[CH3:1][O:2][CH:3]1[O:8][CH2:7][CH:6]([CH2:9][O:10][C:11]2[CH:16]=[CH:15][N+:14]([O-])=[C:13]([CH3:18])[C:12]=2[CH3:19])[CH2:5][O:4]1.C(OC(=O)C)(=[O:22])C>>[CH3:1][O:2][CH:3]1[O:8][CH2:7][CH:6]([CH2:9][O:10][C:11]2[CH:16]=[CH:15][N:14]=[C:13]([CH2:18][OH:22])[C:12]=2[CH3:19])[CH2:5][O:4]1The 4-((2-methoxy-1,3-dioxan-5-yl)methoxy)-2,3-dimethylpyridine 1-oxide (1.8 g, 6.68 ml) obtained in the step (6b) was mixed with acetic anhydride (8 ml). The mixture was stirred at 100° C. for 2 hours. After cooled to room temperature, the reaction mixture was concentrated under reduced pressure. To the residue, methanol (10 ml) and a 5N aqueous sodium hydroxide solution (5 ml) were added and the mixture was stirred at room temperature for 15 hours. The reaction mixture was concentrated and the residue was separated with a saturated saline solution and ethyl acetate. The organic layer was dried over anhydrous magnesium sulfate, filtered, concentrated and the residue was purified by silica gel column chromatography (elution solvent: heptane/ethyl acetate, ethyl acetate/methanol) to obtain the title compound (0.41 g, yield: 22.8%), which is a cis and trans (1:1) mixture, as a yellow oil.
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