Scheme 1. Synthesis of 2′-OCF₃ cytidine building block C7. Reaction conditions: (a) 1.1 equiv. TIPDSCl₂, in pyridine, room temperature, 5 h, 86%; (b) 1.2 equiv. ^tBuLi, 9.0 equiv. CS₂, 3.0 equiv. CH₃I, in THF, −75 °C to room temperature, 18 h, 77%; (c) 3.0 equiv. acetic anhydride, in pyridine, room temperature, 16 h, 90%; (c′) 1.1 equiv. benzoic anhydride, in DMF, room temperature, 16 h, 90%; (d) 5.0 equiv. NBS, in HF pyridine and CH₂Cl₂, –75 °C to 0 °C, 3 h, 11% for C5a, 16% for C5b; (e) 1.4 equiv. DMTCl, 0.4 equiv. DMAP, in pyridine, room temperature, 16 h, 76% for C6a, 86% for C6b; (f) 2.5 equiv. 2-cyanoethyl N,N-diisopropylchlorophosphoramidite, 7.5 equiv. iPr₂NEt, 0.5 equiv. 1-methylimidazole, in CH₂Cl₂, room temperature, 2 h, 74% for C7a, 92% for C7b.