[Cl-].[Al+3].[Cl-].[Cl-].[CH:5]1[C:14]2[C:9](=[CH:10][CH:11]=[CH:12][CH:13]=2)[CH:8]=[CH:7][CH:6]=1.[CH3:15][C:16](=[CH2:20])[C:17](Cl)=[O:18]>ClCCl>[CH3:15][CH:16]1[C:17](=[O:18])[C:5]2[C:14]3[CH:13]=[CH:12][CH:11]=[CH:10][C:9]=3[CH:8]=[CH:7][C:6]=2[CH2:20]1Aluminum chloride powder (240 g) was added to a dry round-bottomed flask under nitrogen, after which 1 l of dichloromethane was added. The suspension was cooled to -30° C., and 100 g of naphthalene and 90.5 g of freshly distilled 2-methyl-2-propenoyl chloride in 950 ml of dichloromethane were added dropwise under nitrogen in 1.5 h. The reaction mixture was stirred for another hour at -30° C., after which the cold reaction mixture was poured into 10 l of ice water. The organic layer was separated, the water layer was extracted with dichloromethane, and the combined organic layers were washed with brine, dried over magnesium sulfate, and evaporated to dryness. The crude product was distilled at 200° C./33 Pa, after which 127 g (84%) of 2,3-dihydro-2-methyl-1H-benz[e]indene-1-one were obtained with a purity >95% (GLC).
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