To a mixture of hexamethyldisilazane (50 mL, 239.77 mmol) and chlorotrimethylsilane (1.0 mL, 21.43 mmol) was added pyridine-4-one-3-carboxamide (1.38 g, 10.0 mmol) (Prepared by the procedure reported: W. C. J. Ross, J. Chem. Soc., C, 1816, (1966); W. Herz and D. R. K. Murty, J. Org. Chem., 26, 122, 1961). The mixture was refluxed with stirring for 2 h and then evaporated to dryness under vacuum and further dried under high vacuum for 2 h at 60° C. The dry gummy residue was suspended in freshly distilled 1,2-dichlorethane (50 mL) and to this suspension was added 1-O-acetyl-2,3,5-tri-O-benzoyl-L-ribofuranose(5.06 g, 10.0 mmol) and freshly distilled SnCl4 (1.0 mL, 8.52 mmol). The reaction mixture was refluxed for 1.5 h, cooled and diluted with CH2Cl2 (100 mL) and sat. aqueous NaHCO3 (25 mL). The mixture was filtered through celite and the bed was washed with CH2Cl2 (20 mL). The organic phase was separated, washed with water until the washings are neutral, dried over anhydrous sodium sulfate. The organic extract was evaporated to dryness to give a gummy residue. The residue was purified by flash chromatography over silica gel using CH2Cl2→EtOAc as the eluent. Pure fractions were pooled and concentrated to provide 0.50 g (9%) of 33 as white foam: 1H NMR (CDCl3): δ4.94 (m, 3H, C4′H and C5′H), 6.12 (m, 1H), 6.20 (m, 1H), 6.32 (d, 1H) and 7.20-8.30 (m, 20H).
