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Abicyclic molecule (from bi 'two' and cycle 'ring') is a molecule that features two joinedrings.[1] Bicyclic structures occur widely, for example in many biologically important molecules likeα-thujene andcamphor. A bicyclic compound can be carbocyclic (all of the ring atoms are carbons), orheterocyclic (the rings' atoms consist of at least two elements), likeDABCO.[2] Moreover, the two rings can both bealiphatic (e.g.decalin andnorbornane), or can bearomatic (e.g.naphthalene), or a combination of aliphatic and aromatic (e.g.tetralin).[3]
Many properties of saturated bicyclic hydrocarbons are those of mono and acyclic analogues: colorless, combustable, and resistant to many reactions. Exceptions apply to the cyclopropane derivatives, which can undergo ring opening. The distinctive feature of the bicyclic compounds is their relative rigidity. Whereas only one stereoisomer exists for methyl cycloheptane, seven are possible for methylnorbornane, including three pairs of enantiomers. Ring fusions are a potential source of isomerism, but for small rings, the fusions are alwayscis. For the bicyclic C10decalin, bothcis- andtrans-fused isomers are observed, the former being chiral, though notresolvable at room temperature due to rapidinversion.[4]
Bridgehead atoms are the carbons that form three C-C bonds. The parent (that is, unsubstituted) bicyclic hydrocarbons have two bridgehead carbon atoms.
Inspiro compounds, two rings share one single atom, the spiro atom, which is usually aquaternary carbon.[5] An example of a spirocyclic compound is thephotochromic switchspiropyran.
The naming of bicyclic molecules is described byIUPAC nomenclature.[6][7] The root of the compound name depends on the total number of atoms in all rings together, possibly followed by asuffix denoting thefunctional group with thehighest priority. Numbering of the carbon chain always begins at one bridgehead atom (where the rings meet) and follows the carbon chain along the longest path, to the next bridgehead atom. Then numbering is continued along the second longest path and so on. Fused andbridged bicyclic compounds get theprefixbicyclo, whereasspirocyclic compounds get the prefixspiro. In between the prefix and the suffix, a pair of brackets with numerals denotes the number of carbon atoms between each of the bridgehead atoms. These numbers are arranged in descending order and are separated by periods.For example, the carbon frame ofnorbornane contains a total of 7 atoms, hence the root nameheptane. This molecule has two paths of 2 carbon atoms and a third path of 1 carbon atom between the two bridgehead carbons, so the brackets are filled in descending order: [2.2.1]. Addition of the prefixbicyclo gives the total name bicyclo[2.2.1]heptane.
The carbon frame ofcamphor also counts 7 atoms, but is substituted with acarbonyl. Numbering of the carbon frame starts at the bridgehead atom with thehighest priority (methyl goes beforeproton), hence the bridgehead carbon in front gets number 1, the carbonyl gets number 2 and numbering continues along the carbon chain following the longest path, until the doubly substituted top carbon (number 7). Norbornane has two paths of 2 carbon atoms and one path of 1 carbon atom between the two bridgehead carbons, so the numbers within the brackets stay [2.2.1].
When naming simple fused bicyclic compounds, the same method as for bridged bicyclic compounds is applied, except the third path between the two bridgehead atoms now consists of zero atoms. Therefore, fused bicyclic compounds have a "0" included in the brackets. For example,decalin is named bicyclo[4.4.0]decane. The numbers are sometimes omitted in unambiguous cases. For example, bicyclo[1.1.0]butane is typically called simplybicyclobutane.
The bicyclic structures emphasized apply to many compounds where CH2 and CH site are replaced by other elements. CH2 is often replaced by O, NH and NR, as well as S. CH is often replaced by N and P. Theheterocyclic moleculeDABCO has a total of 8 atoms in its bridged structure, hence the root nameoctane. The two bridgehead atoms arenitrogen instead ofcarbon atoms. Its official name is 1,4-diazabicyclo[2.2.2]octane.
Of the major classes of biomolecules, only thepurines are bicyclic. Otherwise, bicyclic compounds are restricted to cofactors and secondary metabolites.Terpenes andalkaloids, which are known for their structural complexity, are often bicyclic or still more complex.[citation needed]
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