To a stirred solution of the anomeric mixture 33 (4.0 g, 8.22 mmol) and 4-N-benzoylcytosine (2.79 g, 13.0 mmol) was added N,O-bis(trimethylsilyl)acetamide (8.16 cm3, 33.0 mmol). The reaction mixture was stirred for 1 h at room temperature and cooled to 0° C. Trimethylsilyl triflate (3.0 cm3, 16.2 mmol) was added dropwise and the mixture was stirred at 60° C. for 2 h. Saturated aqueous solutions of sodium hydrogencarbonate (3×20 cm3) and brine (2×20 cm3) were successively added, and the separated organic phase was dried (Na2SO4). The solvent was removed under reduced pressure and the residue was purified by silica gel column chromatography using dichloromethane/methanol (99:1, v/v) as eluent to give compound 54 as a white solid material (3.9 g, 74%). δH (CDCl3), 8.28 (1H, d, J 7.5, 6-H), 7.94-7.90 (2H, m, Bz), 7.65-7.25 (13H, m, Bn, Bz), 7.16 (1H, d, J 7.1, 5-H), 6.22 (1H, d, J 2.8, 1′-H), 5.51 (1H, dd, J 2.8, 5.8, 2′-H), 4.62 (1H, d, J 11.6, Bn), 4.51 (1H, d, J 12.3, 1″-Ha), 4.49-4.34 (4H, m, 3′-H, Bn), 4.21 (1H, d, J 12.3, 1″-Hb), 3.85 (1H, d, J 10.3, 5′-Ha), 3.47 (1H, d, J 10.3, 5′-Hb), 2.13 (3H, s, COCH3), 2.06 (3H, s, COCH3). δC (CDCl3) 170.52, 169.61 (C═O), 166.83, 162.27 (C-4, C═O), 154.26 (C-2), 145.26 (C-6), 137.25, 136.93, 133.18, 129.0, 128.75, 128.51, 128.45, 128.18, 128.10, 127.89, 127.71 (Bn, Bz), 96.58 (C-5), 89.42, 86.52 (C-1′, C-4′), 76.21, 75.10, 74.17, 73.70, 69.70, 63.97 (C-2′, C-3′, Bn, C-5′, C-1″), 20.82 (COCH3). FAB-MS m/z 664 [M+Na]+, 642 [M+H]+. Found: C, 65.0; H, 5.7; N, 6.5; C35H35N3O9 requires C, 65.5; H, 5.5; N, 6.5%.
