Reaction Steps
The nucleoside 20 (2.8 g, 5.7 mmol) was evaporated thrice with dry pyridine and dissolved in the same. To this pre-cooled solution was added DMAP (0.69 g, 5.7 mmol) and then dropwise was added phenyl chlorothionoformate (1.15 mL, 8.55 mmol) and reaction stirred overnight. Reaction was quenched with saturated solution of NaHCO3 and extracted with dichloromethane. Organic layer was dried over MgSO4 concentrated and chromatographed over silica gel (10-30% ethyl acetate in cyclohexane, v/v) to give 21 as yellowish foam (2.1 g, 3.42 mmol, 60%). Rf=0.60 (30% ethyl acetate in cyclohexane, v/v). 1H NMR (600 MHz, CDCl3) δ: 8.32 (1H, br s, NH), 7.48 (1H, s, H6), 7.48-6.99 (15H, m), 6.4 (1H, d, JH1′,H2′=6.3 Hz, H1′), 5.94 (1H, app t, J=6.0 Hz, H2′), 5.77 (1H, m, H8′), 4.97 (2H, m, H9′ and H9″), 4.77 (1H, d, Jgem=12 Hz, CH2Bn), 4.55 (3H, d, Jgem=12 Hz, CH2Bn), 4.53 (1H, d, JH2′,H3′=5.8 Hz, H3′), 3.74 (1H, d, JH5′,H5″=9.9 Hz, H5″), 3.47 (1H, d, JH5′,H5″=9.9 Hz, H5′), 2.22 (1H, m, H7′), 2.06 (1H, m, H7″), 1.94 (1H, m, H6′), 1.66 (1H, m, H6″), 1.53 (3H, thymine CH3); 13C NMR (125.7 MHz, CDCl3) δ: 194.4 (C═S), 163.3 (C4), 153.3, 150.2 (C2), 138.1 (C8′), 137.3, 137.0, 135.6 (C6), 129.5-121.9 (aromatic), 114.8 (C9′), 111.5 (C5), 87.6 (C4′), 85.1 (C1′), 82.9 (C2′), 78.1 (C3′), 74.9 (CH2Bn), 73.7 (C5′), 73.6 (CH2Bn), 31.6 (C6′), 27.6 (C7′), 12.0 (CH3 thymine); MALDI-TOF m/z [M+H]+ found 629.09, calcd 629.73.
