Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

The feasibility of cycloaddition reactions under thermal and photochemical conditions can be predicted using a set of selection rules. If the total number of π electrons involved in the rearrangement is a multiple of 4n, the reaction is photochemically allowed. The reaction is thermally allowed if the total number of π electrons involved is 4n + 2.