(ai) A suspension of 4.85 g of 7β-phenoxyacetamido-3-hydroxy-3-cephem-4-carboxylic acid p-nitrobenzyl ester in 25 ml of carbon tetrachloride and 27 ml of water is treated, at 20°, whilst stirring vigorously, successively with 3.0 g of potassium bicarbonate, 3.8 ml of dimethyl sulphate and 1.93 g of tetrabutylammonium bromide. The mixture is stirred vigorously for 4 hours at 20°. After diluting with 50 ml of water, the mixture is extracted with twice 50 ml of methylene chloride. 7β-Phenoxyacetamido-3-methoxy-3-cephem-4-carboxylic acid p-nitrobenzyl ester is obtained by evaporating the extracts, previously dried over sulphate, and crystallising the residue from methylene chloride/diethyl ether.
