2,2,2-Trichloroethyl chloroformate (3.24 mL, 23.5 mmol) was added dropwise over 5 min to a solution of Intermediate 110a (4.11 g, 22.4 mmol) in NaOH (1M in water, 33.6 mL, 33.6 mmol) and EtOAc (35 mL) cooled at 0° C. under argon atmosphere. The reaction mixture was stirred at RT for 5 h. Additional 2,2,2-trichloroethyl chloroformate (462 μL) was added and stirring at RT was continued for 16 h. Additional NaOH (1M in water, 15 mL) and 2,2,2-trichloroethyl chloroformate (462 μL) were added and stirring at RT was continued for 1 h. The aqueous layer was extracted with EtOAc (×3) and the combined organic layers were washed with brine, dried (Na2SO4) and concentrated in vacuo. The resultant residue was dissolved in cyclohexane (100 mL) and left to stand for 5 days. The resulting suspension was filtered and the solid collected by filtration washing with additional cyclohexane affording the title compound (3.64 g, 45%) as a white solid. The filtrate was concentrated in vacuo and the resultant residue was purified by FCC on silica, using a mixture of 50% EtOAc in cyclohexane, then partitioned between DCM and water. The organic layer was dried through a phase separator and concentrated in vacuo to afford additional title compound (1.43 g, 18%) as an orange gum. LCMS (Method 3): Rt 3.72 min, m/z 358 [MH+].
