To a solution of di-tert-butyl dicarbonate (58 mg, 0.265 mmol) and DMAP (4.6 mg, 0.038 mmol) in dichloromethane (5 mL) was added a dropwise a solution of 21 (100 mg, 0.189 mmol) in dichloromethane (3 mL) and triethylamine (0.131 mL, 0.945 mmol), and the resulting mixture was stirred overnight. After this time, water (10 mL) was added, and the organic phase was separated. The aqueous component was extracted with dichloromethane (5 × 10 mL), and the combined organic extracts were washed with brine (100 mL), dried over anhydrous magnesium sulfate and concentrated under reduced pressure. Purification by flash column chromatography, eluting with 0 -10% (7N ammonia in methanol) in dichloromethane, afforded the title compound as a colorless oil (52 mg, 43% in dichloromethane, afforded the title compound as a pale brown oil (640 mg, 34%).