Pyridine-3-boronic acid (0.59 g, 0.48 mmol) and Cs2CO3 (3.11 g, 9.57 mmol) were added to a solution of 2′,3′,5′-tris-O-(tert-butyldimethylsilyl)-8-bromoadenosine (13) (2.20 g, 3.19 mmol) and Pd(PPh3)4 (0.37 g, 0.32 mmol) in dioxane (50 ml) under argon. Water (10 ml) was added to dissolve all Cs2CO3. The solution was heated under reflux overnight. EtOAc (200 ml) was added to the cold reaction mixture, the resultant solution shaken with water (100 ml), the organic solution dried (MgSO4), evaporated and the residual material subjected to flash chromatography on silica gel using EtOAc:MeOH 99:1; yield 1.76 g (80%). HRESI: M+H 687.3900. Calc. for C33H58N6O4Si3+H, 687.3900. 1H NMR: (300 MHz, CDCl3); δ −0.41/−0.12/0.01/0.02/0.09/0.11 (18H, s, 3×Si(CH3)2), 0.67/0.85/0.86 (27H, s, 3×SiC(CH3)3), 3.71-4.12 (3H, m, H-5′, H-3′ or H-4′), 4.53-4.55 (1H, m, H-4′ or H-3′), 5.61-5.65 (1H, m, H-2′), 5.81 (1H, d, J 6.3 Hz, H-1′), 6.02 (2H, s, NH2), 7.38-7.42 (1H, m, H-5′), 8.09-8.12 (1H, m, H-4″), 8.30 (1H, s, H-2), 8.73-8.75 (1H, m, H-6″), 9.04-9.05 (1H, m, H-2″). 13C NMR: (75 MHz, CDCl3); δ −5.5/−5.4/−5.3/−4.7/−4.6/−4.5 (3×Si(CH3)2), 17.2/18.0/18.3 (3×SiC), 25.6/25.8/25.8 (3×SiC(CH3)3), 62.5 (C-5′), 71.7/72.4 (C-2′, C-3′), 85.5/88.9 (C-4′, C-1′), 120.1/123.1/125.7/136.9/149.9/150.4/151.0/151.1/152.6/155.6 (C-5, C-5″, C-3″, C-4″, C-6, C-8, C-6″, C-2″, C-2, C-4).
