| 544 / | 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) {1} |
| 1 | DF | .~.~ Hetero ring is six-membered having two or more ring hetero atoms of which at least one is nitrogen (e.g., selenazines, etc.) {6} |
| 63 | DF | .~.~.~ Six-membered hetero ring consists of oxygen, nitrogen and carbon (e.g., 1,2-oxazines, etc) {8} |
| 69 |  | .~.~.~.~ Boron or silicon containing |