To obtain the subject new compound exhibiting a wide antibacterial spectrum and an excellent antibacterial action, excellent in chemical stability and biological stability, especially in stability to renal dehydropeptidase-1, and useful as an antibacterial agent.

A compound of formula I [R1 is a (substituted) lower alkyl; R2 is H, a lower alkyl; R3, R5 are each H, R1, etc.; R4 is H, R1; A is a substituted linear alkylcarbonyl, etc.], its ester or salt. For example, (4R,5R,6S)-4- methyl-3-[(3S,5S)-1-(4-nitrobenzylcarbonyl)-5-{[N-(4-nitrobenzyloxycarbonyl) glycyl]aminomethyl}pyrrolidin-3-ylthio-7-oxo-6[(R)-1-(trimethylsilyloxy) ethyl]-1- azabicyclo [3.2.0]hept-2-ene-2-carboxylic acid 4-nitrobenzyl ester. The compound of formula I is obtained by reacting a compound of formula II (L is a releasable group) with a compound of formula III.