Which of the following terms best describes a Diels-Alder reaction?

A) a [4+2] cycloaddition
B) a [2+2] cycloaddition
C) a sigmatropic rearrangement
D) a 1,3-dipolar cycloaddition
E) a substitution reaction

Cycloaddition

A chemical reaction in which two or more unsaturated molecules (or parts of the same molecule) combine to form a cyclic compound.

Sigmatropic Rearrangement

A type of pericyclic reaction in which a σ-bond (sigma bond) changes position relative to a π system (pi system) within a molecule, following specific suprafacial or antarafacial pathways.

Diels-Alder

A chemical reaction between a diene and a dienophile, resulting in the formation of a cyclohexene derivative.