Syntheses of (±)-cis-3-hydroxycarbonyl-7-(phenylacetamido)carbacephem (10 ), (±)-cis-3-methoxycarbonyl-7-(phenylacetamido)carbacephem (11 ), (±)-cis-2-chloro-3-hydroxycarbonyl-7-(phenylacetamido)carbacephem (14 ), and (±)-cis-2-chloro-3-methoxycarbonyl-7-(phenylacetamido)carbacephem (15 ) were accomplished. These four heretofore undescribed compounds 10 , 11 , 14 , and 15 showed notable activity against Staphylococcus aureus FDA 209P, Escherichia coli ATCC 39188, Pseudomonas aeruginosa 1101-75, Klebsiella pneumoniae NCTC 418 as well as the β-lactamase producing organisms E. coli A9675, P. aeruginosa 18S-H and methicillin-resistant organism S. aureus 95. The electronic activation of lactam moieties of 11 and 15 enhanced remarkably their antibacterial activity relative to those of the 10 and 14 . The chlorine atom in 14 and 15 , acting as an effective leaving group, also resulted in an increment in the biological activity in comparison with those of the parent carbacephemes 10 and 11 . The mode of action related to 14 and 15 can be explained by a non-classical [1,4]-elimination process.