To a solution of 1-(3,5-di-O-benzoyl-β-L-xylofuranosyl)uracil 11 (8 g, 17.7 mL) in an anhydrous benzene-DMSO mixture (265 mL, 6:4, v/v) were added anhydrous pyridine (1.4 mL), dicyclohexylcarbodiimide (10.9 g, 53 mmol) and dichloroacetic acid (0.75 mL). The resulting mixture was stirred at room temperature for 4 h, then diluted with ethyl acetate (400 mL) and a solution of oxalic acid (4.8 g, 53 mmol) in methanol (14 mL) was added. After being stirred for 1 h, the solution was filtered. The filtrate was washed with a saturated NaCl solution (2×500 mL), 3% NaHCO3 solution (2×500 mL) and water (2×500 mL). The organic phase was dried over Na2SO4, then evaporated under reduced pressure. The resulting residue was then solubilized in an EtOH absolute-benzene mixture (140 mL, 2:1, v/v). To this solution at 0° C. was added NaBH4 (0.96 g, 26.5 mmol). After being stirred for 1 h, the solution was diluted with ethyl acetate (400 mL), then filtered. The filtrate was washed with a saturated NaCl solution (400 mL) and water (400 mL). The organic phase was dried over Na2SO4, then evaporated under reduced pressure. The resulting crude material was purified by silica gel column chromatography [eluent: stepwise gradient of methanol (0-3%) in dichloromethane to give 12 (5.3 g, 66%) which was crystallized from acetonitrile: mp=182-183° C.; 1H-NMR (DMSO-d6): δ 11.35 (br s, 1H, NH), 8.0-7.5 (m, 11H, 2 C6H5CO, H-6), 6.23 (br s, 1H, OH-2′), 6.15 (d, 1H, H-1′, J1′-2′=4 Hz), 5.54 (d, 1H, H-5, J5-6=8.1 Hz), 5.37 (t, 1H, H-3′, J3′-2′=J3′-4′=2.6 Hz), 4.7-4.6 (m, 2H, H-5′ and H-5″), 4.5 (m, 1H, H-4′), 4.4 (m, 1H, H-2′); MS: FAB>0 (matrix GT) m/z 453 (M+H)+, 341 (S)+, 113 (BH2)+, 105 (C6H5CO)+; FAB<0 (matrix GT) m/z 451 (M−H)−, 121 (C6H5CO2)−, 111 (B)−; Anal. Calcd for C23H20N2O8: C, 61.06; H, 4.46; N, 6.19. Found: C, 60.83; H, 4.34; N, 6.25.
