To a solution of 0.18 g (0.40 mmol) of 11a in CH2Cl2 (3 mL) was added dropwise 10 mL (8.77 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 0.7 g (10.00 mmol) of triethylamine. After 5 minutes, to the solution was added dropwise 0.06 g (0.5 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, 1:1) to yield 0.16 g (98.2%) of 12a as a clear oil. TLC Rf=0.6 (EtOAc/Hexanes, 1:3)
