[OH:1][CH:2]([CH:4]1[CH:7]([C:8]#[C:9][Si](C)(C)C)[NH:6][C:5]1=[O:14])[CH3:3].[F-].[Cs+]>O1CCCC1.CO.O>[OH:1][C@@H:2]([C@H:4]1[C@@H:7]([C:8]#[CH:9])[NH:6][C:5]1=[O:14])[CH3:3]A cis/trans mixture (1.0 g) of 3-(1-hydroxyethyl)-4-trimethylsilylethynyl-2-azetidinone is dissolved in a mixture of tetrahydrofuran (10 ml) and methanol (5 ml), and a solution of cesium fluoride (0.2 g) in water (2 ml) is added to the solution. The mixture is stirred at 60° C. for an hour and concentrated under reduced pressure and the residue is extracted with ethyl acetate (20 ml) and tetrahydrofuran (20 ml). The extract is dried over magnesium sulfate, concentrated under reduced pressure and crystallized from ethyl acetate to give 0.18 g of cis-3-[(R)-1-hydroxyethyl]-4-ethynyl-2-azetidinone.
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