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Structure 1 (1-3butadiene.mol) The Diels alder reaction is one of the best examples of the [4+2] Cycloaddition reaction. In this reaction, dialkene (diene with electon donating groups) and dienophile (alkene with an electron withdrawing group) upon heating react to form a cyclic compound. Always the cis conformer of the diene participates in the reaction. 1,3-Butadiene, the simplest of the diene, exists in s-trans and s-cis conformations. The s-trans conformation is preferred due to steric reasons. Due to very low rotational barrier about the central sigma bond ( 30 kJ/mol at RT), conversion to s-cis conformation is rapid. Hence less favourable s-cis conformer is available for the cycloaddition reaction.

Structure 2 (DAproduct1.mol) Typical example for the Diels alder reaction is, 2,3-dimethyl-1,3-butadiene as diene and 2-propenaldehyde as dienophile react to form a 6-membered cyclic product. Here, two new sigma bonds are formed and a single bonds converts to a double bond. The transition state resembles a 6-membered aromatic ring.

Structure 3 (DAproduct2.mol) Another common example is the reaction between 1,3-butadiene and maleic anhydride. This reaction gives a bicyclic product.