The Diels-Alder reaction is a [4+2] cycloaddition in which a conjugated diene, called the diene, reacts with an alkene group or an alkyne group in a compound, called the dienophile, to give a cycloadduct, called the Diels-Alder adduct.

eg. 1:

eg. 2:

mechanism:  The Diels-Alder reaction is a pericyclic reaction.

see also retro Diels-Alder reaction