Reaction Steps
To a stirred solution of the anomeric mixture 33 (3.0 g, 6.17 mmol) and uracil (1.04 g, 9.26 mmol) in anhydrous acetonitrile (65 cm3) was added N,O-bis(trimethylsilyl)acetamide (9.16 cm3, 37.0 mmol). The reaction mixture was stirred for 1 h at room temperature and cooled to 0° C. Trimethylsilyl triflate (1.8 cm3, 10.0 mmol) was added dropwise and the solution was stirred at 60° C. for 2 h. The reaction was quenched by addition of a saturated aqueous solution of sodium hydrogencarbonate (10 cm3) at 0° C. and extraction was performed with dichloromethane (3×20 cm3). The combined organic phase was washed with brine (2×20 cm3) and was dried (Na2SO4). The solvents were 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 nucleoside 40 as a white solid material (2.5 g, 75%). δH (CDCl3) 9.57 (1H, br s, NH), 7.63 (1H, d, J 8.2, 6-H), 7.40-7.24 (10H, m, Bn), 6.18 (1H, d, J 4.5, 1′-H), 5.39-5.32 (2H, m, 2′-H, 5-H), 4.61 (1H, d, J 11.6, Bn), 4.49-4.40 (5H, m, 3′-H, Bn, 1″-Ha), 4.37 (1H, d, J 12.3, 1″-Hb), 3.76 (1H, d, J 10.1, 5-Ha), 3.49 (1H, d, J 10.1, 5′-Hb), 2.09 (s, 3H, COCH3), 2.04 (3H, s, COCH3). δc (CDCl3) 170.47, 169.94 (C═O), 163.32 (C-4), 150.30 (C-2), 140.24 (C-6), 137.15, 136.95, 128.65, 128.52, 128.32, 128.19, 128.02, 127.77 (Bn), 102.57 (C-5), 87.41, 86.14 (C-1′, C-4′), 77.09, 74.84, 74.51, 73.75, 70.60, 63.73 (C-2′, C-3′, C-5′, C-1″, Bn), 20.79, 20.68 (COCH3). FAB-MS m/z 539 [M]+.
