[CH3:1][O:2][C:3]1[CH:4]=[C:5]2[C:9](=[CH:10][CH:11]=1)[C:8]1([CH2:15][CH2:14][CH2:13][CH2:12]1)[CH2:7][C:6]2=[O:16].[CH3:17][CH:18](O)[CH:19]=[CH2:20].C1(C)C=CC(S(O)(=O)=O)=CC=1.O>COC(OC)(C)C>[CH2:17]([CH:7]1[C:6](=[O:16])[C:5]2[C:9](=[CH:10][CH:11]=[C:3]([O:2][CH3:1])[CH:4]=2)[C:8]21[CH2:15][CH2:14][CH2:13][CH2:12]2)[CH:18]=[CH:19][CH3:20]A solution of 9.73 g of 5'-methoxy-2',3'-dihydro-spiro[cyclo-pentane-1,1'-[1H]indene]-3'-one, 9.3 ml of 3-buten-2-ol and 100 mg of p-toluenesulfonic acid in 100 ml of 2,2-dimethoxy-propane was boiled under reflux for 90 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/ethyl acetate 7:1). In addition to 4.0 g of educt, there were obtained 6.24 g (51%) of (RS)-2'-(2-buten-1-yl)-5'-methoxy-2',3'-dihydro-spiro[cyclopentane-1,1'-[1H]indene]-3'-one as a yellow oil.
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