Classification of Polycyclic Compounds
Bicyclic or polycyclic compounds are molecules that contain two or more
rings that are joined together. There are three different ways that rings can be joined.
The most common are fused rings, where two atoms and the bond between them are
shared. When more than two atoms are shared, the result is a bridged system, so-called
because the shared atoms form a "bridge". Finally, when two rings are joined at a single
atom the result is a spiro linkage.
Figure 3.1: Fused, bridged, and spiro bicyclic molecules with their trivial
names.
Conformations of Decalin
Ring fusion can occur either in a cis- or trans- fashion.
Cis-fused rings are joined with bonds that are cis with respect to both rings, and trans-
fused rings are joined with trans bonds. While cis fusion is possible with rings of all
sizes, trans fusion occurs only on sufficiently large rings that are able to accommodate the increased
distance between two trans bonds. For instance, trans-fused cyclopentanes
are much less stable than trans-fused cyclohexanes, or trans-decalin.
The most stable conformation of both isomers of decalin is one in which both
rings take on chair conformations. Drawing the cis isomer is a bit tricky because
one of the chairs has to be drawn sideways. While the cis isomer can take on two
conformations via simultaneous chair flips, the trans isomer is conformationally rigid.
Neither ring of the trans isomer can undergo a chair flip because doing so would make
the other ring impossibly strained.
Figure 3.2: Conformations of cis- and trans-decalin
