Reaction Steps
To a stirred suspension of 2-bromo-5,6-dichlorobenzimidazole (which may be made according to U.S. Pat. No. 5,248,672) (1b, 1.33 g, 5 mmol) in dry MeCN (25 ml) was added 1.8 ml (7.5 mmol) of BSA. The reaction mixture was stirred at room temperature for 15 min to give a clear solution. This solution was treated with a solution of 5-deoxy-1,2,3-tri-O-acetyl-D-ribofuranose (1.4 g, 5.5 mmol) in dry acetonitrile (5 mL) and 1.3 mL (6.5 mmol) of TMSOTf at room temperature for 30 min. A fresh solution of 5-deoxy-1,2,3-tri-O-acetyl-D-fibofuranose (0.26 g, 1 mmol) in dry MeCn (2 mL) was added and stirring was continued at room temperature for 30 minutes. The reaction mixture was diluted with EtOAc (100 mL). The EtOAc solution was washed with a sat. NaHCO3 solution (100 mL×2), dried (Na2SO4), and evaporated. The residue was recrystallized from MeOH to give 1.2 g (53%) of 2b as white crystals. The mother liquor was evaporated and the residue was chromatographed on a silica column (2.2×35 cm, eluted with CHCl3). Evaporation of fractions 16-42 (20 mL per fraction) and recrystallization from MeOH gave an additional 0.6 g (27%) of 2b as white crystals. The total yield of 2b was 1.86 g (80%). MP: 145-148° C. HRMS: (EI, with DCI probe) m/z 463.9582 (6%, M+=463.9541). 1H NMR (DMSO-d6): 8.11 (s, 1, 7-H), 7.99 (s, 1, 4-H), 6.14 (d, 1, 1′-H, J1′-2′=6.5 Hz), 5.65 (t, 1, 2′-H, J2′-3′=7.0 Hz), 5.27 (dd, 1, 3′-H, J3′-4′=5.5 Hz), 4.25 (m, 1, 4′-H, J4′-5′=6.5 Hz), 2.13, 2.02 (2×s, 6, 2×Ac), 1.51 (d, 3, 5′-H). 13C NMR (DMSO-d6): δ169.61, 168.20, (2×COCH3), 142.28 (C3a), 132.80 (C7A), 131.49 (C2),126.35, 126.07 (C5 and C6), 120.22 (C4),113.37 (C7), 87.93 (C1′), 77.98 (C4′), 72.89 (C3′), 70.91 (C2′), 20.33, 20.07 (2×COCH3), 17.78 (C5′). Anal. (C16H15BrCl2N2O5) C, H, N. Evaporation of fractions 45-58 gave 0.38 g of 3b which was further purified on a silica column (2.2×8 cm, eluted with hexane, 30% EtOAc/hexane). Evaporation of fractions 9-12 (20 mL per fraction) gave 0.26 g (11 %) of 3b as a colorless syrup. HRMS: (EI) m/z 463.9529 (2%, M+=463.9541). 1H NMR (DMSO-dhd 6): δ7.95 (s,1, 4-H), 7.89 (s, 1, 7-H), 6.68 (d, 1, 1′-H,J1′-2′−4.5 Hz), 5.70 (t, 1, 2′-H, J2′-3′=5.0 Hz), 5.28 (dd, 1, 3′-H, J3′-4′=7.0 Hz), 4.72 (m, 1, 4′-H, J4′-5′=6.5 Hz), 2.03, 1.51 (2×s, 6, 2×Ac), 1.39 (d, 3, 5′-H). 13C NMR (DMSO-d6): δ169.46, 168.30, (2×COCH3), 142.13 (C3a), 133.58 (C7a), 131.10 (C2), 125.74, 125.42 (C5 and C6), 119.95 (C4), 114.24 (C7), 86.87 (C1′), 76.89 (C4′), 74.55 (C3′), 71.18 (C2′), 20.21, 19.58 (2×COCH3), 18.12 (C5′).
