A 4-(1,1-dialkoxycarbonylalkyl )azetidin-2-one derivative represented by formula (I): ##STR1## wherein R.sup.1 and R.sup.2 are identical or different and each represents an alkyl group, an alkenyl group, or an aralkyl group, R.sup.3 represents a lower alkyl group, R.sup. 4 represents a hydrogen atom or a hydroxyl-protective group, and R.sup.5 represents a hydrogen atom or an amino-protective group, and a process for producing a 4-(1-carboxyalkyl)azetidin-2-one derivative represented by formula (II) useful as an intermediate for 1&bgr;-alkylcarbapenem-type antibacterials: ##STR2## wherein R.sup.3 represents a lower alkyl group and R.sup.4 represents a hydrogen atom or a hydroxyl-protective group, which comprises de-esterifying and decarboxylating the derivative represented by formula (I), and in the case where an amino-protective group is present in the derivative of formula (I), eliminating the protective group from the derivative, are disclosed.
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