[CH2:1]([CH:3]([CH2:6][CH3:7])[CH:4]=O)[CH3:2].[CH3:8][C:9]([CH:11]=[CH2:12])=[O:10].OS(O)(=O)=O>C(Cl)(Cl)Cl>[CH2:1]([C:3]1([CH2:6][CH3:7])[CH2:4][CH2:8][C:9](=[O:10])[CH:11]=[CH:12]1)[CH3:2]A mixture of 2-ethylbutyraldehyde (12.3 mL, 100 mmol), methylvinylketone (5.6 mL, 67.3 mmol) and H2SO4 (0.07 mL) is stirred at 40° C. overnight. Another portion of methylvinylketone (5.6 mL, 67.3 mmol) and H2SO4 is added and stirring is continued at 40° C. for 2 d. The yellow solution is diluted with chloroform and the solvent is removed again under reduced pressure. The crude product is purified by vacuum distillation to give 4,4-diethyl-cyclohex-2-enone (10.7 g) as a colourless oil; Bp11 mbar=88° C.; 1H NMR (CDCl3): δ 6.71 (d, J=10.0 Hz, 1H), 5.92 (d, J=10.5 Hz, 1H), 2.42 (t, J=7.0 Hz, 2H), 1.84 (t, J=7.0 Hz, 2H), 1.57-1.40 (m, 4H), 0.87 (t, J=7.6 Hz, 6H).
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