| 548 / | 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) {4} | 
| 400 | DF | .~.~ Hetero ring is five-membered consisting of one nitrogen and four carbons (e.g., halopyrrolidines, etc.) {20} | 
| 416 | DF | .~.~.~ Polycyclo ring system having the five-membered hetero ring as one of the cyclos {5} | 
| 420 | DF | .~.~.~.~ Tetracyclo ring system having the five-membered hetero ring as one of the cyclos {3} | 
| 421 |  | .~.~.~.~.~ Plural ring hetero atoms in the tetracyclo ring system {2} | 
| 422 | DF | .~.~.~.~.~.~> Three-membered nitrogen containing hetero ring is cyclo in the tetracyclo ring system (e.g., mitomycin C, etc.) | 
| 423 | DF | .~.~.~.~.~.~> Plural chalcogens bonded directly to ring carbons of the five-membered hetero ring (e.g., cyclic imides, etc.) |