Bicyclic molecule

1. At two mutually bonded atoms or
2. Across a sequence of atoms (bridgehead) - an example is norbornane, bicyclo[2.2.1]heptane - or
3. At a single atom (spirocyclic, forming a spiro compound)

All these systems occur in naturally-occurring organic compounds, in about the order of commonness of the list above.

Bicyclic molecules have a strict nomenclature. On its simplest level the parent hydrocarbon is the equivalent open carbon alkane. For bridged compounds, the prefix bicyclo is added, followed by, between brackets and in descending order, the number of carbon atoms in each of the rings not counting the bridgeheads. For example in bicyclo[2,2,1]heptane the carbon frame contains a total of 7 atoms hence the parent name heptane. This molecule has three bridges having 2, 2 and 1 carbon atoms hence the prefix bicyclo[2,2,1]. For spiro compounds, terms like spiro[2,4] are used, indicating that there are 3- and 5-membered rings (because the spiro atom itself is not counted) meeting at the spiro atom.

A bridge is an unbranched chain of atoms or an atom or a covalent bond connecting two bridgeheads in a polycyclic compound.

The main bridge is a bridge which connects the two main bridgeheads.

A secondary bridge is any bridge not included in the main ring or the main bridge.

An independent secondary bridge links bridgeheads which are part of the main ring or main bridge.

A dependent secondary bridge links at least one bridgehead which is part of a secondary bridge.

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