Reaction Steps
A mixture of compound 17 (0.95 g, 3.03 mmol) and 4,4′-dimethoxytrityl chloride (1.54 g, 4.77 mmol) was dissolved in anhydrous pyridine (20 cm3) and stirred for 4 h at room temperature. The reaction mixture was evaporated to give an oily residue which was co-evaporated with toluene (2×20 cm3). Dichloromethane (50 cm3) and a saturated aqueous solution of sodium hydrogencarbonate (50 cm3) were added, the organic phase was separated and evaporated, and the residue purified by silica gel HPLC (the residue was dissolved in the minimum amount of dichloromethane containing 0.5% triethylamine (v/v) and applied to the column equilibrated by the same solvent. The column was washed (ethylacetate:petroleum ether:triethylamine; 15:84.5:0.5 (v/v/v, 1000 cm3) and the product was eluted in a gradient of methanol (0-2%) in dichloromethane containing 0.5% of triethylamine (v/v/v) to give compound 18 (1.71 g, 92.8%) as white solid material. δH (CDCl3) 7.51-7.17 (10H, m, DMT, 6-H), 6.79-6.85 (4H, m, DMT), 6.04 (1H, s, 1′-H), 4.12-3.98 (3H, m, 5′-Ha, 5′-Hb, 4′-H), 3.77 (6H, s, CH3-DMT), 3.49 (3H, s, CH3—O), 3.45-3.32 (2H, m, 2″-Ha, 2″-Hb), 2.11-2.01 (1H, m,1″-Ha), 1.94-1.87 (1H, m, 1″-Hb), 1.93 (3H, s, CH3). δC (CDCl3) 164.2 (C-4), 158.6, 144.7, 135.7, 130.1, 128.2, 127.9, 126.8, 113.2 (DMT), 150.7 (C-2), 137.7 (C-6), 109.8, 109.7 (C-5, C-2′), 86.5, 85.3, 85.0, 81.4 (DMT, C-1′, C-3′, C-4′), 69.2, 62.4 (C-5′, C-2″), 55.2 (CH3-DMT), 51.7 (CH3—O), 35.5 (C-1″), 12.7 (CH3). FAB-MS m/z 617.26 [M+H]+, 639.23 [M+Na]+ (Found: C, 66.4; H, 6.1; N, 4.2; C34H36O9N2 requires C, 66.2; H, 5.9; N, 4.5%).
