| 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} |
| 484 | DF | .~.~ The hetero ring contains seven members including nitrogen and carbon {14} |
| 544 | DF | .~.~.~ The hetero ring contains chalcogen {2} |
| 546 | DF | .~.~.~.~ Polycyclo ring system which contains the hetero ring as one of the cyclos {2} |
| 547 | DF | .~.~.~.~.~ Tricyclo ring system which contains the hetero ring as one of the cyclos {4} |
| 551 |  | .~.~.~.~.~.~ Nitrogen bonded directly to ring carbon of the hetero ring |