[O:1]1[C@H:5]2[O:6][CH2:7][CH2:8][C@H:4]2[C@@H:3]([OH:9])[CH2:2]1.O1[C@H]2OCC[C@H]2[C@H](O)C1.P([O-])([O-])(O)=O.[K+].[K+].Cl[O-].[Na+]>C(OCC)(=O)C.O1[C@@H]2OCC[C@@H]2[C@H](O)C1.O1[C@@H]2OCC[C@@H]2[C@@H](O)C1.[Br-].[K+].CC(O)C>[O:1]1[C@H:5]2[O:6][CH2:7][CH2:8][C@H:4]2[C:3](=[O:9])[CH2:2]1The mixture (18.9 g) containing (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol (7.93 g, 60.9 mmol), (3S,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol (2.07 g, 15.9 mmol), (3S,3aR,6aS)-hexahydrofuro[2,3-b]furan-3-ol (0.05 g, 0.4 mmol) and (3R,3aR,6aS)-hexahydrofuro[2,3-b]furan-3-ol (0.05 g, 0.4 mmol) obtained in Example 4 was dissolved in ethyl acetate (112 ml), and dipotassium hydrogen phosphate (27.1 g, 155.3 mmol), potassium bromide (0.5 g, 3.9 mmol) and 2,2,6,6-tetramethylpiperidinyl-1-oxy (61 mg, 0.4 mmol) were added, and the mixture was cooled to 0. Into this was dropped a sodium hypochlorite aqueous solution (123.9 g, effective chlorine concentration 14%, 0.23 mol) at 15° C. or less, and after completion of dropping, the mixture was stirred for 1 hour. After completion of the reaction, 2-propanol (10 ml) was added, and the mixture was stirred for 30 minutes and liquid-partitioned. Further, the aqueous layer was extracted with ethyl acetate (50 ml) and the organic layers were combined, and dried over anhydrous sodium sulfate (1.0 g) added. Sodium sulfate was filtrated, then, the solvent was distilled off, and 2-propanol (30 ml) was added to the concentrated residue and recrystallization was performed, to obtain a pale brown white crystal (3aR,6aR)-tetrahydrofuro[2,3-b]furan-3(2H)-one (7.4 g, purity 98%, yield 73%). At this stage, the enantiomer excess was 100% ee.
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