Scheme 1.

The general synthesis procedure of 2′-F,4′-C-OMe–araU phosphoramidite 8. Reagents and conditions: (i) imidazole, Ph₃P, I₂, tetrahydrofuran (THF), 0 °C–room temperature (rt), 4 h, 78%; (ii) NaOMe–MeOH, reflux, 3 h, 85%; (iii) methanol, PbCO₃, I₂, THF (dry), 0 °C–rt, 3 h, 90%; (iv) benzoyl chloride (BzCl), pyridine (dry), dichloromethane (DCM), 0 °C–rt, 3 h, 95%; (v) (a) m-chloroperbenzoic acid (m-CPBA), DCM/H₂O, 0 °C–rt, 3 h, (b) NH₃–MeOH, rt, 12 h, 64% two steps; (vi) dimethoxytrityl chloride (DMTrCl), pyridine (dry), rt, 12 h, 81%; (vii) 2-cyanoethyl N,N,N′,N′-tetraisopropylphosphoramidite, 1H-tetrazole, DCM (dry), room temperature, 3 h, 74%.