[CH2:1]([O:8][CH2:9][C:10]([CH2:12][O:13][CH2:14][C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1)=[O:11])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.[CH2:21](O)[CH2:22][OH:23].C(OCC)(OCC)OCC.C(=O)([O-])O.[Na+]>O.C1(C)C=CC(S(O)(=O)=O)=CC=1.C(OCC)(=O)C>[CH2:1]([O:8][CH2:9][C:10]1([CH2:12][O:13][CH2:14][C:15]2[CH:16]=[CH:17][CH:18]=[CH:19][CH:20]=2)[O:23][CH2:22][CH2:21][O:11]1)[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1The 1,3-bis(benzyloxy)acetone (30 g, 111 mmol) obtained in the step (4a) was mixed with ethylene glycol (64 ml, 1,148 mmol) and triethyl orthoformate (19 ml, 114 mmol), and p-toluenesulfonic acid monohydrate (591 mg, 3.11 mmol). The mixture was stirred at 50° C. for 14 hours. To the mixture, a saturated aqueous solution of sodium hydrogen carbonate and ethyl acetate were added. The organic layer was washed with water and a saline solution, dried over anhydrous sodium sulfate, and concentrated. The obtained crude product was purified by silica gel column chromatography (elution solvent: heptane/ethyl acetate=1/0-4/1 gradient) and desired fractions were concentrated to obtain the title compound (28.46 g, yield: 81.6%) as a colorless oil.
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