In the cephalosporin field of β-lactam antibiotics, the electronegativity of the C-3' substituent affects antibacterial activity by altering the stability of the β-lactam bond to nucleophilic cleavage, and a good leaving group at C-3' usually enhances activity, especially in cephamycins. This led us to consider constructing penicillin nuclear analogues (bisnorisopenicillins I) with a potential leaving group (sulfur atom, which could be expelled as mercaptide similar to the vinylogous acetate of cephalosporins) incorporated directly into the ring system, expecting them to exhibit the broad antibacterial spectrum of cephalosporins. Herein, we report the total synthesis of bisnorisopenicillin and its monocyclic precursor, both showing potent antibacterial action. The synthesis was based on the selective cleavage of the thioacetate ester in monocycle III to the corresponding thiol, which cyclized via intramolecular halogen displacement to form the 3-thia-1-azabicyclo[3.2.0]heptane nucleus IV. Key steps included acid-catalyzed addition of benzyl glyoxylate to 2 to afford hydroxy iodide 3, treatment with thionyl chloride and potassium thioacetate to get monocycle 4, cyclohexylamine-mediated cleavage-cyclization of 4 to yield bicyclic ester 8, hydrolysis to obtain acid 9 and its sodium salt 10, and conversion of monocyclic salt 7 to bisnorisopenicillin salt 11. Antibacterial testing revealed that the monocyclic precursor 6 (sodium salt of acid 5) and bisnorisopenicillin 11 had activity against gram-negative bacteria comparable to thienylpenicillin but were significantly less active against gram-positive bacteria (e.g., Staphylococcus aureus, Streptococcus faecalis) than their penicillin and cephalosporin counterparts. Since 6 and 11 are racemic and mixtures of carboxylate epimers, a single stereoisomer might exhibit better in vitro activity.