| 540 / | HD | ORGANIC COMPOUNDS -- PART OF THE CLASS 532-570 SERIES | 
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| * | DD | ORGANIC COMPOUNDS (Class 532, Subclass 1) {1} | 
| 1 | DF | .~ HETEROCYCLIC CARBON COMPOUNDS CONTAINING A HETERO RING HAVING CHALCOGEN (I.E., OXYGEN, SULFUR, SELENIUM OR TELLURIUM) OR NITROGEN AS THE ONLY RING HETERO ATOMS {6} | 
| 200 | DF | .~.~ Hetero ring is four-membered containing nitrogen and having chalcogen double bonded directly to a ring carbon which is adjacent to the ring nitrogen {7} | 
| 203 | DF | .~.~.~ Polycyclo ring system containing the hetero ring as one of the cyclos {4} | 
| 302 | DF | .~.~.~.~ The ring nitrogen is shared by a five-membered ring {2} | 
| 303 | DF | .~.~.~.~.~ The five-membered ring contains an additional hetero atom {2} | 
| 304 | DF | .~.~.~.~.~.~ 1-thia-4-aza-bicyclo(3.2.0)hep-tane (including unsaturated; e.g., penam, etc.) {6} | 
| 310 | DF | .~.~.~.~.~.~.~ Having -C(=X)-, wherein X is chalcogen, bonded directly at the 3-position of the bicyclo ring system {2} | 
| 312 | DF | .~.~.~.~.~.~.~.~ Nitrogen bonded directly at the 6-position of the bicyclo ring system {3} | 
| 314 | DF | .~.~.~.~.~.~.~.~.~ Having -C(=X)-, wherein X is chalcogen, single bonded directly to the nitrogen (e.g., penicillin F, etc.) {9} | 
| 331 | DF | .~.~.~.~.~.~.~.~.~.~ Acyclic nitrogen or azide attached indirectly to the -C(=X)- group by acyclic nonionic bonding {3} | 
| 335 |  | .~.~.~.~.~.~.~.~.~.~.~ Additional acyclic nitrogen or acyclic chalcogen in the 6-position substituent |