N-Hydroxyphthalimide → 5'-O-(t-Butyldimethylsilyl)-3'-O-Phthalimidothymidine

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To a solution of 5'-O-t-butyldimethylsilylthymidine [1, 21.36 g, 60 mmol, prepared according to the procedure of Nair, et al., Org. Prep. Procedures Int. 1990, 22, 57 in dry THF (750 ml)], triphenylphosphine (17.28 g, 66 mmol) and N-hydroxyphthalimide (10.74 g, 66 mmol) were added. The solution was cooled to 0° C. and diisopropylazodicarboxylate (15.15 g, 75 mmol) was added dropwise over a period of 3 hr while stirring under nitrogen. The reaction mixture was then stirred at room temperature for 12 hr. The solution was evaporated and the residue was dissolved in CH2 Cl2 (750 ml), extracted with sat. NaHCO3 (200 ml), and water (200 ml), dried (MgSO4), filtered and concentrated to furnish yellow oily residue. Silica gel column chromatography (100% hexanes, and then hexanes:Et2O gradient to 90% Et2O) of the residue gave compound 2 as a colorless glass (18.68 g, 62%); 1H NMR (CDCl3) δ 0.05 [2s, 6, (CH3)2 ], 0.91 [s, 9, (CH3)3 ], 2.0 (s, 3, CH3), 2.5-2.65 (m, 2, 2'CH2), 4.05-4.2 (m, 2, 5'CH2), 4.25-4.35 (m, 1, 4'H), 5.0 (m, 1, 3'H), 6.15 (m, 1'H), 8.6 (br s, 1, NH), and aromatic protons. Anal. Calcd. for C24H3 N3O7Si: C, 57.46;H, 6.23; N, 8.37. found : C, 57.20; H, 6.26; N, 8.27.

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