Compound 6 (0.62 g, 1.24 mmol) in dry pyridine (25 mL) was treated with 4-monomethoxytrityl chloride (0.46 g, 1.49 mmol) at room temperature for 16 h. After addition of methanol (5 mL), the mixture was concentrated to dryness. The residue was dissolved in dichloromethane (60 mL) and washed successively with water (40 mL), a saturated aqueous solution of NaHCO3 (40 mL) and water (3×40 mL). The organic layer was dried, filtered, concentrated and co-evaporated with toluene and methanol. The residue was purified by silica gel column chromatography (0-10% MeOH in dichloromethane) to give 7 (0.71 g, 72%) as a foam: 1H NMR (DMSO-d6): δ 2.21 (d, 1H, H-2′ J2′,2″=14.3 Hz), 2.6-2.7 (m, 1H, H-2″), 3.1-3.3 (2m, 2H, H-5′ and H-5″), 3.64 and 3.65 (2s, 6H, 2×OCH3), 4.1-4.2 (m, 1H, H-4′), 4.2-4.3 (m, 1H, H-3′), 5.68 (d, 1H, OH-3′, JH,OH=5.2 Hz), 6.24 (d, 1H, H-1′, J1′,2″=7.0 Hz), 6.7-6.8 and 7.1-7.3 (2m, 29H, 2 MMTr and NH-6) 7.83 and 8.21 (2s, 2H, H-2 and H-8); ms: matrix G/T, (FAB+) m/z 796 [M+H]+, 408 [BH2]+, (FAB−) m/z 794 [M−H]−, 406 [B]−; UV (95% ethanol): λmax 275 nm (ε 30900), λmin 246 nm (ε 12800); [α]D20=+14 (c 1.03, DMSO).
