Diels-Alder: s-cis conformation

Methyl acrylate and butadiene

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Acylic dienes always react in the s-cis conformation. Butadiene normally prefers to be in the s-trans conformation for steric reasons. However it must adopt the unfavorable s-cis conformation, by rotating through its middle single bond to achieve a flat planar structure, before it reacts with the dienophile.

J. Sauer, Angew. Chemie Int. Ed. English, 1966, 5, 211–230.