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To a mixture of 2-[di-(t-butoxycarbonyl)]amino-9-hydroxy-6-methoxypurine (154 mg, 404 μmol), diisopropyl (E)-4-hydroxybut-1-enylphosphonate (89 mg, 404 μmol) and triphenylphosphine (159 mg, 606 μmol) in dry tetrahydrofuran (4 ml) stirred at 0° C. was added diethyl azodicarboxylate (105 mg, 606 μmol). The mixture was allowed to warm to room temperature and stirred for 2.3 hr. The solvent was removed and the residue was purified by column chromatography on silica gel eluting with ethyl acetate-methanol (20:1) to give (E)-2-[di-(t-butoxycarbonyl)]amino-9-[4-(diisopropoxyphosphoryl)but-3-enyloxy]-6-methoxypurine as a colourless gum (150 mg, 62%); λmax (EtOH) 255 (12,300)nm; υmax (KBr) 3440, 3220, 2975, 1790, 1600, 1370, 1280 and 1100 cm-1 ; δH [(CD3)2SO] 1.22 (12H, dd, J 6 Hz and 7 Hz, CH(CH3)2), 1.40 (18H, S, C(CH3)3), 2.70 (2H, m, CH2), 4.08 (3H, s, CH3O), 4.53 (4H, m, CH2O and CH(CH3)2), 5.98 (1H, dd, J 17 Hz and 19 Hz, PCH=CH), 6.25 (1H, ddt, J 6 Hz, 17 Hz and 22 Hz, PCH=CH), 8.71 (1H, s, 8-H) (Found: M+ 599.2724. C26H42N5O9P requires M+ 599.2720).

1. temperatureto warm to room temperature
2. stirringstirred for 2.3 hr
3. customThe solvent was removed
4. customthe residue was purified by column chromatography on silica gel eluting with ethyl acetate-methanol (20:1)