[Br:1][C:2]1[C:3]([CH:9]([OH:15])[C:10]([O:12][CH2:13][CH3:14])=[O:11])=[C:4]([CH3:8])[S:5][C:6]=1[CH3:7].Cl(O)(=O)(=O)=O.C(=O)(O)[O-].[Na+]>C(OC(=O)C)(C)(C)C>[Br:1][C:2]1[C:3]([CH:9]([O:15][C:3]([CH3:9])([CH3:4])[CH3:2])[C:10]([O:12][CH2:13][CH3:14])=[O:11])=[C:4]([CH3:8])[S:5][C:6]=1[CH3:7]To a suspension of ethyl 2-(4-bromo-2,5-dimethylthiophen-3-yl)-2-hydroxyacetate (28d) (830 mg, 2.83 mmol) in tert-butylacetate (29 mL) at −10° C. was added perchloric acid (1 mL). The mixture was stirred at −5° C. for 1 hour. The mixture was then basified with a saturated aqueous solution of sodium bicarbonate until pH 8. The aqueous layer was extracted with ethyl acetate. The organic layer was washed with brine, dried over sodium sulfate, filtered and concentrated under reduced pressure. The residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 90/10) to afford the desired product (28e) as a colorless oil (870 g, 2.49 mmol, 88%).
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