In the absence of an indication to the contrary, and with the exception referred to below, a compound is always classified in the last appropriate place in the classification system, e.g. a compound containing an acyclic chain and a heterocyclic ring is classified only as a heterocyclic compound, and a sterol is classified only as a cyclopentanophenanthrene compound. In general, and in the absence of an indication to the contrary (such as groups Fulltext... Hierarchy... Expanded...C07C 59/24, Fulltext... Hierarchy... Expanded...C07C 59/26), the terms "acyclic" and "aliphatic" are used to describe compounds in which there is no ring; and, if a ring were present, the compound would be taken by the "last place" rule to a later group for cycloaliphatic or aromatic compounds, if such a group exists. Where a compound or an entire group of compounds exists in tautomeric forms, it is classified as though existing in the form which is classified last in the system, unless the other form is specifically mentioned earlier in the system.
Albumens, proteins, alkaloids of unknown structure, glucosides of unknown structure are examples of compounds to be found unter Fulltext... Hierarchy... Expanded...C07G.
Inventions dealing with the preparation of chemical compounds might be classified both according to the type of reaction employed and according to the type of compound prepared, if suitable groups or sub-groups exits.