[C:1]([O:5][C:6]([NH:8][C@H:9]1[CH2:12][NH:11][C:10]1=[O:13])=[O:7])([CH3:4])([CH3:3])[CH3:2].C([Li])CCC.[N:19]([CH2:22][C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24])=[C:20]=[S:21]>O1CCCC1>[C:1]([O:5][C:6]([NH:8][C@H:9]1[CH2:12][N:11]([C:20](=[S:21])[NH:19][CH2:22][C:23]([O:25][C:26]([CH3:28])([CH3:27])[CH3:29])=[O:24])[C:10]1=[O:13])=[O:7])([CH3:4])([CH3:2])[CH3:3]A solution of (S)-3-[[(t-butyloxy)carbonyl]amino]-2-azetidinone (186 mg, 1 mmol) in 6 ml of dry tetrahydrofuran at -75° C. was treated with 0.65 ml (1.1 mmol) of 1.68N n-butyl lithium. After 30 minutes, a solution of t-butyl isothiocyanatoacetate (0.17 ml, 1.1 mmol) in 1 ml of dry tetrahydrofuran was added to the reaction mixture. After stirring at -75° C. for 2 hours, the reaction mixture was placed at -70° C. overnight.
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