[CH2:1]1[C:4]2([O:9][CH2:8][CH:7]([CH2:10][O:11][C:12]3[C:17]([CH3:18])=[CH:16][N+:15]([O-])=[C:14]([CH3:20])[C:13]=3[CH3:21])[CH2:6][O:5]2)[CH2:3][CH2:2]1.C(OC(=O)C)(=[O:24])C.[OH-].[Na+]>CO>[CH2:1]1[C:4]2([O:9][CH2:8][CH:7]([CH2:10][O:11][C:12]3[C:17]([CH3:18])=[CH:16][N:15]=[C:14]([CH2:20][OH:24])[C:13]=3[CH3:21])[CH2:6][O:5]2)[CH2:3][CH2:2]1The 4-(5,9-dioxaspiro[3.5]non-7-ylmethoxy)-2,3,5-trimethylpyridine 1-oxide (1.97 g, 6.72 mmol) obtained in the step (13b) was mixed with acetic anhydride (20 ml, 212 mmol). The mixture was stirred at 85° C. for 1.5 hours. After cooled to room temperature, the reaction mixture was concentrated. To the residue, methanol (20 ml) and a 5N aqueous sodium hydroxide solution (20 ml, 100 mmol) were added and the mixture was stirred at room temperature for 45 minutes. The reaction mixture was concentrated and the residue was partitioned between water and ethyl acetate. The organic layer was washed with a 2N aqueous sodium hydroxide solution, dried over anhydrous magnesium sulfate, filtered and concentrated to obtain the title compound (1.69 g, yield: 85.7%) as a brown oil.
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