[Al+3].[Cl-].[Cl-].[Cl-].[O:5]1[CH:9]=[CH:8][CH:7]=[C:6]1[C:10]([O:12][CH3:13])=[O:11].[C:14](Br)([CH3:17])([CH3:16])[CH3:15].Cl>C(=S)=S>[C:14]([C:9]1[O:5][C:6]([C:10]([O:12][CH3:13])=[O:11])=[CH:7][CH:8]=1)([CH3:17])([CH3:16])[CH3:15]AlCl3 (25.4 g, 190 mmol) is added batchwise to methyl furan-2-carboxylate (20.0 g, 159 mmol) and tert.-butylbromide (21.4 mL, 190 mmol) in CS2 (200 mL) while cooling with ice and the mixture is stirred for 6 h at RT. The reaction mixture is poured onto ice water, mixed with conc. hydrochloric acid (25 mL) and stirred for 10 min. Then the aqueous phase is extracted 2× with 200 mL EA. The combined organic phase is dried on Na2SO4, filtered and evaporated down using the rotary evaporator. The methyl 5-tert.-butyl-furan-2-carboxylate thus obtained is used further without any additional purification steps.
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