[CH:1](NC(C)C)([CH3:3])[CH3:2].C([Li])CCC.[O:13]=[C:14]1[N:21]2[CH:16]([CH2:17][CH2:18][O:19][C:20]2([CH3:23])[CH3:22])[CH2:15]1.C(I)(C)C>CCCCCC.C1COCC1.CCOC(C)=O.CN(C)P(N(C)C)(N(C)C)=O>[O:13]=[C:14]1[N:21]2[CH:16]([CH2:17][CH2:18][O:19][C:20]2([CH3:23])[CH3:22])[CH:15]1[CH:1]([CH3:3])[CH3:2]
THF, 20 ml is placed under N2, treated with 1.54 ml diisopropylamine and cooled to -78° C. A solution of n-butyl lithium 1.97M in hexane 5.6 ml is added dropwise over 5 min. The reaction mixture is stirred at -78° C. for 10 min and then treated with 8-oxo-2,2-dimethyl-3-oxa-1-azabicyclo[4.2.0]octane 1.55 g in 15 ml THF added dropwise over 5 min. After another 10 min hexamethylphosphoramide 1.97 ml is added. The mixture is stirred another 10 min, then treated with 2 ml of isopropyl iodide. The reaction mixture is stirred at -78° C. for 15 min and allowed to warm to 25° C. and stirred for 15 min. The reaction mixture is diluted with EtOAc, washed once with pH 7 phosphate buffer then dried and evaporated. The residue is chromatographed on silica gel using 25% EtOAc/C 6 H6 as eluant to give 8-oxo-2,2-dimethyl-7α-isopropyl-3-oxa-1-azabicyclo[4.2.0]octane.μ i.r.: 5.7 (β-lactam).
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