Scheme 7.

Linker attachment and triphosphate synthesis with all four 3′-O-(2-cyanoethyl) modified nucleosides 10, 13, 16, and 21. Reagents and conditions: a) dry DMF, triethylamine, CuI (0.2 equiv), [Pd(PPh₃)₄] (0.1 equiv), alkyne 23 (1.2–2.0 equiv), rt, 3.5–5 h; b) 1. dry pyridine/dry dioxane (1:30, 2-chloro-4H-1,2,3-benzodioxaphosphorin-4-one, rt, 15 min; 2. dry DMF, bis(tributylammonium)pyrophosphate, tributylamine, rt, 20 min; 2. iodine in pyridine/water (2 %, 98:2), rt, 20 min; 4. 5 % Na₂SO₃ solution; c) TFA (40 equiv) in water, rt, 5–6 h. TFA=trifluoroacetic acid.