Scheme 2. Synthesis of Modified 2′-Deoxyadenosine Phosphoramidite Containing a Two-Carbon Linker with Azide.

Reagents and conditions: (i) acetic anhydride, pyridine, rt overnight then 60°C for 6 h; (ii) compound 5, Cs₂CO_3, CH₃CN, 60°C, overnight; (iii) Et₃N in MeOH/H₂O, rt, 15 min; (iv) 4,4′-dimethoxytrityl chloride, pyridine, 0°C to rt, overnight; (v) N,N-diisopropylamino-2-cyanoethoxychlorophosphine (CEP-Cl), Et₃N, dry DCM, 0°C.