A 250-mL single-neck round-bottomed flask equipped with a magnetic stirrer was purged with nitrogen and charged with 25 (1.50 g, 4.66 mmol), THF (45 mL), 1-hydroxy-7-azabenzotriazole (1.27 g, 9.32 mmol) and 1-ethyl-3-[3-dimethyl amino)propyl]-carbodiimide hydrochloride (1.79 g, 9.32 mmol), and the reaction mixture was stirred for 15 min. Ethanol (429 mg, 9.32 mmol) and 4-dimethylamino pyridine (57 mg, 0.47 mmol) were added, and the reaction mixture was stirred at room temperature for a further 16 h. After this time, the reaction was partitioned between water (40 mL) and methylene chloride (50 mL). The layers were separated, and the aqueous phase was extracted with methylene chloride (2×30 mL). The organic extracts were combined and dried over sodium sulfate. The drying agent was removed by filtration, and the solvent was evaporated under reduced pressure. The resulting residue was dried to a constant weight under vacuum to afford 26 in 73% yield (1.20 g) as a white solid: mp 87-88° C.; 1H NMR (500 MHz, DMSO-d6) δ 9.35 (s, 1H), 7.40 (d, 2H, J=7.5 Hz), 7.28 (d, 2H, J=7.5 Hz), 4.16 (s, 1H), 4.05 (m, 2H), 2.80 (septet, 1H, J=6.5 Hz), 2.02 (s, 3H), 1.47 (s, 9H), 1.11 (t, 3H, J=7.0 Hz), 0.94 (d, 3H, J=6.5 Hz), 0.88 (d, 3H, J=6.5 Hz); MS (ESI+) m/z 351 (M+H).
