p-Toluenesulfonic anhydride (33 mg, 0.1 mmol) and Et3N (17 μl, 0.12 mmol) are added to a solution of benzyl 1-azabicyclo[3.2.0]heptan-3,7-dione-2-carboxylate (26 mg., 0.1 mmol) in anhydrous CH2Cl2 (2 ml). The resulting solution is stirred at room temperature for 2 hours. The mixture is diluted with CH2Cl2 (10 ml), washed with H2O (2×5 ml), pH 3 phosphate buffer (5 ml) and 5% NaHCO3 (5 ml), dried with MgSO4, filtered and evaporated i.v. to provide benzyl 3-toluenesulfonyloxy-1-azabicyclo[3.2.0]hept-2-en-7-one-2-carboxylate.
