in which the condensed system contains four or more hetero rings

Full Title

Heterocyclic compounds containing in the condensed system at least one hetero ring having nitrogen and oxygen atoms as the only ring hetero atoms (4-oxa-1-azabicyclo [3.2.0] heptanes, e.g. oxapenicillins C07D503/00; 5-oxa-1-azabicyclo [4.2.0] octanes, e.g. oxacephalosporins C07D505/00; analogues thereof having ring oxygen atoms in other position C07D507/00) > in which the condensed system contains four or more hetero rings