Endo and Exo are used to designate the stereochemistry of an "R" group in a bicyclic ring system. Lets do an example problem.

QUESTION: Is the "R" group in the below structure in the endo or exo position?

SOLUTION:

• First, identify the longest and shortest bridges relative to the "R" group.

• Second, if the R group is on the same side as the SHORTEST bridge then it is considered EXO

if the R group is on the same side as the LONGEST bridge then it is considered ENDO

Thus, the R group is this example is exo.