A spiro compound is a bicyclic organic compound with rings connected through just one atom. The rings can be different in nature or identical. The connecting atom is also called the spiroatom, most often a quaternary carbon ("spiro carbon"). All spiro compounds have the infix spiro followed by square brackets [] containing the number of atoms in the smaller ring and the number of atoms in the larger ring excluding the spiroatom itself. For example compound A is called 1-bromo-3-chlorospiro[4.5]decan-7-ol and compound B is called 1-bromo-3-chlorospiro[3.6]decan-7-ol. The spiro compound consisting of a cyclohexane ring and a cyclopentane ring is called spiro[4.5]decane. This nomenclature was proposed by Adolf von Baeyer in 1900 [1].
![1-Brom-3-Chlor-Spiro[4.5]-decan-7-ol B: 1-Brom-3-Chlor-Spiro[3.6]-decan-7-ol](http://en.wikipedia.org/wiki/File:Spiroverbindung_Nomenklatur.svg)
An example of a spiro compound with a trivial name: spiropentadiene.
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Polyspiro compounds
If there are two or more spiroatoms in a compound (a polyspiro compound), the prefixes di-, tri-, tetra-, etc. are to represent the amount of spiroatoms. A polyspiro compound is numbered starting from a ring that holds only one spiroatom (i.e. a terminal ring) starting from the first atom on that ring away from the spiroatom and around the compound, ending on that spiroatom. The compound is numbered so that the spiroatom numbers are the lowest possible. For example if the spiroatom numbers are 3 and 5 for one numbering and 3 and 6 for another way of numbering, 3 and 5 are preferred; the first spiroatom number for each pair is the same, but for the second number, 5 is preferred. When comparing sequences of spiroatom numbers, in the first instance of different numbers, whichever spiroatom number is lower (or lowest) is contained by the preferred numbering, regardless of any following spiroatom numbers.
In polyspiro nomenclature the first number within the brackets is the amount of atoms starting from the atom numbered 1 to the last atom on the first ring before the first numbered spiroatom. The rest of the numbers represent the amount of atoms between the spiroatoms, or the amount of atoms at a terminal ring, going in the direction of the numbering. Each number is separated by a period. In the example to the left there are two atoms (numbered 1 and 2) before the first spiroatom (3). Between spiroatoms 3 and 4 there are zero, and likewise between 5 and 4, and 6 and 5. At the second terminal ring there are two; there is one between 6 and 5, 5 and 4, and 4 and 3. This gives the sequence 2,0,0,0,2,1,1,1. The total number of atoms determines the acyclic alkane name that follows, in this case, undecane. For clarity, superscripts of the spiroatom numbers appear after the number of atoms that are the last to encounter the spiroatom, going in the direction of the numbering.
In the example to the right, there are two spiroatoms, so the name starts with dispiro-. In the first terminal ring, there are two atoms, showing so far the name dispiro[2. Between the next two spiroatoms there is one atom, and four at the other terminal ring, revealing the name dispiro[2.1.35. The superscript for the second spiroatom, numbered 5, was added as this is the last time it will be encountered after the three atoms at the terminal ring, following the path of numbered atoms. After adding atoms 9 and 10 between spiroatoms 3 and 5, then adding the name of the alkane that has the same amount of atoms as this spiro compound, decane, the name dispiro[2.1.35.23]decane is revealed. The superscript for spiroatom 3 is added after the the atoms 9 and 10 are represented (by a 2) since after these two atoms are encountered the spiroatom 3 is encountered for the last time.
Chirality
Some spiro compounds exhibit axial chirality. Spiroatoms can be centers of chirality even when they lack the required four different substituents normally observed in chirality. When two rings are identical the priority is determined by a slight modification of the CIP system assigning a higher priority to one ring extension and a lower priority to an extension in the other ring. When rings are dissimilar the regular rules apply.
Spiro forms of lactones and oxazines are frequently used as leuco dyes, frequently displaying chromism - reversible change between their colorless and color form.
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References
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