Br[C:2]1[CH:7]=[C:6]([CH3:8])[CH:5]=[C:4]([CH3:9])[CH:3]=1.C([Li])CCC.[CH3:15][CH:16]1[CH2:20][CH2:19][CH2:18][C:17]1=O.Cl>CCCCCC.C(OCC)C>[CH3:15][C:16]1[CH2:20][CH:19]=[C:18]([C:2]2[CH:7]=[C:6]([CH3:8])[CH:5]=[C:4]([CH3:9])[CH:3]=2)[CH:17]=1
1-Bromo-3,5-dimethylbenzene (13.6 g, 73.6 mmol) was dissolved in a mixed solution of hexane (150 mL) and diethyl ether (20 mL) and added dropwise at 0° C. with a solution (46.6 mL, 1.58M) of n-butyllithium in hexane. The solution was immediately warmed to room temperature, stirred night and day and added dropwise at −78° C. to a solution of 3-methyl-2-cyclopentanone (7.3 mL, 74 mmol) in diethyl ether (100 mL). The resultant solution was warmed to room temperature immediately after dropwise addition and stirred for 2 hours. After termination of the reaction, the reaction solution was added with dilute hydrochloric acid, stirred at room temperature for 30 minutes and then subjected to layer separation. The organic layer was dried with magnesium sulfate and then had the solvent distilled off under reduced pressure. The crude product was distilled under reduced pressure (125 to 127° C./0.5 mmHg) to obtain 1-methyl-3-(3,5-dimethyl-phenyl)-cyclopentadiene (6.2 g, yield: 46%).
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