(also spirane), any of the compounds having one carbon atom or, more rarely, one atom of another element, such as Si, P, or As, common to two rings. The methods for obtaining spirans are based for the most part on the cyclization of bifunctional derivatives, in which both hydrocarbon residues containing functional groups are joined to the same atom in a ring. An example of this process is seen in the reaction
1 ,l-Bis-(methyl bromide)-cyclohexane Spiro [2.5] octane
Since the joined rings lie in mutually perpendicular planes, asymmetrically substituted spirans can be separated into optical antipodes.