4.1.6  9-[(2R,5R)-3-[2-({[(4R,6R)-6-(6-amino-9H-purin-9-yl)-2,2-dimethyl-tetrahydro-2H-furo[3,4-d] [1,3]dioxol-4-yl]methyl}amino)ethoxy]-4-[(tert-butyldimethylsilyl)oxy]-5-{[(tert-butyldimethylsilyl)oxy]methyl}oxolan-2-yl]-9H-purin-6-amine (20)
To a solution of 19 (210 mg, 0.20 mmol, 1.00 eq) in anhydrous DMF (10 mL) was added K2CO3 (85 mg, 0.61 mmol, 3.00 eq) and thiophenol (11 μL, 1.14 mmol, 5.70 eq). After stirring for 2 h at room temperature under argon, the reaction mixture was diluted with DCM (20 mL) and washed with 1 M aqueous NaOH solution (10 mL). The aqueous layer was extracted with DCM (3 × 20 mL) and the combined organic extracts were washed with brine, dried over Na2SO4 and concentrated under vacuum. The residue was purified by flash column chromatography (silica gel, linear gradient 0–15% MeOH in DCM) as the eluent to give 20 (129 mg, 76%) as a white solid. Rf 0.31 (8:92 MeOH/DCM). 1H NMR (500 MHz, CDCl3) δ 8.32 (s, 1H), 8.31 (s, 1H), 8.17 (s, 1H), 7.93 (s, 1H), 6.17–6.06 (m, 5H), 6.03 (d, J = 2.9 Hz, 1H), 5.39 (dd, J = 6.5 Hz, J = 3.0 Hz, 1H), 4.99–4.92 (m, 1H), 4.51–4.45 (m, 1H), 4.33–4.25 (m, 2H), 4.08 (dt, J = 5.6 Hz, J = 3.2 Hz, 1H), 3.98 (dd, J = 11.5 Hz, J = 3.6 Hz, 1H), 3.75 (dd, J = 11.6 Hz, J = 2.8 Hz, 1H), 3.71–3.63 (m, 2H), 2.95–2.69 (m, 4H), 1.61 (s, 3H), 1.37 (s, 3H), 0.92 (s, 9H), 0.88 (s, 9H), 0.13–0.04 (m, 12H). 13C NMR (125 MHz, CDCl3) δ 155.8, 155.7, 153.3, 153.1, 149.7, 149.6, 139.8, 139.4, 120.5, 120.3, 114.8, 90.7, 87.2, 85.8, 85.1, 83.9, 82.6, 82.4, 70.3, 70.13, 62.0, 51.5, 49.5, 27.4, 26.2, 25.8, 18.6, 18.2, −4.5, −4.8, −5.2, −5.3. HRMS (ESI+): m/z calcd for C37H62N11O7Si2 [M+H]+: 828.4372, found: 828.4371.