To a solution of benzyl α-[3-phenoxymethyl-7oxo-4-thia-2,6-diazabicyclo[3,2,0]hept-2-en-6-yl]-α-isopropenylacetate (4.22 g) in dichloromethane and methanol (5:1) is introduced ozonized oxygen until the blue color of the solution does not fade out. Then the solution is mixed with dimethyl sulfide, washed with water, dried, and concentrated. The obtained residue is purified by chromatography over silica gel containing 10% water to give benzyl α-[3-phenoxymethyl-7-oxo-4-thia-2,6-diazabicyclo[3,2,0]hept-2-en-6-yl]-α-(1-hydroxyethylidene)acetate (2.98 g: 70.28%).
