To a solution of compound 4 (0.88 g, 1.92 mmol) in dry pyridine (40 mL) was added 4-monomethoxytrityl chloride (1.18 g, 3.84 mmol). The mixture was stirred at 60° C. for 24 h. After addition of methanol (5 mL), the solution was concentrated to dryness, the residue was dissolved in dichloromethane (50 mL) and washed successively with water (30 mL), aqueous saturated NaHCO3 (30 mL) and water (30 mL). The organic layer was dried, filtered, concentrated and co-evaporated with toluene to give pure 5 (1.01 g, 72%) as a foam: 1H NMR (CDCl3): δ 2.9-3.0 (m, 2H, H-2′ and H-2″), 3.62 (s, 3H, OCH3), 4.6-4.8 (m, 3H, H-4′, H-5′ and H-5″), 5.85 (pt, 1H, H-3′), 6.44 (dd, 1H, H-1′, J1′,2′=3.1 Hz, J1′,2″=7.3 Hz), 6.9 (br s, 1H, NH-6), 6.7-6.8 and 7.2-7.4 (2m, 24H, 2 benzoyls and MMTr), 7.97 and 8.13 (2s, 2H, H-2 and H-8); ms: matrix G/T, (FAB+) m/z 732 [M+H]+, (FAB−) m/z 730 [M−H]−; UV (95% ethanol): λmax 274 nm (ε 12100), 225 nm (ε 24200), λmin 250 nm (ε 5900); [α]D20=−16 (c 1.12, DMSO).
