C1O[C:4]2([CH2:9][CH2:8][CH:7]([CH2:10][C:11]([O:13][CH2:14][C:15]3[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=3)=[O:12])[CH2:6][CH2:5]2)[O:3]C1.O.O.C1(C)C=CC(S(O)(=O)=O)=CC=1.C(=O)([O-])O.[Na+]>CC(C)=O.CCCCCC.C(OCC)(=O)C>[O:3]=[C:4]1[CH2:9][CH2:8][CH:7]([CH2:10][C:11]([O:13][CH2:14][C:15]2[CH:16]=[CH:17][CH:18]=[CH:19][CH:20]=2)=[O:12])[CH2:6][CH2:5]1
To a solution of 4-benzyloxycarbonylmethylcyclohexanone ethylene acetal (7.62 g, 26.2 mmol) in acetone (235 mL)-water (10 mL) was added p-toluenesulfonic acid monohydrate (221 mg, 1.16 mmol), and the mixture was stirred at 55° C. for 10 h. The reaction mixture was adjusted to pH 5 with sodium hydrogen carbonate with stirring under ice cooling, and concentrated under reduced pressure. The residue obtained by azeotropic distillation with the addition of toluene was subjected to silica gel column chromatography (hexane-ethyl acetate 4:1) to give 5.57 g of benzyl 2-(4-oxocyclohexyl)acetate as oil (yield: 86%).
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