A Rapid, High-Yield Method for 5′-Hydroxyl Protection in Very Reactive and Amino Group Modified Nucleosides Using Dimethoxytrityl Tetrafluoroborate

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• Compound 7a was prepared as follows: aminomethylpyrene19 (0.119 g, 0.515 mmol) and 1a (0.153 g, 0.567 mmol) were heated with Et3N (0.21 mL, 1.51 mmol) in DMF (2 mL) at 50–55°C for 16 h. Another portion of 1a (27.7 mg, 0.103 mmol) was added and the heating was continued for 5 h. The mixture was cooled, diluted with EtOAc, washed with water and brine. The aqueous layers were extracted once with EtOAc. The combined organic layer was dried over Na2SO4 and evaporated. The product was dissolved in 2:1 EtOAc-MeOH with a small volume of added DMF, slurried with silica gel and evaporated to dryness. The product impregnated silica gel was loaded onto a short dry-packed silica gel column. Sequential elution with EtOAc and 5% MeOH in EtOAc afforded the adduct as a colorless powder which was washed with Et2O (0.156 g, 65%). TLC (CH2Cl2/MeOH 9:l) Rf= 0.39
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• The sodium bicarbonate wash during workup is essential to remove traces of citric acid from the organic layer. In one instance, during the preparation of 2b, the bicarbonate wash was intentionally omitted. Subsequently, significant product decomposition occurred during evaporation of the organic layer as evidenced by appearance of the characteristic red color of the DMT cation.
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• Pretreatment of the catalyst was performed as follows: 5% Pd on C (50 mg) was washed with 2% 2,6-lutidine in MeOH (2 × 2.5 mL). Traces of 2,6-lutidine were removed by washing with THF and the catalyst was dried in vacuo.
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