5-methylseleno-3′-t-butyldimethylsilyl-5′-O-(4,4-dimethoxytrityl)-2′-deoxycytidine (compound 2) 300 mg (MW 751.3, 0.4 mmol) was placed in a flask and dried on high vacuum for one hour before addition of anhydrous pyridine 10 ml. Benzoyl chloride 0.03 ml (0.6 mmol, 1.5 eq.) was added dropwise before the reaction solution was stirred for 1 hour. After the reaction was completed (monitored on TLC, 30% ethyl acetate in hexane, Rf=0.5), the crude product was extracted with saturated NaCl solution and ethyl acetate. The organic phase was removed and aqueous phase was extracted with ethyl acetate three times. The combined organic phase was dried over anhydrous MgSO4, the salt was filtered, and the organic solvents were evaporated under reduced pressure. The residue was purified on a silica gel column (equilibrated with hexane) and eluted with an ethyl acetate/hexane gradient (EtOAc in hexane, 10-50%). Compound 3 was isolated and dried to afford a yellow-green solid 287 mg. (yield=84%). 1H-NMR (400 MHz, DMSO) δ: 0.09 and 0.04 [2×s, 6H, Si(CH3)2], 0.86 (s, 9H, t-butyl), 2.02 (s, 3H, SeCH3), 2.18-2.20 and 2.43-2.52 (2×m, 2H, H-2′), 3.29-3.33 and 3.43-3.46 (2×m, 2H, H-5′), 3.81 (s, 6H, 2×OMe), 4.05-4.08 (m, 1H, H-3′), 4.41-4.45 (m, 1H, H-4′), 6.30-6.34 (t, J=6.4 Hz, 1H, H-1′), 6.86-7.56 (m, 18H, arom-H), 7.88 (s, 1H, NH), 8.32 (s, 1H, H-6). 13C NMR (100 MHz, DMSO) δ: −4.89 and −4.69 [Si(CH3)2], 1.03 (SeCH3), 17.95 [SiC(CH3)], 25.73 [SiC(CH3)], 41.86 (C-2′), 55.26 (OMe), 63.05 (C-5′), 72.41 (C-3′), 86.30 (C-4′), 86.77 (C-5), 87.24 (C-1′), 113.27 (arom-C), 127.07 (arom-C), 128.13 (arom-C), 128.27 (arom-C), 129.38 (arom-C), 130.09 (arom-C), 132.67 (arom-C), 133.68 (C-6), 135.51 (NHCO), 144.41 (arom-C), 144.36 (C-2), 158.69 (C-4). HR-MS (ESI-TOF, negative ion mode): molecular formula, C44H51N3O7SeSi: [M−H]−: 840.2570 (calc. 840.2583)
