The synthesis of p-O-(tetra-O-acetyl-α-D-glucopyranosyl)-phenylacetic acid from tetra-O-acetyl-2-deoxy-2-nitroso-α-D-glucopyranosyl chloride and t-butyl-p-hydroxyphenylacetate and its coupling with 6-aminopenicillanic acid by way of the corresponding acid chloride to give p-O-(tetra-O-acetyl-α-D-glucopyranosyl)-penicillin G is described.