Reaction Steps
For the preparation of 1-O-chloro-2,3,5-tri-O-benzyl-D-ribofuranose, HCl gas was allowed to pass through a solution of 1-O-p-nitrobenzoyl-2,3,5-tri-O-benzyl-β-D-ribofuranose (1.479 g, 2.6 mmol) in 10 mL of dry CH2Cl2 at 0° C. for 15 min. The resulting suspension was quickly filtered and the filtrate was evaporated. The residue was coevaporated with dry MeCN and then dissolved in 5 mL of dry MeCN for immediate use in the subsequent glycosylation reaction. To a mixture of 13 (0.512 g, 2 mmol) in 10 mL of dry MeCN, was added 0.5 mL (2 mmol) of BSA. The reaction mixture was stirred at 75° C. for 20 min. This solution was treated with the above MeCN solution of the blocked carbohydrate and 0.464 mL (2.4 mmol) of TMSOTf at 70° C. for 20 min. The reaction mixture was cooled and diluted with EtOAc (50 mL). The EtOAc solution was washed with sat. NaHCO3 solution (50 mL×2), sat. NaCl solution (50 mL), dried (Na2SO4), and evaporated. The residue was chromatographed on a silica column (1.9×38 cm, eluted with hexane, 10%, 15% EtOAc/hexane). Evaporation of fractions 30-37 (15 mL per fraction) gave 0.593 g (45%) of 91a as a colorless syrup. MS: (EI) m/e 658.0780 (2%, M+ =658.0774). 1H NMR (DMSO-d6): d 7.93 (s, 1, 7-H), 7.35, 6.92 (2×m, 15, 3×PhCH2), 5.99 (d, 1, 1'-H, J1'-2' =8.0 Hz), 4.72, 4.58, 4.35 (3×m, 9, 2'-H, 3'-H, 4'-H, and 3×PhCH2), 3.76 (dd, 1, 5'-H, J4'-5' =2.0 Hz, J5'-5' =11.0 Hz), 3.65 (dd, 1, 5"-H, J4'-5" =2.5 Hz). 13C NMR (DMSO-d6): d 142.00 (C2), 138.40 (C3a), 137.81, 137.44, 136.74 (3×PhCH2), 131.34 (C7a), 128.08, 128.03, 127.59, 127.40, 127.32, 127.28, 126.97 (3×PhCH2 and C4), 124.64, 122.52 (C5 and C6), 112.65 (C7), 87.46 (C1'), 83.03, 77.08, 74.92 (C2', C3', and C4'), 72.43, 71.44, 71.28 (3×PhCH2), 69.22 (C5').
