Naming Bridged Bicyclic Molecules
The naming of bridged bicycles has its own special kind of funk. Unlike the molecules you’ve likely come across so far, which will have a clear “longest chain” or “largest ring” to start from, trying to find the place to start based on those criteria alone will likely have you going in circles. Instead, bridged bicycles are named according to a unique system of their own. based on the length of their bridges, and then the overall number of carbons in the bicycle. The figure below walks through the process.
Once you’ve run through a few examples by yourself, I think you’ll find that naming bridged bicyclic molecules is actually fairly intuitive – as long as you can interpret the diagrams correctly. [Try making a model if you’re still stuck!] See if you can follow the naming of these compounds.
See that last example? We can also use bridged bicycle nomenclature to name fused rings as well! So bicyclo[4.4.0]decane is simply another name for “decalin” (without specifying the stereochemistry, of course).
Naming Spiro Compounds
Let’s wrap up by briefly covering “spiro” fused compounds. Since both “bridgehead” positions are on the same carbon, we won’t be able to use the same “bicyclo” nomenclature as before- but the process is very similar. We simply substitute “spiro” for “bicyclo” , insert the two bridge lengths, and place the suffix as before. So the molecule below is spiro[5.4]decane. Included next door are two other examples of spiro compounds, spiro[4.3]octane and spiro[5.2]octane.
In the next post – and last in this series – we’ll talk about one final, very interesting consequence of the fact that carbons can form rings: Bredt’s Rule.
Did you find the other bridged bicyclic isomer of decane that contains a six-membered ring? Here it is: bicyclo[4.2.2]decane. Note that the key difference is the presence of two 2-carbon bridges (in addition to the 4 carbon bridge) in contrast to the 1 and 3-carbon bridges seen in bicyclo[4.3.1]decane.
Secondly, a note about bridged ring fusions. Like with Bredt’s rule (more on that next post) once the size of the ring perimeter gets large enough [11 seems to be the minimum], the rules can be bent a bit. There are known examples of molecules with trans bridgehead ring fusions, sometimes known as “inside-outside” isomerism. A very prominent example is the natural product ingenol, isolated from Euphorbia species. A close look at the [4.4.1]undecane ring structure reveals that the two carbons on one of the bridges are in fact trans to each other. This does lead to ring strain [about 5.9 kcal/mol according to one calculation], but not enough to render closure of the ring impossible.