To a solution of 0.24 g (0.55 mmol) of 3 in CH2Cl2 (2 mL) was added dropwise 0.13 mL (1.65 mmol) of trifluoroacetic acid, and the mixture was allowed to stir for 2 hours. The solution was diluted with CH2Cl2 (10 mL) and cooled to 0° C. followed by dropwise addition of 0.3 g (3.00 mmol) of triethylamine. After 5 minutes, to the solution was added dropwise 0.08 g (0.61 mmol) of chlorooxoacetate, and the mixture was stirred for 2 hours. The solution was quenched with H2O and extracted with CH2Cl2 (2×200 mL). The organic phase was dried (MgSO4) and evaporated to a clear oil which was subject to column chromatography (EtOAc/Hexanes, 1:3) to yield 0.19 g (81.9%) of 5 as a clear oil. TLC Rf=0.5 (EtOAc/Hexanes, 1:3)
