Spiro compound

In organic chemistry, spiro compounds are compounds that have at least two molecular rings sharing one common atom. Simple spiro compounds are bicyclic (having just two rings).^{: SP-0 : 653, 839} The presence of only one common atom connecting the two rings distinguishes spiro compounds from other bicyclics.^{: 653ff  : 839ff} Spiro compounds may be fully carbocyclic (all carbon) or heterocyclic (having one or more non-carbon atom). One common type of spiro compound encountered in educational settings is a heterocyclic one— the acetal formed by reaction of a diol with a cyclic ketone.

The common atom that connects the two (or sometimes three) rings is called the spiro atom.^: SP-0 In carbocyclic spiro compounds like spiro[5.5]undecane, the spiro-atom is a quaternary carbon, and as the -ane ending implies, these are the types of molecules to which the name spirane was first applied (though it is now used general of all spiro compounds).^: 1138ff The two rings sharing the spiro atom are most often different, although they can be identical [e.g., spiro[5.5]undecane and spiropentadiene, at right].^{: 319f.846f}