| 540 / | HD | ORGANIC COMPOUNDS -- PART OF THE CLASS 532-570 SERIES |
| * | 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} |
| 543 | | .~.~.~ Spiro |