This invention relates to compounds of the formula I ##STR1## wherein: R1 is alkyl, cycloalkyl (which alkyl and cycloalkyl groups may be substituted) aryl, arylalkyl or silyl (bearing alkyl, aryl or arylalkyl substituents);R2 is hydrogen or alkyl;R3 is hydrogen, alkyl, cycloalkyl (which alkyl and cycloalkyl groups may be substituted), alkenyl, aryl, arylalkyl, amino, mono--or di--alkylamino or a group of the formula --OR5 wherein R5 is optionally substituted alkyl, aryl or arylalkyl;R4 is alkyl optionally substituted by one or more alkoxy groups or halogen atoms;Y is optionally substituted alkoxy or cycloalkoxy, arylalkoxy or a group of formula --NR6R7 wherein R6 is hydrogen or alkyl and R7 is alkyl, cycloalkyl (which alkyl and cycloalkyl groups may be substituted), aryl or arylalkyl or R7 is a group of formula --COOR8 is alkyl or arylalkyl, which are useful as intermediates in the synthesis of beta-lactam antibiotics, particularly carbapenems, and to a process for the manufacture of the said compounds.