Scheme 1. Preparation of dinucleotide phosphoramidites 5a–d. Reagents and conditions: (a) (i) KSeCN (1.5 eq.), MeCN, MW power 20 W, 100 °C, 90 min; (ii) BnBr (0.6 eq.), MeOH, rt, 60 min. (b) 3a–e (1.5 eq.), 2,6-lutidine (5 eq.), MeCN, rt, 30–60 min. (c) (i) ClP(OCE)NiPr₂ (2 eq.), iPr₂NEt (3 eq.), DCM, rt, 45 min; (ii) MeOH (1.5 eq.), rt, 15 min. *in MeCN (4d) or DCM, ‡in DMSO-d₆. Key. Base^PG. Ade^PAc: 9-(N⁶-phenoxyacetyladeninyl). Cyt^Ac: 1-(N⁴-acetylcytosinyl). Gua^iBu: 9-(N²-isobutyrylguaninyl). Thy: 1-thyminyl. DMTr: 4,4′-dimethoxytrityl. MMTr: 4-methoxytrityl CE: 2-cyanoethyl. p: P(O)OMe. Ts: p-toluenesulfonyl.