A process for preparing an azetidinone derivative represented by the formula (I)
wherein R1 represents an aryl group, arylmethyl group or aryloxymethyl group, R2 represents hydrogen atom or a carboxyl-protecting group and R3 represents an alkyl group or aryl group, the process comprising reacting an azetidinone derivative represented by the formula (II)
wherein R1, R2 and R3 are as defined above with a chlorinating agent.
A process for preparing an azetidinone derivative represented by the formula (I) ##STR1## wherein R.sup.1 represents an aryl group, arylmethyl group or aryloxymethyl group, R.sup.2 represents hydrogen atom or a carboxyl-protecting group and R.sup.3 represents an alkyl group or aryl group, the process comprising reacting an azetidinone derivative represented by the formula (II) ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 are as defined above with a chlorinating agent.