Scheme 1. Synthesis of the Solid Support 3 for 3′-End 2′-O-(2-azidoethyl) Modified RNA.

Reaction conditions: (a) 5 equiv HOCH₂CH₂N₃, 2.5 equiv BF₃·OEt₂ in dimethylacetamide, 120 °C, 16 h, 55%; (b) 1.1 equiv DMT-Cl, in pyridine, 16 h, RT, 75%; (c) 3.5 equiv PfpOOC(CH₂)₄COOPfp, 1.2 equiv DMAP, in DMF/pyridine (1:1), room temperature, 1 h, 47%; (d) 3 equiv (w/w) amino-functionalized support (GE Healthcare, Custom Primer Support 200 Amino), 2 equiv pyridine, in DMF, room temperature, 48 h, loading: 60 mmol g^–1.