Cl(O)(=O)(=O)=O.[OH:6][CH:7]([C:12]1[N:13]=[C:14]([CH3:17])[S:15][CH:16]=1)[C:8]([O:10][CH3:11])=[O:9].C(O[C:22]([CH3:25])([CH3:24])[CH3:23])(=O)C>>[C:22]([O:6][CH:7]([C:12]1[N:13]=[C:14]([CH3:17])[S:15][CH:16]=1)[C:8]([O:10][CH3:11])=[O:9])([CH3:25])([CH3:24])[CH3:23]Under a nitrogen atmosphere, perchloric acid (70%, 3.75 mL) was added at −10° C. to a solution of methyl 2-hydroxy-2-(2-methyl-1,3-thiazol-4-yl)acetate (15b) (590 mg, 3.15 mmol) in tertbutyl acetate (25 mL). After 1 hour, the reaction was quenched with a saturated aqueous solution of sodium carbonate and extracted with dichloromethane (2×20 mL). The organic layer was washed with brine (10 mL), dried over sodium sulfate, filtered and evaporated under reduced pressure. The residue was purified by flash chromatography on silica gel (dichloromethane/ethyl acetate 75/25) to afford the desired product (15c) as a white solid (160 mg, 0.66 mmol, 21%).
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