Scheme 3. Synthesis of R-and S-dAzep nucleosides. Reagents and conditions: (i) NH₂OH·HCl, NaOH, H₂O, MeOH, r.t., 10 min; (ii) tosyl chloride, 4N NaOH, acetone/H₂O r.t., 2 h then 4 N HCl, r.t., overnight; (iii) Hoffer's chlorosugar, K-^tBuO, 1,4-dioxane, r.t., 2 h; (iv) NaBH₄, MeOH, 0 °C → r.t., overnight; (v) benzyl chloride, Et₃N, DMF, 0 °C → r.t., overnight; (vi) 1 N HCl, THF, r.t., overnight; (vii) 10% Pd/C, H₂, EtOH, r.t., 6 h; (viii) 30% aq. NH₃, MeOH, r.t., 3 days; (ix) TBDPS chloride, imidazole, DCM, 0 °C → r.t., overnight; (x) 4,4′-dimethoxytrityl chloride (DMTCl), pyridine, 0 °C → r.t., overnight; and (xi) N,N-diisopropylamino-2-cyanoethoxychlorophosphine, Et₃N, DCM, r.t., 20 min.