C[O-].[Na+].[C:4](OC)(=O)[CH2:5][C:6]([O:8][CH3:9])=[O:7].[CH2:13]([C:18]1C(=O)[CH2:20][CH2:21][C:22]=1[O:23]C)[CH2:14][CH2:15][CH2:16][CH3:17].Cl>CO>[CH3:9][O:8][C:6](=[O:7])[CH2:5][C:4]1[CH2:20][CH2:21][C:22](=[O:23])[C:18]=1[CH2:13][CH2:14][CH2:15][CH2:16][CH3:17]The air in a reactor (a volume of 0.3 L) was preliminarily replaced with nitrogen at ordinary pressure. The reactor was charged with 13.5 g (70 mmol) of a 28% methanol solution of sodium methoxide and 140 ml of absolute methanol. Thereto was added 6.6 g (50 mmol) of dimethyl malonate, and the mixture was stirred for 30 minutes at 25° C. Thereto was added 6.6 g (36 mmol) of 2-pentyl-3-methoxy-2-cyclopentenone, and the mixture was heated under reflux for 24 hours in a nitrogen atmosphere. The resulting mixture was ice-cooled and neutralized by gradually adding 6.1 ml of concentrated hydrochloric acid, and methanol was distilled off. Thereto were added 50 ml of water and 100 ml of toluene, and then the mixture was stirred sufficiently and allowed to stand to separate the organic phase. The toluene in the organic phase was distilled off in vacuo followed by vacuum distillation, yielding methyl(2-pentyl-3-keto-1-cyclopentenyl-)acetate (a compound in which R22 represents an n-pentyl group, and R23 represents a methyl group in the above formula (15)). Gas chromatographic analysis showed that the yield of methyl(2-pentyl-3-keto-1-cyclopentenyl-)acetate relative to 2-pentyl-3-methoxy-2-cyclopentenone was 90%.
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