Into a -30° C. cold mixture of 0.66 g (1.5 mmol) of (Z)-2-amino-α-[[1-(diphenylmethoxycarbonyl)-1-methylethoxy]imino]-4-thiazoleacetic acid in 13 ml of dry dimethylformamide, 0.63 ml (4.5 mmol) of triethylamine was dropped, followed by 0.33 ml (1.5 mmol) of diphenylchlorophosphate. After stirring at -30° C. for 1 hour, 0.42 ml (3.0 mmol) of triethylamine was dropped in, followed by the addition of 0.80 g (1.5 mmol) of (S)-3-amino-1-[[[[2-[[(3,4-dihydroxyphenyl)amino]carbonyl]hydrazino]sulfonyl]amino]carbonyl]-2-oxoazetidine. The mixture was stirred for 2 hours at -10° C. and an additional 2 hours at 0° C. The solvent was removed in vacuo and the residue was taken up in a few ml of ethyl acetate and ice water. The pH of the mixture was corrected to pH=2 the addition of dilute hydrochloric acid. The insoluble material was collected by suction1) and stirred with a few ml of ethyl acetate until it became crystalline; yield after drying in vacuo: 0.31 g. This material was used in the next step without further purification.
