β-内酰胺环 Common nucleus CH3 Ampicillin CH-C-NH-CH-CH C o=c—N—cH-cooH 舌 s、Ch3 Methicillin C-NH--CH-CH C OCH O=c-N—cH-cooH 乏°虽 Carbenicillin \/CH3 CH---C-NH-CH--CH C CH3 COOH o=c—N—CH-CooH g Oxacillin C—c—c-NH-CH-CHC CH cN—cH-cooH O CH3 B-lactam ring Semisynthetic penicillins9 β-内酰胺 环