[C:1]([O:5][C:6]([NH:8][C@H:9]1[C@H:12]([CH3:13])[NH:11][C:10]1=[O:14])=[O:7])([CH3:4])(C)C.FC(F)(F)C(O)=O.[C:22]1(OC)[CH:27]=[CH:26]C=[CH:24][CH:23]=1>ClCCl>[CH2:1]([O:5][C:6]([NH:8][C@H:9]1[C@H:12]([CH3:13])[NH:11][C:10]1=[O:14])=[O:7])[C:4]1[CH:26]=[CH:27][CH:22]=[CH:23][CH:24]=1A solution of 3.3 g of (3S-trans)-3-t-butoxycarbonylamino-4-methylazetidinone in 10 ml each of dichloromethane and anisole was cooled to 0° C. and 112 ml of trifluoroacetic acid was added. The solution was stirred for 90 minutes and evaporated in vacuo (benzene added and evaporated three times). The residue was dissolved in 70 ml of acetone and the solution was adjusted to pH 7 with 5% sodium bicarbonate solution. A total of 5.33 g of benzyl chloroformate was added over 1 hour at pH 6.5-7.5. The mixture was stirred for 30 minutes at pH 7, diluted with 100 ml of saturated salt, and extracted with ethyl acetate (three 400 ml portions). The residue obtained by evaporation was chromatographed on a 1 liter silica gel column. Elution with chloroform-ethyl acetate (4:1) gave 2.19 g of compound. Crystallization from ether-hexane yielded 1.125 g of the title compound.
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