Rule A-42. Compounds: Method 2 42.1 - (Alternate to Rules A-41.1 and A-41.2) - When two dissimilar cyclic components are united by a spiro union, the name of
the larger component is followed by the affix "spiro" which, in turn, is followed by the name of the smaller component. Between the affix "spiro" and the name of each component system is inserted the
number denoting the spiro position in the appropriate ring system, these numbers being as low as permitted by any fixed enumeration of the component. The components retain their respective
enumerations but numerals for the component mentioned second are primed. Numerals I may be omitted when a free choice is available for a component.
42.2 - (Alternate to A-41.3) - Rule A-41.3 applies also with appropriate different enumeration, where nomenclature is according to Rule A-42.1, but the spiro
junction has priority for lowest numbers over unsaturation.
42.3 - (Alternate to A-41.5) - The nomenclature of Rule A-41.5 is applied also to monocyclic components with identical saturation, the spiro union being numbered 1.
42.4 - (Alternate to A-41.6 and A-41.7) - Polycyclic compounds containing more than one spiro atom are named in accordance with Rule A-42.1) starting from
the senior end-component irrespective of whether the components are simple or fused rings.
See Recommendations'93 R-2.4.3
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