Is there a molecular orbital/Woodward-Hoffmann explanation as to why Diels-Alder reactants don't commonly undergo electrocyclization instead of cycloaddition?
closed as off-topic by Tyberius, Mithoron, Mathew Mahindaratne, Karsten Theis, Todd Minehardt Jul 31 at 23:39
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The Woodward-Hoffmann rules only tell you whether a reaction is thermally allowed or not, they do not give any indication of the relative rates of multiple allowed reactions.
I guess you're asking why the 4π component in a [4+2]-cycloaddition doesnt undergo a conrotatory 4π electrocyclic ring-closure? This is plausible, but I suppose even if it did happen the cyclobutene product is highly strained so the equilibrium would lie in favour of the ring-opened diene anyway.