Cis–Trans Isomerism of Fused and Bridged Ring Systems
Fused bicyclic systems are those in which two rings share two and only two atoms.
In such systems, there is no new principle. The fusion may be cis or trans, as illustrated
by cis- and trans-decalin. However, when the rings are small enough, the
trans configuration is impossible and the junction must be cis. The smallest trans
junction that has been prepared when one ring is four membered is a four–five junction;
trans-bicyclo[3.2.0]heptane (78) is known.232 For the bicyclo[2.2.0] system