[CH2:1]([CH:6]1[C:10](=[O:11])[CH2:9][CH2:8][C:7]1=[O:12])[CH2:2][CH2:3][CH2:4][CH3:5].Cl.[C:14](=O)([O-])O.[Na+]>CO>[CH2:1]([C:6]1[C:10](=[O:11])[CH2:9][CH2:8][C:7]=1[O:12][CH3:14])[CH2:2][CH2:3][CH2:4][CH3:5]To 32.5 g (0.19 mol) of 2-n-pentyl-1,3-cyclopentanedione was added 650 ml of methanol containing 1 wt % of hydrogen chloride (prepared by preliminarily blowing hydrogen chloride therein), followed by heating under reflux for 5 hours and cooling to 25° C. Then, thereto was gradually added 15.4 g of sodium hydrogencarbonate followed by stirring for 30 minutes at 25° C. Methanol was distilled off in vacuo and then 200 mL of water and 200 mL of isobutanol were added. The resulting mixture was stirred for 10 minutes at 25° C. and then allowed to stand to separate the organic phase. Isobutanol in the organic phase was distilled off, yielding 2-pentyl-3-methoxy-2-cyclopentenone (a compound in which R20 represents an n-pentyl group, and R21 represents a methyl group in the above formula (14)). Gas chromatographic analysis showed that the yield of 2-pentyl-3-methoxy-2-cyclopentenone relative to 2-n-pentyl-1,3-cyclopentanedione was 80%.
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