Reaction SMILES: [CH3:1][Se:2][C:3]1[C:4](=O)[NH:5][C:6](=[O:47])[N:7]([CH:46]=1)[C@@H:8]1[O:38][C@H:12]([CH2:13][O:14][C:15]([C:32]2[CH:37]=[CH:36][CH:35]=[CH:34][CH:33]=2)([C:26]2[CH:31]=[CH:30][CH:29]=[CH:28][CH:27]=2)[C:16]2[CH:21]=[CH:20][C:19]([O:24][CH3:25])([O:22][CH3:23])[CH2:18][CH:17]=2)[C@@:10]([Si:39]([C:42]([CH3:45])([CH3:44])[CH3:43])([CH3:41])[CH3:40])([OH:11])[CH2:9]1.C([N:52](C(C)C)CC)(C)C.C(C1C=CC(S(Cl)(=O)=O)=C(C(C)C)C=1C(C)C)(C)C.[OH-].[NH4+]>C(#N)C.CN(C)C1C=CN=CC=1.C(Cl)Cl.CO.C(OCC)(=O)C.CCCCCC>[CH3:1][Se:2][C:3]1[C:4]([NH2:52])=[N:5][C:6](=[O:47])[N:7]([CH:46]=1)[C@@H:8]1[O:38][C@H:12]([CH2:13][O:14][C:15]([C:26]2[CH:27]=[CH:28][CH:29]=[CH:30][CH:31]=2)([C:32]2[CH:33]=[CH:34][CH:35]=[CH:36][CH:37]=2)[C:16]2[CH:21]=[CH:20][C:19]([O:22][CH3:23])([O:24][CH3:25])[CH2:18][CH:17]=2)[C@@:10]([Si:39]([C:42]([CH3:43])([CH3:45])[CH3:44])([CH3:40])[CH3:41])([OH:11])[CH2:9]1
Reactant: CCN(C(C)C)C(C)C
Reactant: COC1(OC)C=CC(C(OC[C@H]2O[C@@H](n3cc([Se]C)c(=O)[nH]c3=O)C[C@@]2(O)[Si](C)(C)C(C)(C)C)(c2ccccc2)c2ccccc2)=CC1
Agent: CC(C)c1ccc(S(=O)(=O)Cl)c(C(C)C)c1C(C)C
Agent: CN(C)c1ccncc1
Agent: [NH4+]
Solvent: CC#N
Solvent: ClCCl
Solvent: CCOC(C)=O
Product: COC1(OC)C=CC(C(OC[C@H]2O[C@@H](n3cc([Se]C)c(N)nc3=O)C[C@@]2(O)[Si](C)(C)C(C)(C)C)(c2ccccc2)c2ccccc2)=CC1
Reaction time: 16 h
Yield: 64.9 %

Procedure:
After 5-methylseleno-3′-t-butyldimethylsilyl-5′-O-(4,4-dimethoxytrityl)-2′-deoxyuridine 480 mg (MW 752.2, 0.64 mmol) was dissolved in acetonitrile 10 ml, diisopropylethyl amine (0.67 ml, 3.19 mmol, 5 eq.), triisopropylbenzenesulfonyl chloride (576 mg, 1.59 mmol, 2.5 eq.) and p-dimethylamino pyridine (96 mg, 0.64 mmol, 1 eq.) were added sequentially. The reaction mixture was stirred for 16 hours and monitored on TLC (ethyl acetate/hexane, 3:7). After completion, excessive concentrated ammonium hydroxide solution was added dropwise and the reaction was stirred under room temperature for 1 hour. After the reaction was completed (monitored on TLC, 5% MeOH in CH2Cl2, Rf=5), the organic solvent and ammonium hydroxide were evaporated under reduced pressure. A silica gel column was equilibrated with CH2Cl2 before the crude product was loaded and purified on the column. The product was eluded by MeOH in CH2Cl2 (1-7%). The fractions containing the product were combined, evaporated and dried on high vacuum overnight to yield yellow solid 307 mg. (yield=64%). 1H-NMR (400 MHz, DMSO) δ: 0.05 (s, 6H, SiCH3), 0.87 (s, 9H, t-butyl), 1.95 (s, 3H, SeCH3), 2.18-2.25 and 2.58-2.62 (2×m, 2H, H-2′), 3.30-3.33 and 3.46-3.49 (2×m, 2H, H-5′), 3.84 (s, 6H, 2×OMe), 4.03-4.08 (m, 1H, H-3′), 4.42-4.46 (m, 1H, H-4′), 6.11 (s, 1H, NH2), 6.28-631 (t, J=5.6 Hz, 1H, H-1′), 6.50 (s, 1H, NH2), 6.86-7.48 (m, 13H, arom-H), 8.30 (s, 1H, H-6). 13C NMR (100 MHz, DMSO) δ: −4.86 and −4.69 [Si(CH3)2], 7.31 (SeCH3), 17.96 [SiC(CH3)], 25.74 [SiC(CH3)], 41.74 (C-2′), 55.26 (OMe), 62.88 (C-5′), 72.23 (C-3′), 85.55 (C-4′), 87.05 (arom-C), 87.16 (C-1′), 103.45 (C-5), 113.23 (arom-C), 126.94 Carom-C), 128.15 (arom-C), 130.11 (arom-C), 135.50 (arom-C), 141.74 (C-6), 144.41 (arom-C), 149.89 (C-2), 155.46 (arom-C), 158.66 (C-4), 161.46 (arom-C), 165.55 (arom-C). HR-MS (ESI-TOF, positive ion mode): molecular formula, C32H47N3O6SeSi: [M+H]−: 738.2468 (calc. 738.2478).