549 mg of the syn isomer of 2-[2-tritylaminothiazol-4-yl]-2-ethoxyimino acetic acid were dissolved in 5.5 ml of acetone and 0.18 ml of triethylamine and 228 mg of tosyl chloride were added immediately with stirring for 45 minutes. A solution of 285 mg of the product of Step C in 1 ml of water, 1.5 ml of a 1M sodium bicarbonate solution and 2 ml of acetone was added to the reaction mixture with stirring for 35 minutes. The acetone was distilled off under reduced pressure, and extraction was effected with methylene chloride. The organic phases were dried and concentrated to dryness. The residue was chromatographed on silica under pressure and elution with a methylene chloride-ethyl acetate mixture (75-25) to obtain 602 mg of syn isomer of racemic cis 1,1-dimethylethyl 7-[2-(2-tritylaminothiazol-4-yl)-2-ethoxyimino-acetamido]-3-hydroxymethyl-8-oxo-4-thia-1-azabicyclo[4,2,0]oct-2-en-2-carboxylate.
