To a solution of triphosgene (0.246 g, 0.804 mmol) in THF (5 mL) at 0° C. was added a mixture of 1-6 (0.78 g, 2.29 mmol), triethylamine (0.36 mL, 3.09 mmol) and THF (1 mL) over 10 minutes. After 30 minutes, the reaction was allowed to warm to ambient temperature and stir for 1 hour. The mixture was then purged with argon for 30 minutes, and to it was added a mixture of 3-6 (0.435 g, 2.52 mmol), triethylamine (0.36 mL, 3.09 mmol) and THF (5 mL). The mixture was then heated at 0° C. for 2 hours, allowed to cool to ambient temperature. 2M H2SO4 (2.5 mL) was then added, and the mixture stirred for 15 hours. The reaction was then quenched with a 10% w/v solution of K2CO3 (10 mL), diluted with ethyl acetate, washed with water and brine, dried over MgSO4, and concentrated to give 850 mg of 3-7 as a yellow oil, used crude in the next step.
