A solution of compound 17 (4.37 g, 16.72 mmol) in THF (30 mL) was cooled to 0° C. and treated successively with Et3N (4.6 mL, 3.38 g, 33 mmol) and benzylchloroformate (2.8 mL, 3.42 g, 20 mmol). The reaction mixture was allowed to stir for 2 hrs at 0° C., quenched by adding an aqueous saturated NH4Cl solution (30 mL) and extracted with EtOAc. The extract was dried over Na2SO4, filtered and concentrated. Crude product was purified by flash chromatography using 30% EtOAc in hexanes as an eluent to provide title compound 115 (3 g, 45% yield). LRMS (ESI): (calc) 396.20 (found) 397.2 (MH)+.
