[Br:1][C:2]1[CH:6]=[C:5]([Br:7])[S:4][C:3]=1[CH:8]([OH:14])[C:9]([O:11][CH2:12][CH3:13])=[O:10].Cl(O)(=O)(=O)=O.C(=O)([O-])[O-].[K+].[K+].C(O[C:30]([CH3:33])([CH3:32])[CH3:31])(=O)C>>[C:30]([O:14][CH:8]([C:3]1[S:4][C:5]([Br:7])=[CH:6][C:2]=1[Br:1])[C:9]([O:11][CH2:12][CH3:13])=[O:10])([CH3:33])([CH3:32])[CH3:31]Ethyl 2-(3,5-dibromothiophen-2-yl)-2-hydroxyacetate (7b) (100 mg, 0.29 mmol) was dissolved in tert-butyl acetate (5.4 mL) at −20° C. and 70% perchloric acid (0.7 mL) was rapidly added. The mixture was stirred at 0° C. for 3 hours then poured in a saturated solution of potassium carbonate. Layers were separated. The aqueous layer was extracted with ethyl acetate (2×20 mL). The organic layers were washed with brine (10 mL), dried over sodium sulfate and concentrated in vacuo. The residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 90/10) to provide ethyl 2-(tert-butoxy)-2-(3,5-dibromothiophen-2-yl)acetate (100 mg, 0.40 mmol, 86%).
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