Cl[C:2](=[O:8])[C:3]([O:5][CH2:6][CH3:7])=[O:4].[Cl-].[Al+3].[Cl-].[Cl-].[Br:13][C:14]1[CH:18]=[C:17]([CH3:19])[S:16][C:15]=1[CH3:20].O>ClCCl>[Br:13][C:14]1[C:18]([C:2](=[O:8])[C:3]([O:5][CH2:6][CH3:7])=[O:4])=[C:17]([CH3:19])[S:16][C:15]=1[CH3:20]
Under a nitrogen atmosphere, ethyl chlorooxoacetate (1.04 mL, 9.27 mmol) and aluminum chloride (per 3 portions) (2.47 g, 18.54 mmol) was added successively at −5° C. to a solution of 3-bromo-2,5-dimethylthiophene (28b) (886 mg, 4.64 mmol) in dichloromethane (20 mL). After 3 hours at −5° C., the reaction mixture was slowly hydrolyzed at 0° C. with water. The organic layer was separated, washed with water, brine, dried over sodium sulfate, filtered, and evaporated under reduced pressure. The residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 95/5) to give the desired keto-ester (28c) (725 mg, 2.50 mmol, 50%).
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