| 549 / | HD | ORGANIC COMPOUNDS -- PART OF THE CLASS 532-570 SERIES | 
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| * | DD | ORGANIC COMPOUNDS (Class 532, Subclass 1) {1} | 
| * | DD | .~ HETEROCYCLIC CARBON COMPOUNDS CONTAINING A HETERO RING HAVING CHALCOGEN (I.E., OXYGEN, SULFUR, SELENIUM, OR TELLURIUM) OR NITROGEN AS THE ONLY RING HETERO ATOMS (Class 540, subclass 1) {2} | 
| 1 | DF | .~.~ Sulfur containing hetero ring (e.g., thiiranes, etc.) {9} | 
| 13 | DF | .~.~.~ The hetero ring is six-membered {3} | 
| 23 | DF | .~.~.~.~ Polycyclo ring system having the hetero ring as one of the cyclos {2} | 
| 26 | DF | .~.~.~.~.~ Tricyclo ring system having the hetero ring as one of the cyclos {1} | 
| 27 |  | .~.~.~.~.~.~ Chalcogen attached directly to the hetero ring by nonionic bonding |