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.
(MOM) moiety, used as protecting group for the carboxyl function of penicillin derivatives, their sulfoxides and sulfones, could be easily cleaved in aqueous methanol at room temperature. The rate of deprotection by solvolysis is not sensitive to the nature of the substituent in position 6, but depends on the state of oxidation of the thiazolidine sulfur.