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.