Scheme 1. In intramolecular [2+2] photocycloaddition reactions, two regioisomeric products A and B are possible. The photoexcited olefin is often embedded in a ring to avoid competitive E/Z isomerization (dotted line). If the tether is attached to the reactive olefin components as shown, product A is called the straight and product B the crossed photocycloaddition product.