 2-chloro- 5,6-dinitrobenzimidazole. A part of the sample (0.18 g) was purified on a silica column (2.4×10 cm, eluted with pure EtOAc). Evaporation of the appropriate fractions and crystallization by addition of CHCl3 gave 0.084 g of 61 as a white solid. MP: 132°-135° C. MS: (El) m/e 374.0276 (1%, M+ =374.0265). 1H NMR (DMSO-d6): δ9.18 (s, 1, 7-H), 8.60 (s, 1, 4-H), 6.00 (d, 1, 1'-H, J1'-2' =7.5 Hz), 5.59 (d, 1, 2'-OH, J2'-2'OH =6.0 Hz), 5.52 (t, 1, 5'-OH, J5'-5'OH =4.5 Hz), 5.38 (d, 1, 3'-OH, J3'-3'OH =4.5 Hz), 4.01 (m, 1, 2'-H, J2'-3' =5.5 Hz), 4.16 (m, 1, 3'-H, J3'-4' =2.0 Hz), 4.07 (m, 1, 4'-H, J4'-5" =J4'-5" =2.5 Hz), 3.73 (m, 2, 5'-H and 5"-H, J5'-5" =12.0 Hz). 13C NMR (DMSO-d6): δ146.55 (C2), 142.63 (C3a), 138.90, 138.60 (C5 and C6), 134.12 (C7a), 116.67 (C4), 111.62 (C7), 89.98 (C1'), 86.91 (C4'), 72.86 (C2'). 69.84 (C3'), 60.85 (C5'). Anal. Calcd. for C12H11ClN4O8 : C, 38.47; H, 2.96; N, 14.95. Found: C, 38.46; H, 2.98; N, 14.62.
