The specification discloses a process for preparing methoxymethyl D-6-(2,2-dimethyl-5-oxo-4-phenyl-1-imidazolidinyl)penicillanate and the use of this compound as a well absorbed antibacterial agent and also as an intermediate in the preparation of other antibiotic derivatives of 6-aminopenicillanic acid such as hetacillin. The methoxymethyl ester of ampicillin is also disclosed.