Abstract In accordance with this invention, it has been found that carbon and oxygen analogs of 6β-aminopenicillanic acid and biologically active derivatives thereof can be formed from esters of 6-oxopenicillanic acid. For example, 6β-phenoxyacetoxypenicillanic acid -- an oxygen analog of penicillin V and 6β-phenoxyacetylmethylpenicillanic acid -- a carbon analog of penicillin V, may be formed from an ester of 6-oxopenicillanate. The ester of 6-oxopenicillanic acid is formed by a diiospropyl carbodiimide/dimethyl sulfoxide oxidation of the corresponding ester of 6α-hydroxypenicillanic acid. The oxygen analogs are formed by reducing the ester of 6-oxopenicillanic acid to the corresponding 6β-hydroxypenicillanate and then forming the desired analog by acylation. The carbon analogs are formed by a Wittig reaction of the ester of 6-oxopenicillanic acid with a suitable acylmethylenetriphenylphosphorane followed by saturation of the newly formed double bond and removal of the protective ester group.