4.1.35  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}[(4-chloro-3-nitrophenyl)methyl]amino)ethoxy]-4-[(tert-butyldimethylsilyl)oxy]-5-{[(tert-butyldimethylsilyl)oxy]methyl}oxolan-2-yl]-9H-purin-6-amine (35)
To a solution of 20 (100 mg, 0.12 mmol, 1.00 eq) in anhydrous DCE (3.5 mL) was added a solution of 4-chloro-3-nitrobenazaldehyde (27 mg, 0.18 mmol, 1.50 eq) in anhydrous DCE (3.5 mL) and acetic acid (10 μL, 0.18 mmol, 1.50 eq). After stirring for 20 min at 40 °C under argon, sodium triacetoxyborohydride (38 mg, 0.18 mmol, 1.50 eq) was added. After stirring at 40 °C for 16 h, the solution was diluted with DCM (30 mL) and washed with saturated aqueous NaHCO3. The aqueous layer was extracted with DCM (3 × 30 mL) and the combined organic extracts were washed with brine (60 mL), dried over Na2SO4 and concentrated under vacuum. The residue was purified by flash column chromatography (silica gel, linear gradient 0–5% MeOH in DCM) to give 35 as a white solid (105 mg, 87%). Rf 0.71 (MeOH/DCM 5:95). 1H NMR (500 MHz, CDCl3) δ 8.27 (s, 1H), 8.18 (s, 1H), 8.15 (s, 1H), 7.82 (br. s, 2H), 7.29–7.27 (m, 2H), 6.22 (br. s, 2H), 6.20 (br. s, 2H), 6.10 (d, J = 3.6 Hz, 1H), 5.99 (d, J = 2.1 Hz, 1H), 5.37 (dd, J = 6.5 Hz, J = 2.1 Hz, 1H), 4.91 (dd, J = 6.5 Hz, J = 3.7 Hz, 1H), 4.48 (t, J = 5.1 Hz, 1H), 4.30 (td, J = 6.7 Hz, J = 3.6 Hz, 1H), 4.22 (t, J = 4.2 Hz, 1H), 4.07 (dt, J = 5.8 Hz, J = 3.0 Hz, 1H), 3.99 (dd, J = 11.5 Hz, J = 3.4 Hz, 1H), 3.81–3.72 (m, 2H), 3.71–3.66 (m, 1H), 3.65–3.61 (m, 2H), 2.90–2.70 (m, 4H), 1.57 (s, 3H), 1.36 (s, 3H), 0.92 (s, 9H), 0.86 (s, 9H), 0.14 – - 0.02 (m, 12H). 13C NMR (125 MHz, CDCl3) δ 155.8, 155.7, 153.1, 153.0, 149.6, 149.0, 147.8, 140.6, 140.1, 139.3, 133.0, 131.4, 125.6, 125.2, 120.3, 120.1, 114.6, 90.8, 87.0, 85.7, 84.9, 84.0, 83.4, 82.8, 69.6, 69.3, 61.8, 58.2, 56.4, 53.5, 27.2, 26.2, 25.8, 25.4, 18.6, 18.2, −4.5, −4.7, −5.2, −5.3. HRMS (ESI+): m/z calcd for C44H66N12O9Si2Cl [M+H]+: 997.4297, found: 997.4327.