8-Bromoguanosine

White Solid

To a suspension of guanosine 5 (4.0 g, 14.4 mmol) in water (100 mL) was added bromine (0.8 mL, 14.6 mmol) at such a rate that reaction mixture became colourless between each addition. The addition was stopped when the reaction mixture stayed slightly yellow due to unreacted bromine. The colourless solid was quickly filtered off, washed with 60 mL of cold water and 30 mL of cold acetone, recrystallized from 150 mL of hot water and dried in vacuo for 6 h at 60 °C. Yield 4.4 g (11.8 mmol, 85percent) of colourless, needle-shaped crystals 13, mp 198-200 °C; Typical procedure for the bromination of unprotected nucleosides: DBH (323 mg, 1.13 mmol) was added to a stirred solution of 1d (500 mg, 2.05 mmol) in DMF (5 mL). The resulting pale-yellow solution was stirred at room temperature for 20 minutes or until TLC showed absence of starting material and formation of less polar product. Volatiles were evaporated and the residue was coevaporated with MeCN. The resulting pale solid was crystallized from hot acetone to give 2d (500 mg, 75percent) as colorless crystals with data as reported.General procedure: 5'-O-Monomethoxytrityl-N2-phenoxyacetylguanosine (33, 0.138 g, 0.2 mmol) was dissolved inaqueous DMF solution (H2O:DMF 1:4, 5 mL) under stirring. SMBI (1.1 equiv., 0.051 g, 0.22 mmol)was added at r.t. and the mixture stirred. Progress of the reaction was followed by TLC. An additionalamount of the reagent (0.15 equiv., 0.007 g) was added into the reaction mixture after 1.5 h. Oncompletion of the reaction by 2 h, the reaction mixture was filtered, evaporated to dryness underreduced pressure and coevaporated with water (2 × 2 mL). The crude reaction mixture was purified bycolumn chromatography (4percent–5percent MeOH in DCM, v/v) to afford nucleoside 34 (0.148 g, 96percent) in pureform as a white solid.8-Bromo-5'-O-monomethoxytrityl-N2-phenoxyacetylguanosine (34).