The Diels-Alder cycloaddition reaction is our only example of a pericyclic reaction: one in which the bond-making and bond-breaking in the transition state occur in a cyclic fashion. Notice that the three curly arrows in the diagram below proceed in a circle, the head of one toward the tail of the next:
In writing the mechanism focus on the two double bonds in the diene and the one double bond of the dienophile; don't be distracted by the nature of the substituents or the presence of "extra" carbons that will form a bridge in the product:
