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Recently we did an experiment of Diels–Alder reaction and we left the adduct overnight. I've read a paper of Woodward that says the adduct of Diels–Alder thought to be endo, whereas in our experiment it turned out to be exo.

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    $\begingroup$ The kinetic product is often endo while the thermodynamic product is often exo. Kinetic product is endo due to favourable secondary orbital interactions during reaction while thermodynamic product favoured due to decreased steric hindrance in the exo product. $\endgroup$ Commented Nov 30, 2018 at 0:00
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    $\begingroup$ See this earlier Q&A. $\endgroup$
    – ron
    Commented Aug 27, 2019 at 15:01

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As you probably know, the Diels–Alder (DA) reaction is reversible — the easiest example is the reaction of cyclopentadiene, which polymerizes at room temperature, which — if needed as monomer — has to be cracked just in advance.

In terms of the selectivity you observed, the endo-oriented product of the DA reaction often is the kinetic one, the exo-oriented product the thermodynamic preferred one. Which is, if the rate of a DA reaction is to be increased while still obtaining endo-preference, an increase of the reaction temperature is skipped in favour a Lewis-catalysis, addition of a perchlorate, or high (external) pressure.

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