[CH3:1][S:2](Cl)(=[O:4])=[O:3].[CH:6]([C@H:19]1[CH2:24][C@H:23]([OH:25])[CH2:22][CH2:21][O:20]1)([C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)[C:7]1[CH:12]=[CH:11][CH:10]=[CH:9][CH:8]=1.C(N(CC)CC)C>C(Cl)Cl>[CH:6]([C@H:19]1[CH2:24][C@H:23]([O:25][S:2]([CH3:1])(=[O:4])=[O:3])[CH2:22][CH2:21][O:20]1)([C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)[C:7]1[CH:8]=[CH:9][CH:10]=[CH:11][CH:12]=1Methanesulfonyl chloride (0.62 g, 5.41 mmol) in dry methylene chloride (10 ml) was added dropwise to a mixture of trans-2-diphenylmethyl-4-hydroxypyran 3a (0.73 g, 2.70 mmol), triethylamine (0.41 g, 4.06 mmol) in methylene chloride (10 ml) and was cooled to 0° C. After one hour, the reaction was gradually allowed to reach room temperature over a period of four hours. Additional methylene chloride (20 ml) was added to the reaction mixture, and the mixture was washed in turn with saturated aqueous sodium bicarbonate, brine and water, then dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and purification by flash chromatography gave compound 4a (0.93 g, 99.9% yield) as an oil.
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