Numbering spiro junctions

A-42.1, but the spiro junction has priority for lowest numbers over un-saturation. 

Other chiral geometries are also possible, e g. those that possess perpendicular disymmetric planes. Optical activity will occur if there are two rings that contain an even number of atoms joined in a spiro manner, with each ring substituted at the carbon furthest from the spiro junction. In this case, an elongated tetra-hedron is formed. 

A spiro junction exists when two rings are connected through a single (carbon) center. Monospirocycles containing only two aliphatic rings are specified by the term spiro in front of the name of the corresponding acyclic system with the same number of carbon atoms. To discern possible... 

Linear polyspiro compounds of this kind are identified as dispiro-, trispiro- etc. -alkanes here too the numerical descriptors are obtained by enclosing the sums of atoms between the spiro centers in square brackets and separating them by full stops. The smaller peripheral ring is numbered first, and smaller numbers are preferred for the internal path lengths. This leads automatically to smallest possible locants for spiro centers when overall numbering starts on the smaller outer ring at a C atom next to the first spiro junction. 

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