| 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} | 
| 2 | DF | .~.~ Cyclopentanohydrophenanthrene ring system containing {10} | 
| 6 | DF | .~.~.~ Spiro {3} | 
| 28 | DF | .~.~.~.~ The spiro includes the cyclopentanohydrophenanthrene ring system and a hetero ring {6} | 
| 30 |  | .~.~.~.~.~ Sulfur in the spiro hetero ring |