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). The presence of only one common atom connecting the two rings distinguishes spiro compounds from other bicyclics. 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. 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). 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].