[Cl:1][C:2]1[CH:3]=[C:4]2[C:8](=[CH:9][C:10]=1[Cl:11])[C:7](=[O:12])[CH2:6][CH2:5]2.[CH3:13][CH:14](O)[CH:15]=[CH2:16].C1(C)C=CC(S(O)(=O)=O)=CC=1.O>COC(OC)(C)C>[CH2:13]([CH:6]1[CH2:5][C:4]2[C:8](=[CH:9][C:10]([Cl:11])=[C:2]([Cl:1])[CH:3]=2)[C:7]1=[O:12])[CH:14]=[CH:15][CH3:16]A solution of 12.3 g of 5,6-dichloro-1-indanone, 12.6 ml of 3-buten-2-ol and 125 mg of p-toluenesulfonic acid in 125 ml of 2,2-dimethoxy-propane was boiled under reflux for 68 hours on a water separator filled with molecular sieve (0.4 nm, 2 mm pearl shaped). The reaction mixture was subsequently concentrated in a vacuum and purified by column chromatography on silica gel (hexane/diethyl ether 4:1). In addition to 4.3 g of educt, there were obtained 10.8 g (69%) of (RS)-2-(2-buten-1-yl)-5,6-dichloro-1-indanone as a yellow oil.
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