[CH3:1][C:2]1([CH3:14])[O:9][CH2:8][CH2:7][C@H:6]2[N:3]1[C:4](=[O:13])[C@@H:5]2[NH:10][CH:11]=O.N1C(C)=CC=CC=1C.P(Cl)(Cl)(Cl)=O>C(OCC)(=O)C>[CH3:1][C:2]1([CH3:14])[O:9][CH2:8][CH2:7][C@H:6]2[N:3]1[C:4](=[O:13])[C@@H:5]2[N+:10]#[C-:11]To a solution of (6R,7R)-2,2-dimethyl-7-formamido-1-aza-3-oxabicyclo[4.2.0]octan-8-one (345 mg) were added 2,6-lutidine (2.05 ml) and phosphorus oxychloride (0.52 ml) at 0° C. After stirring for 3 hours at 0° C., the mixture was poured into a mixture of ethyl acetate (80 ml) and ice-water. The organic layer was separated, washed in turn with 10% phosphoric acid, water, brine, an aqueous mixture of sodium bicarbonate and sodium chloride (1:1), and brine, dried over magnesium sulfate, and evaporated in vacuo. The crystalline residue was washed with a mixture of petroleum ether and ether (1:1) and dried over phosphorus pentoxide to give (6R,7R)-2,2-dimethyl-7-isocyano-1-aza-3-oxabicyclo[4.2.0]octan-8-one (205 mg).
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