Question

Can someone explain what exocyclic bouble bond used in woodward fieser rules is (with pictures would be better)?

Answer 1

An exocyclic double bond would be the one where one of the double bond atoms is part of the ring. An example is shown below. Each exocyclic double bond adds 5 nm to the UV calculation.

In the first picture it is very clear that double bond is exocyclic. In the second picture, the double bond labelled as exocyclic is not inside the first ring and thus is an exocyclic bond to the first ring (attached to outside of the ring). This exocyclic double would be though an endocyclic double bond to the second ring.