2,4,6-Triisopropylbenzenesulfonyl Chloride

Experimental procedure: 2,4,6-Triisopropylbenzenesulfonyl chloride (144 mg, 0.48 mmol) is added in three portions every 30 min to a mixture of 1 (100 mg, 0.31 mmol), triethylamine (0.13 mL, 0.93 mmol), DMAP (5 mg) and 2 (104 mg, 0.63 mmol) in chloroform (ethanol free, 5 mL) at reflux temperature. After heating at reflux for 4 h, the reaction mixture is cooled to room temperature, poured into 1 mol/L HCl, and extracted with dichloromethane. The extracts are successively washed with dilute aq. NaHCO₃, water and brine, and then dried over Na₂SO₄. Evaporation of the solvent affords a residue, which is purified over silica gel column chromatography (dichloromethane : acetone = 5:1 - 7:3) to yield a residue, which is crystallized from a mixture of acetone and ether to afford 147 mg (100% yield) of the product 3 as crystals.

Typical procedure (protection of the O⁶-position of guanosine): To a suspension of 3’,5’-O-di-tert-butylsilanediylguanosine (2.12 g) in dry CH₂Cl₂ are added DMAP (0.122 g), Et₃N (4.18 mL), and 2,4,6-triisopropylbenzenesulfonyl chloride (4.68 g) under argon. The reaction mixture is stirred at room temperature for 3 h and then the solvent is removed under reduced pressure. The residue is dissolved in ethyl acetate and the solution is washed with saturated NaHCO₃ solution and brine, and dried over Na₂SO₄. After filtration, the solvent is evaporated in vacuo. The residue is purified by column chromatography (eluent: 50-60 % CHCl₃ in hexane) to give the O⁶-protected guanosine as a colorless foam (3.34 g, 97%).