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. The common atom is designated as the 'spiro atom'. According to the number of spiro atoms present, the compounds are distinguished as
monospiro, dispiro, trispiro, etc., ring systems. The following recommendations apply only to the naming of parent hydrides containing free spiro unions.
R-2.4.3.1 Monospiro parent hydrides consisting of two homogeneous saturated monocyclic rings are named by placing 'spiro' before the name of the acyclic parent hydride with the same total number of skeletal atoms; heteroatoms, if any, in an otherwise hydrocarbon structure are designated by replacement nomenclature, i.e., by 'a' prefixes (see R-9.3) placed before the spiro prefix. The numbers of skeletal atoms linked to the spiro atom in each ring are indicated by arabic numbers separated by a full stop, cited in ascending order, and enclosed in square brackets; this descriptor is placed between the spiro prefix and the name of the parent hydride.
Numbering starts with a ring atom next to the spiro atom and proceeds first through the smaller ring, if one is smaller, and then through the spiro atom and around the second ring.
The method is also used for monospiro systems of alternating skeletal atoms.Examples to R-2.4.3.1
R-2.4.3.2 Polyspiro parent hydrides consisting of unbranched assemblies of three or more saturated homogeneous monocyclic rings are named by using the prefix 'dispiro-', 'trispiro-', 'tetraspiro-', etc., instead of 'spiro-' in front of the name of the acyclic parent hydride that has the same total number of skeletal atoms. Heteroatoms in an otherwise hydrocarbon system are designated by replacement nomenclature using 'a' prefix (see R-9.3). The numbers of skeletal atoms linked to the spiro atom in each terminal ring and between the spiro atoms in the other rings are given by arabic numbers separated by full stops cited in the same order as the numbers proceed around the ring and enclosed in square brackets. Numbering begins with a ring atom next to the terminal spiro atom of the smaller terminal ring, proceeding around that terminal ring through its terminal spiro atom and, by the shortest path, through each of the other spiro atoms, around the other terminal ring, and then back to the first terminal ring.Example to R-2.4.3.1
Note: Extension of this procedure to branched polyspiro systems may lead to ambiguity.Example to R-2.4.3.2
R-2.4.3.3 Spiro parent hydrides containing polycyclic ring systems, such as a fused ring system, are named by placing the prefix 'spiro-', 'dispiro-', 'trispiro-', etc., in front of the names of the components, which are cited in order of occurrence beginning with the terminal component earliest in alphabetical order, and enclosed in square brackets. Established numbering of each component is retained, but those of the second cited and succeeding components are primed serially.
Examples to R-2.4.3.3
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