A solution of Intermediate 152b (69 mg, 0.24 mmol) in EtOAc (1.2 mL) was treated with aqueous 1N NaOH (0.4 mL, 0.43 mmol) then 2,2,2-trichloroethyl chloroformate (0.034 mL, 0.25 mmol) and the mixture was stirred at RT for 2.5 h. The mixture was treated with another portion of 2,2,2-trichloroethyl chloroformate (0.010 mL, 0.07 mmol) and stirred at RT for a further 0.75 h. The mixture was diluted with EtOAc and water and the phases were separated. The aqueous layer was extracted with EtOAc (×2) and the combined organic phase was washed with brine, dried (Na2SO4), and concentrated in vacuo to give a pale red gum (121 mg). The gum was purified by FCC, using 0-40% EtOAc in cyclohexane, to give the title compound as a pale golden gum (62 mg, 56%). LCMS (Method 3): Rt 4.15 min, m/z 464.2, 466.2 [MH+].
