Scheme 3.

Synthesis of Nucleoside-5′-Carboxamides and Deprotection of the 2′,3′-Position^a

^a Reagents and conditions: (a) (i) coupling reagent (PyBOP, HCTU, or HBTU; see Experimental Section), diisopropylethylamine, DMF, room temp, 12 h, (ii) silica gel flash chromatography, CH₂Cl₂/MeOH = 40:1; (b) (i) 3–5% TFA, CH₂Cl₂, H₂O, 2 h, room temp, (ii) C-18 RP-HPLC, gradient of MeOH/H₂O = 25:75 to 100:0.