Compounds of the formula (III): ##STR1## wherein R.sub.1 is alkyl of up to 3 carbon atoms and R.sub.2 is alkyl of up to 3 carbon atoms optionally joined to R.sub.1 so that R.sub.1 and R.sub.2 and the carbon atom to which they are attached form a spiro-cyclopentyl or spiro-cyclohexyl ring, and wherein the configuration about the .beta.-lactam ring is trans, are useful in the preparation of antibacterial .beta.-lactam compounds.A process for the preparation of the compounds of the formula (III), is also desired.