Triethylamine (10 ml) in 200 ml THF was injected into a solid mixture of 6.8 g (28.0 mmol) thymidine and 10.2 g (28.6 mmol) 4,4'-dimethoxytrityl chloride under nitrogen with stirring. The solution was stirred at room temperature for 2 hours. After completion of the reaction, 10 ml methanol was added to consume the excess DMTrCl. The mixture was stirred for 5 minutes and the solvent removed by rotary evaporation. The residue was dissolved in 250 ml of ethyl acetate and the solution was washed with saturated NaHCO3 and dried over MgSO4. The solid was recrystallized from ethyl acetate/hexane to obtain 13.0 g 5'-protected thymidine 16 (85.6%).: m.p. 124°-126° C.; 1H NMR (200 MHz, CDCl3) δ8.97 (s, 1H, NH), 7.60 (m, 1H, H-6), 6.72-7.42 (m, 13H, Ph), 6.42 (m, 1H, H-1'), 4.56 (m, 1H, H-3'), 4.05 (m, 1H, H-4'), 3.78 (s, 6H, OMe2), 3.41 (m, 2H, H-5'), 2.62 (m, 1H, OH), 2.46 (m, 2H, H-2'), 1.46 (s, 3H, Me); 13C NMR (50 MHz, CDCl3) δ163.0 (C-4), 157.6, 149.8 (C-2), 143.5, 135.0 (C-6), 134.7, 134.6, 129.4, 127.4, 127.3, 126.5, 112.8, 110.9 (C-5), 86.7 (C-4'), 86.2, 84.7 (C-1'), 72.5 (C-3'), 63.8 (C-5'), 55.5 (OCH3), 41.3 (C-2'), 12.5 (CH3).
