Recently, I read about the nomenclature of bicyclic compounds, by mentioning the number of carbons between two bridgeheads. But, then I came across the structure of a weird type of compound, whose IUPAC name is tricyclo-[2.2.1.02,6]-heptane. As the name suggests it must be a polycyclic compound, having three alicycles. 2.2.1 suggests that there are 2, 2 and 1 carbons between the bridgeheads. But what about the next part? What is the actual rule for naming tricyclo compounds?
The main ring of a polycyclic hydrocarbon ring system is selected so as to include as many skeletal atoms of the structure as possible. The main bridge is the bridge that includes as many of the atoms as possible that are not included in the main ring. Bridges other than the main bridge are called secondary bridges. Thus, tricyclic hydrocarbons consist of the main ring, the main bridge, and one secondary bridge.
The nomenclature of tricyclic hydrocarbons is based on the nomenclature of bicyclic systems. The additional steps for the naming of tricyclic hydrocarbons are described in the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) as follows.
P-23.2.5 Naming and numbering tricyclic alicyclic hydrocarbons
P-126.96.36.199 Tricyclic hydrocarbons having an independent secondary bridge are named on the basis of a bicyclic system (…). Rings not described by the bicyclic system are defined by citing the number of atoms in the independent secondary bridge as an arabic number. The locants of the two attachment points of the independent secondary bridge to the main ring are cited as a pair of superscript arabic numbers (lower number is cited first) separated by a comma.
The name of the tricyclic system is then constructed by citing:
(a) the prefix ‘tricyclo’, in place of ‘bicyclo’, indicating the presence of three rings in the polyalicyclic system;
(b) numbers indicating the bridge lengths, starting with the two branches of the main ring (…), followed by the main bridge (…), and the secondary bridge (with superscript locants separated by commas indicating its points of attachment to the main ring), all separated by full stops and placed in brackets,
(c) the name of the acyclic hydrocarbon having the same total number of skeletal atoms.
For example tricyclo[2.2.1.02,6]heptane
(The main ring is shown in red, the main bridge is shown in black, and the secondary bridge is shown in blue.)