Compound 9.8 (430 mg, 0.907 mmol) and adenine (135 mg, 1.0 mmol) was dissolved in 10 mL anhydrous CH3CN. The mixture was treated with SnCl4 (0.234 mL, 2.0 mmol) and stirred at r.t. for 20 h. The mixture was cooled to 0° C., treated with NaHCO3 (2 g), water (2 mL), and stirred for 2 h. The mixture was diluted with CH3CN, filtered, and concentrated under reduced pressure. The residue was subjected to a silica gel column chromatography eluting with EtOAc to 20% MeOH in EtOAc to yield compound 12.1 (210 mg, 48% yield).
