4.1.9  Methyl (2S)-4-({[(4R,6R)-6-(6-amino-9H-purin-9-yl)-2,2-dimethyl-tetrahydro-2H-furo[3,4-d] [1,3]dioxol-4-yl]methyl}(2-{[(2R,5R)-2-(6-amino-9H-purin-9-yl)-4-[(tert-butyldimethylsilyl)oxy]-5-{[(tert-butyldimethylsilyl)oxy]methyl}oxolan-3-yl]oxy}ethyl)amino)-2-{bis[(tert-butoxy)carbonyl]amino}butanoate (22)
To a solution of 20 (250 mg, 0.30 mmol, 1.00 eq) in anhydrous DCM (9 mL) was added aldehyde 21 (120 mg, 0.36 mmol, 1.20 eq) in anhydrous DCM (9 mL) and acetic acid (20 μL, 0.36 mmol, 1.20 eq). After stirring for 20 min at room temperature under argon, sodium triacetoxyborohydride (96 mg, 0.45 mmol, 1.5 eq) was added. After stirring for 2.5 h, the reaction mixture was diluted with DCM (30 mL) and washed with saturated aqueous NaHCO3 (30 mL). The aqueous layer was extracted with DCM (3 × 30 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 0–8% MeOH in DCM) to give 22 (321 mg, 93%) as a white solid. Rf 0.71 (MeOH/DCM 5:95). 1H NMR (600 MHz, CDCl3) δ 8.33 (s, 1H), 8.30 (s, 1H), 8.17 (s, 1H), 7.93 (s, 1H), 6.09 (d, J = 3.3 Hz, 1H), 6.01 (d, J = 2.4 Hz, 1H), 5.97 (m, 4H), 5.42 (dd, J = 6.5 Hz, J = 2.4 Hz, 1H), 4.93–4.86 (m, 2H), 4.54–4.49 (m, 1H), 4.33 (td, J = 6.8 Hz, J = 3.3 Hz, 1H), 4.29–4.24 (m, 1H), 4.08 (dt, J = 6.2 Hz, J = 3.2 Hz, 1H), 3.99 (dd, J = 11.5 Hz, J = 3.7 Hz, 1H), 3.78–3.63 (m, 6H), 2.84–2.74 (m, 3H), 2.71–2.51 (m, 3H), 2.33–2.23 (m, 1H), 1.91–1.82 (m, 1H), 1.58 (s, 3H), 1.46 (s, 18H), 1.36 (s, 3H), 0.91 (s, 9H), 0.90 (s, 9H), 0.10–0.05 (m, 12H). 13C NMR (125 MHz, CDCl3) δ 171.6, 155.8, 155.7, 153.3, 153.2, 152.3, 149.7, 149.5, 140.1, 139.8, 120.5, 120.4, 114.5, 91.1, 87.4, 85.45, 84.8, 84.1, 83.3, 83.3, 82.5, 69.9, 69.1, 61.9, 56.3, 56.3, 53.7, 52.4, 52.1, 28.1, 28.1, 27.3, 26.2, 26.0, 25.6, 18.7, 18.3, −4.3, −4.6, −5.2, −5.2. HRMS (ESI+): m/z calcd for C52H87N12O13Si2 [M+H]+: 1143.6049, found: 1143.6043.