Observe the structure (a). Doesn’t it look like our old friend housane after a tornado? It kept its roof but only just.
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| (a) |
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Let us number it in the following fashion:
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| (a) |
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The atom 3 is a quaternary carbon while the rest are secondary carbons. Or, using the graph theory language, we can say that in the graph (a) the degree of vertex 3 is 4 and the degrees of the rest of vertices are 2. The atom 3 is also known as a spiro atom [1, SP-0] while the whole structure is an example of spiro union.
The structure (a) is named spirohexane or, more systematically, spiro[2.3]hexane. The ‘spiro’ and ‘hexane’ parts, respectively, refer to the spiro union and the fact that (a) contains six carbon atoms*. The ‘[2.3]’ bit looks very similar to von Baeyer’s bridge descriptor, to the degree that the IUPAC recommendations [1] call it “von Baeyer descriptor”. In fact, it was von Baeyer who proposed the name spirocyclans† for systems containing two rings which contain one quaternary carbon in common, in the same paper where he first described the nomenclature of bicyclic hydrocarbons [2]. However, you’ll notice some important differences between bridge descriptor and spiro descriptor (as I call it; I think it is a bit weird to call the path from point A to point A “a bridge”).
There are two circular paths from vertex 3 back to 3, viz. 3–1–2–3 and 3–4–5–6–3. Just like a bridge descriptor, a spiro descriptor contains the numbers of intervening carbon atoms. So, ‘2’ for 3–1–2–3 and ‘3’ for 3–4–5–6–3, hence ‘[2.3]’ in (a). Since the minimum number of atoms in a ring is three, a spiro descriptor cannot have either ‘0’ or ‘1’ in it. Also, the numbers are cited from lower to higher, so spiro[2.3]hexane, not spiro[3.2]hexane.
You may recall that in von Baeyer names, we number the main (i.e. larger) ring first, starting from one of bridgehead atoms. Well, the numbering of a spiro system is done very differently [1, SP-1.2]:
Monospiro hydrocarbons with two monocyclic rings are numbered consecutively starting in the smaller ring at an atom next to the spiro atom, proceeding around the smaller ring back to the spiro atom and then round the second ring.
For me, it does not make any sense. I would give the locant number 1 to the most interesting atom, viz. the spiro atom.
What if there is more than one spiro junction? Consider the structure (b):
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| (b) |
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Since there are two spiro junctions now, we have to add a multiplier to ‘spiro’, thus ‘dispiro’. The spiro descriptor becomes more complex too. We probably should call it spiro-bridge descriptor, because it contains alternating cycles and bridges: cycle 3–1–2–3 (‘2’), bridge 3–4 (‘0’), cycle 4–5–6–4 (‘2’) and bridge 4–7–8–3 (‘2’), hence dispiro[2.0.2.2]octane.
More spiro junctions make descriptors messier as now the locants have to be added [1, SP-1.4]:
For trispiro and higher spiro systems each time a spiro atom is reached for the second time its locant is cited as a superscript number to the corresponding number of linking atoms.
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| (c) |
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Although trispiro[2.0.2.0.2.0]nonane seems to be a reasonable name for (c), the preferred IUPAC name would be trispiro[2.0.24.0.27.03]nonane, where ‘24’, ‘27’ and ‘03’ correspond to cycle 4–5–6–4, cycle 7–8–9–7 and bridge 7–3, respectively.
There’s only so much that one can do naming the structures as spiroalkanes. Already in 1911, Dan Rădulescu proposed to name each ring system in a spiro union separately [3]. For example, it is obvious that the structure (d) consists of two identical components:
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| (d) | (e) |
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The component (e) is called 9H-fluorene, or simply fluorene. Two components are joined at the position 9. The locants of the second components are primed, therefore the spiro atom is indicated as ‘9,9′’, which is a short way to say “the common atom is found at the position 9 of the first component and the position 9′ of the second component”. The resulting name recommended by IUPAC is 9,9′-spirobi[fluorene]. The CAS name is almost identical except it also contains the indicated hydrogen within square brackets, thus 9,9′-spirobi[9H-fluorene].
Can we combine the names of non-dentical ring components in a spiro union? Yes we can.
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| (f) | (g) | (h) |
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For instance, fluoran (f) — not to be confused with fluorene (e) — is named systemetically as the derivative of a spiro union of 2-benzofuran (g) and xanthene (h). The spiro atom is found at the position 1 of 2-benzofuran and position 9 of xanthene, thus spiro[2-benzofuran-1,9′-xanthene]. When the oxo group is added at the position 3 of 2-benzofuran, the whole structure is named as a ketone, i.e. acquires the terminal ‘one’, with elision of the terminal ‘e’ in ‘xanthene’: 3H-spiro[2-benzofuran-1,9′-xanthen]-3-one. IUPAC favours citing indicated hydrogen in front of the complete name, while CAS index names have indicated hydrogen in parentheses after the locants, as in spiro[isobenzofuran-1(3H),9′(9′H)-xanthen]-3-one ‡.
As you can see, the spiro nomenclature is a jumble of different methods: spiro[x.y]alkane names are a variation on theme of von Baeyer names; spiro[component A-locant,locant′-component B] names are built in the fashion of fused ring names; and the spirobi[component] names are very much like identical-ring assembly names.
| * | The name ‘spirohexane’, as well as that of the simpler (and the simplest possible) spiro structure, spiropentane, does not really need a descriptor because there is only one way to construct a spiro union containing six (or five) carbon atoms. Similarly, von Baeyer descriptor ‘[1.1.0]’ in bicyclo[1.1.0]butane is redundant, as ‘bicyclobutane’ is unambiguous. |
| † | To quote: “Welche ein beiden Ringen gemeinschaftliches quaternäres Kohlenstoffatom enthalten: Spirocyclane, von »spira« die Brezel” (That <where> both rings contain a common quaternary carbon atom: spirocyclans, from spira “the pretzel”) [2]. |
| ‡ | Somewhat confusingly, indicated hydrogens are there not to indicate the presence of actual hydrogen atoms but to fix the positions of double bonds in mancude systems. For instance, in spiro[isobenzofuran-1(3H),9′(9′H)-xanthen]-3-one (f) neither position 3 nor 9′ has any hydrogen atoms attached. |
References
- Moss, G.P. (1999) Extension and revision of the nomenclature for spiro compounds (IUPAC Recommendations 1999). Pure and Applied Chemistry 71, 531—558.
- Baeyer, A. (1900) Systematik und Nomenclatur bicyclischer Kohienwasserstoffe. Berichte der Deutschen Chemischen Gesellschaft 33, 3771—3775.
- Radulescu, D. (1911) Über die Nomenklatur der Spirane. Berichte der Deutschen Chemischen Gesellschaft 44, 1023—1026.
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