[C:1]1(=[CH:6][CH2:7][CH2:8][C:9](=[O:11])[CH3:10])[CH2:5][CH2:4][CH2:3][CH2:2]1.CC1C=CC(S(O)(=O)=O)=CC=1>C1(C)C=CC=CC=1.CC(OC)(C)C>[C:1]1([CH2:6][CH2:7][CH2:8][C:9](=[O:11])[CH3:10])[CH2:5][CH2:4][CH2:3][CH:2]=1A mixture of 5-cyclopentylidene-2-pentanone (9.92 g; 65 mmol) and p-TsOH (100 mg; 0.53 mmol) in toluene (150 ml) was heated at 90° C. for 8 hours, then cooled down to room temperature, diluted with MTBE (100 ml), washed with saturated NaHCO3 (50 ml), H2O (50 ml), brine (50 ml), dried over MgSO4 and concentrated in vacuo. Distillation under reduced pressure (48° C./0.065 torr) yielded 7.13 g of 5-(cyclopent-1-enyl)-2-pentanone (purity about 80%), which was further purified by flash chromatography to give 6.1 g (61% yield) of a 89% pure product containing 11% of 5-cyclopentylidene-2-pentanone.
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