To a solution of 2′,3′,5′-tri-O-acetyl-2-N-methylguanosine 3 (3.0 g, 7.1 mmol, 1.0 eq) in dry pyridine (50 mL) were added diphenylcarbamoyl chloride (3.45 g, 15 mmol, 2.1 eq) and diisopropylethylamine (1.9 mL, 11.4 mmol, 1.6 eq). The dark brown reaction mixture was then stirred at room temperature for 1 h to obtain 2′,3′,5′-tri-O-acetyl-6-O-(diphenylcarbamoyl)-2-N-methylguanosine 4. TLC showed complete conversion at this stage of the reaction. The reaction mixture was then diluted with pyridine (15 mL) and EtOH (30 mL). To this solution cooled at 0° C. was added 2 M NaOH (25 mL), which was precooled at 0° C. The reaction mixture was stirred for 10 min at 0° C. Acetic acid (c.a. 5 mL) was then added to neutralize the solution. Extraction with CH2Cl2 followed by chromatography on silica gel afforded 5 in 55% yield (1.92 g). TLC (CH2Cl2:MeOH, 9:1 v/v): Rf=0.5; 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 2.81 (d, 3H), 3.51-3.56 (m, 1H), 3.61-3.66 (m, 1H), 3.90 (m, 1H), 4.02 (q, 1H), 4.15 (br.d, 1H), 4.62 (br.s, 1H), 4.99 (br.s, 1H), 5.21 (d, J=4.8 Hz, 1H), 5.48 (d, J=5.6 Hz, 1H), 5.82 (d, J=5.6 Hz, 1H), 7.28-7.32 (m, 4H), 7.4-7.44 (m, 6H), 8.19 (s, 1H); 13C NMR (DMSO-d6, 400 MHz) δ (ppm): 28.3, 61.5, 70.5, 73.1, 85.5, 116.4, 127.1, 129.4, 140.9, 141.8, 150.4, 155.6, 155.9, 159.6; ESI-MS (ES+) calculated for C24H24N6O6 492.1757, found 493.2 (M+H+), 515.2 (2M+Na+), 1007.4 (2M+Na+).
