[Br:1][C:2]1[CH:10]=[C:9]2[C:5]([CH2:6][CH2:7][C:8]2=[O:11])=[CH:4][CH:3]=1.[CH3:12][CH:13](O)[CH:14]=[CH2:15].C1(C)C=CC(S(O)(=O)=O)=CC=1.O>COC(OC)(C)C>[CH2:12]([CH:7]1[CH2:6][C:5]2[C:9](=[CH:10][C:2]([Br:1])=[CH:3][CH:4]=2)[C:8]1=[O:11])[CH:13]=[CH:14][CH3:15]A solution of 10.0 g of 6-bromo-1-indanone, 9.79 ml of 3-buten-2-ol and 100 mg of p-toluenesulfonic acid in 100 ml of 2,2-dimethoxypropane was boiled under reflux for 71 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 1:1). In addition to 3.07 g of educt, there were obtained 9.86 g (78%) of (RS)-2-(2-buten-1-yl)-6-bromo-1-indanone as a yellow oil.
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