| 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} |
| 200 | DF | .~.~ Oxygen containing hetero ring (e.g., dioxirane, etc.) {16} |
| 263 | DF | .~.~.~ Lactones (i.e., -C(=X)O-, wherein X is chalcogen, is part of the hetero ring) {6} |
| 328 |  | .~.~.~.~ Four-membered lactone ring formed {1} |
| 329 | DF | .~.~.~.~.~> From a ketene (HCH=C=O, wherein H may be substituted) |