[OH:1][CH2:2][C:3]1[N:11]([CH3:12])[C:10]2[C:9](=[O:13])[N:8]([CH2:14][CH2:15][CH3:16])[C:7](=[O:17])[N:6]([CH2:18][CH2:19][CH3:20])[C:5]=2[N:4]=1>C(Cl)(Cl)Cl.[O-2].[O-2].[Mn+4]>[CH3:12][N:11]1[C:10]2[C:9](=[O:13])[N:8]([CH2:14][CH2:15][CH3:16])[C:7](=[O:17])[N:6]([CH2:18][CH2:19][CH3:20])[C:5]=2[N:4]=[C:3]1[CH:2]=[O:1]Manganese dioxide (2.48 g, 28.5 mmol) was added to a solution of 800 mg (2.85 mmol) of Compound b obtained in Reference Example 2 in 80 ml of chloroform, and the mixture was stirred at room temperature for 12 hours. Then, the reaction mixture was filtered through Celite and the filtrate was concentrated under reduced pressure. The residue was purified by column chromatography (eluent: 1% methanol/chloroform) to give 440 mg (yield 56%) of Compound c as a pale yellow powder.
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