To a mixture of N4 -acetylcytosine (0.18 g, 1.19 mmol) and compound 2 (0.50 g, 0.99 mmol) in anhydrous MeCN (30 ml) were successively added HMDS (0.17 ml, 0.79 mmol), ClSiMe3 (0.10 ml, 0.79 mmol) and SnCl4 (0.14 ml, 1.19 mmol). The resulting clear solution was refluxed for one hour. Then the solvent was evaporated and the residue dissolved in EtOAc (100 ml), washed with NaHCO3 and H2O. After evaporation of the solvent the residue was purified on a silica gel column using EtOAc/petroleum ether (70%) to give pure 7 (0.41 g, 70%) as a white foam.
