The Diels-Alder reaction of buta-1,3-diene with ethene gives

The Diels-Alder reaction is a [4+2] cycloaddition in which the four diene carbons and the two dienophile carbons form a new six-membered ring. Two new sigma bonds form and one new pi bond remains between C-2 and C-3 of the former diene. The product is cyclohexene.