The Diels–Alder reaction in Chapter 16 is one example of a cycloaddition.
Two features determine the course of the reactions: the number of π bonds involved and
whether the reaction occurs in the presence of heat (thermal conditions) or light (photochemical conditions). These reactions follow a set of rules based on orbitals and symmetry ! rst proposed
by R. B. Woodward and Roald Hoffmann in 1965, and derived from theory described by Kenichi
Fukui in 1954.