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Diels-Alder reaction was discovered by the German chemists Otto Diels and Kurt-Alder in 1928. Diels-Alder reaction is an important organic reaction which includes a substituted alkene in the form of a dienophile and a conjugated diene. It proceeds via concerted mechanism. Diels alder reaction can be used to generate 6 membered rings since there is a simultaneous construction of two new Carbon-carbon bonds.

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Diels-Alder reaction is a pericyclic reaction. It is a reaction between a conjugated diene ( two double bonds) and a dienophile( a substituted alkene with electron withdrawing group). The reaction is stereospecefic i.e., the cis dienophile will give cis stereochemistry in product and trans dienophile will generate product with cis stereochemistry.

Diene is an organic compound which comprises of two conjugated pi bonds (4 pi bonds) while dienophile is a diene loving compound containing 2 pi electrons. When the interaction between them, occurs a transition state without any additional energy barrier, is obtained.

The diene rotates to its s-cis conformer to ease the occurrence of reaction with dienophile. Since, you know that the stereochemistry of the dienophile is retained in the product, trans realationship of two esters will be retained in the product.