A spiro compound
is a bicyclic organic compound
with rings connected through just one atom. The rings can be different in nature or identical. the connecting atom is also called the spiroatom
, most often a quaternary carbon ("spiro carbon"). All spiro compounds have the infix spiro
followed by square brackets  containing the number of atoms in the smaller ring and the number of atoms in the larger ring excluding the spiroatom itself. For example compound A is called 1-bromo-3-chloro-Spiro[4.5]-decan-7-ol
and compound B is called 1-bromo-3-chloro-spiro[3.6]-decan-7-ol
. The spiro compound consisting of a cyclohexane
ring and a cyclopentane
ring is called spiro[4.5]decane
. This nomenclature
was proposed by Adolf von Baeyer
in 1900 
Some spiro compounds exhibit axial chirality. Spiroatoms can be centers of chirality even when they lack the required four different substituents normally observed in chirality. When two rings are identical the priority is determined by a slight modification of the CIP system assigning a higher priority to one ring extension and a lower priority to an extension in the other ring. When rings are dissimilar the regular rules apply.
Spiro forms of lactones and oxazines are frequently used as leuco dyes, frequently displaying chromism - reversible change between their colorless and color form.
-  A. Baeyer, Systematik und Nomenclatur Bicyclischer Kohlenwasserstoffe, Ber. Dtsch. Chem. Ges. 33, 3771-3775 (1900).