= Solution
\b[C has the original cyclohexadiene connectivity but trans, rather than cis, adjacent methyl groups.] Draw one methyl wedge and the other dash, keeping both phenyl groups at the diene termini. One <enantiomer> is shown below; reversing both stereobonds gives its mirror image. For the symmetric achiral starting framework under achiral conditions, the two trans <enantiomers> are not distinguished by this orbital-symmetry argument.
\Image[/past-exam-of-the-chemistry-course-of-the-year-3-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-3-electrocyclic-product.png]
{title=Trans dimethyl cyclohexadiene from thermal closure of the photogenerated triene}
{description=Original product drawing. Opposite stereobonds specify the two methyl faces; the mirror image is obtained by reversing both.}
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