Potent antibacterial cephalosporin analogues of the following formula in forms of free acids, pharmaceutically acceptable salts, or esters preparable from penicillins through azetidinone derivatives by way of a new process, and utilizable as active ingredients in bactericidal compositions for pharmaceutical and veterinary use: ##STR1## (wherein Acyl is selected from phenylacetyl, D-mandeloyl, .alpha.-phenylmalonyl, D-.alpha.-(3-methanesulfonyl-2-oxoimidazolidin-l-yl)carbonamido-.alpha.-phenylacetyl, and 2-thienylacetyl).