[CH2:1]([O:8][CH2:9][CH2:10][C:11]([O:13][CH2:14][C:15]1([CH3:19])[CH2:18][O:17][CH2:16]1)=[O:12])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.FB(F)F.C(OCC)C.C(N(CC)CC)C.C(OCC)C>ClCCl>[CH2:1]([O:8][CH2:9][CH2:10][C:11]12[O:12][CH2:16][C:15]([CH3:19])([CH2:14][O:13]1)[CH2:18][O:17]2)[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=13-Methyl-3-oxetanylmethyl 3-benzyloxypropionate (521 mg) was dissolved in dichloromethane under an argon atmosphere, and trifluoroborane-diethylether was added thereto at −15° C. The mixture was stirred for 5 hours at −15° C. Triethylamine (280 μl) was added to the reaction mixture, and stirring was continued for 15 minutes. Subsequently, diethylether (3 ml) was added. The precipitating crystals were removed from the reaction mixture by filtration, then the mother liquid was concentrated under reduced pressure. The residue was purified by alumina column chromatography (ethyl acetate-hexane 1:3), to give 385 mg of 1-(2-benzyloxyethyl)-4-methyl-2,6,7-trioxabicyclo-[2.2.2]octane as a colorless solid.
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