Sodium borohydride (625 mg, 16.5 mmol, 1.82 eq) was added to a solution of 2′,3′,5′-tri-O-acetyl-2-N-(p-methylphenylthiomethyl)guanosine 2 (5.0 g, 9.2 mmol, 1.0 eq) in dimethyl sulfoxide (25 mL). The mixture was heated at 100° C. for 1 h and then cooled to room temperature. Reaction products were neutralized with 1 M aqueous potassium dihydrogen phosphate solution. The precipitate obtained was collected by filtration and further washed with acetone. The residue was chromatographed on a column of silica gel to give 3 in 75% yield (2.93 g, 6.93 mmol). TLC (CH2Cl2:MeOH, 9:1 v/v): Rf=0.45; 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 1.92 (s, 3H), 2.03 (s, 3H), 2.04 (s, 3H), 2.8 (d, J=4 Hz, 3H), 4.11-4.15 (dd, J=6, 11.6 Hz, 1H), 4.23-4.25 (m, 1H), 4.33-4.37 (dd, J=4, 11.6 Hz, 1H), 5.7 (t, 1H), 5.8 (t, 1H), 5.9 (d, J=4 Hz, 1H), 6.4 (q, 1H), 7.79 (s, 1H), 10.79 (s, 1H); 13C NMR (DMSO-d6, 400 MHz) δ (ppm): 20.2, 20.3, 27.7, 40.4, 62.7, 69.7, 72.1, 78.5, 86.4, 117.3, 136.9, 150.2, 153.3, 156.6, 169.3, 169.4, 169.9; ESI-MS (ES+) calculated for C17H21N5O8 423.139, found 424.2 (M+H+), 446.2 (M+Na+), 847.3 (M+H+), 869.3 (2M+Na+).
