| 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 |  | .~.~.~.~ The ring nitrogen is shared by a five-membered ring {2} |
| 303 | DF | .~.~.~.~.~> The five-membered ring contains an additional hetero atom {2} |
| 350 | DF | .~.~.~.~.~> The ring system is 4-aza-bicyclo(3.2.0)heptane (including unsaturated) and has sulfur bonded directly at the 2-position {1} |