[C:1]([C:4]1[O:8][C:7]2[C:9](=[O:19])[C:10]3[C:15]([C:16](=[O:17])[C:6]=2[CH:5]=1)=[C:14]([OH:18])[CH:13]=[CH:12][CH:11]=3)(=[O:3])[CH3:2].[BH4-].[Na+]>C(Cl)(Cl)Cl.C(O)C>[OH:3][CH:1]([C:4]1[O:8][C:7]2[C:9](=[O:19])[C:10]3[C:15]([C:16](=[O:17])[C:6]=2[CH:5]=1)=[C:14]([OH:18])[CH:13]=[CH:12][CH:11]=3)[CH3:2]To a solution of 2-acetyl-5-hydroxynaphtho[2,3-b]furan-4,9-dione (694 mg, 2.73 mmol) obtained in Example 5 in chloroform (100 mL) and ethanol (25 mL) is added sodium borohydride (515 mg, 13.7 mmol) at 0° C. The mixture is stirred for 30 minutes, and then the reaction is quenched by adding 10% hydrochloric acid to the mixture. The aqueous layer is extracted with chloroform twice, and the extract is continuously washed with water and brine. The mixture is evaporated in vacuo and purified by silica gel column chromatography (eluent: chloroform) to give 2-(1-hydroxyethyl)-5-hydroxynaphtho[2,3-b]furan-4,9-dione (516 mg, 74%) as a racemic mixture of a yellow crystal.
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