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Diels-Alder reaction is a "cycloaddition reaction" (4+2 addition) in which generally diene donatesan electron to dienophile to form the cyclic adductas a final product.

Diels alder reaction diene is electron-rich, and dienophile is electron deficient; this condition increases the Diels-Alder reaction rate.

The diene 1-1’-bi(cyclopentene) is s-trans, but to undergo the Diels-Alder reaction, the s-trans have to rotate to the s-cis arrangement; thus, the 1-1’-bi(cyclopentene) diene is rotated to s-cis.

In the formation of the Diels-Alder product, the diene is drawn so that the ends of its two double bonds are near the dienophile 3-buten-2-one double bond. Then form two single bonds between the partner's diene and dinophile, convert the three double bonds into single bonds, and convert the former single bond of the diene into a double bond.\ to form the desired Diels-Alder product 1-((3Ar,4R,5aS)-1,2,3,3a,4,5,5a,6,7,8 decahydroas-indacen-4-yl) ethenone.

Formation of Diels-Alder product