[CH3:1][C:2]1([CH3:14])[CH2:13][CH2:12][C:5]2=[C:6]([C:9]([OH:11])=[O:10])[S:7][CH:8]=[C:4]2[CH2:3]1.[CH2:15](I)[CH3:16].O.CO.C(O)(=O)CC(CC(O)=O)(C(O)=O)O>C1COCC1.C([Li])(C)(C)C>[CH2:15]([C:8]1[S:7][C:6]([C:9]([OH:11])=[O:10])=[C:5]2[CH2:12][CH2:13][C:2]([CH3:14])([CH3:1])[CH2:3][C:4]=12)[CH3:16]To a cooled solution (−78° C.) of 5,5-dimethyl-4,5,6,7-tetrahydro-benzo[c]thiophene-1-carboxylic acid (960 mg, 4.57 mmol) in THF (19 mL), tert-butyllithium (8 mL, 1.5 M solution in pentane) is added. The mixture is stirred at −78° C. for 10 min before ethyliodide (3.80 g, 24.37 mmol) is added. The reaction mixture is stirred at −78° C. for 3 h. Water/methanol 1:1 (8 mL) followed by 10% aq. citric acid solution is added and the mixture is extracted with EA. The combined organic extracts are washed with brine, dried over Na2SO4 and evaporated. The remaining solid is suspended in acetonitrile (6 mL), heated to 60° C., cooled to rt, filtered and dried to give 3-ethyl-5,5-dimethyl-4,5,6,7-tetrahydro-benzo[c]thiophene-1-carboxylic acid (640 mg) as a slightly beige solid; LC-MS: tR=1.01 min, [M+1+CH3CN]=280.10.
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