Five-membered Ring Shares A N And C Atom With A Four-membered Ring Wherein The Six-membered Ring Contains A C Or Hetero Atom In Position 4 And No Other Hetero Atom, The Four-membered Ring Contains The Single N As Hetero Atom; E.g., Penicillins And Analogs, Etc. (9/6561a) Patents (Class 987/53)