[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.CN([CH:18]=[O:19])C.C(O)(=O)CC(CC(O)=O)(C(O)=O)O>C1COCC1.C([Li])(C)(C)C>[CH:18]([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)=[O:19]To a cooled (−78° C.) solution of 5,5-dimethyl-4,5,6,7-tetrahydro-benzo[c]thiophene-1-carboxylic acid (2.00 g, 9.51 mmol) in THF (40 mL), tert.-butyllithium (15.8 mL, 1.5 M in pentane) is added. The mixture is stirred at −30° C. for 30 min and cooled again to −78° C. before DMF (2 mL, 27.4 mmol) is added. Stirring is continued for 5 min. The reaction mixture is poured onto 10% aq. citric acid solution and extracted twice with EA. The combined organic extracts are washed with brine, dried over Na2SO4 and evaporated go give crude 3-formyl-5,5-dimethyl-4,5,6,7-tetrahydro-benzo[c]thiophene-1-carboxylic acid (2.30 g) as a brownish foam, LC-MS: tR=0.92 min, [M+1+CH3CN]=280.22; 1H NMR (CDCl3): δ 10.06 (s, 1H), 4.02 (s br, 1H), 3.06 (t, J=6.7 Hz, 2H), 2.84 (s, 2H), 1.63 (t, J=6.7 Hz, 2H), 1.03 (s, 6H).
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