A solution of 148 mg (0.2 mmol) of a mixture consisting of 2-[4-(p-toluenesulphonylthio)-3-phenoxyacetamido-2-oxoazetidin-1-yl]-3-(N-methylcyclohexylamino)-crotonic acid p-nitrobenzyl ester and the corresponding isocrotonic acid ester, in 3 ml of dry acetonitrile, is heated under nitrogen for about 4 hours at 80° C. until no further starting material can be detected by thin layer chromatography (silica gel: toluene/ethyl acetate, 1:1). The heating bath is removed, 38 mg (0.2 mmol) of p-toluenesulphonic acid and about 0.2 ml of water are added to the reaction mixture and the mixture is stirred for a further 2 hours at room temperature. The reaction mixture is diluted with benzene, washed with water, dried over sodium sulphate and evaporated in vacuo. The residue is triturated at 0° C. with diethyl ether and gives pale yellow 7β-phenoxyacetamido-3-hydroxy-3-cephem-4-carboxylic acid p-nitrobenzyl ester. IR spectrum (methylene chloride): characteristic bands at 2.95; 3.3; 5.6; 5.75 (sh); 5.9; 5.95 (sh); 6.55, 7.45, 8.15 and 8.3μ; NMR spectrum (deuterochloroform): δ in ppm: 3.4 (zH, q, J=17 Hz); 4.57 (2H, s); 5.06 (1H, d; J=5 Hz); 5.35 (2H, q, J=14 Hz); 5.7 (1H dd, J=5, 10 Hz); 6.8-8.4 (10H, c) and 11.4 (1H, br.s.).
