New racemic thienamonobactams (21) and (28), designed on the basis of hybridization between thienamycin (5) and aztreonam (6), were successfully synthesized by exploiting the [2+2] cycloaddition reaction of diketene with imine (7). Compound (28) exhibited significant inhibitory activity (ID 5 0 = 37 μM) against Citrobacter freundii cephalosporinase.