= Solution
Number the dioxane O1–C2–O3–C4–C5–C6, with methyl at C2 and tert-butyl at C5. Their cis relationship is maintained on <ring inversion>, but because they occupy opposite ring positions, one is axial and the other equatorial in either chair. The two alternatives are \b[C2–Me equatorial/C5–tert-butyl axial], and \b[C2–Me axial/C5–tert-butyl equatorial].
The key signal is \b[U, the C5 methine, with only small <vicinal couplings>, 3.7 and 1.3 Hz]. An axial H5 would have large trans-diaxial couplings to axial <hydrogens> on C4 and C6. Their absence identifies H5 as equatorial, hence \b[the preferred conformer has axial tert-butyl and equatorial methyl]. It is not sufficient to guess from tert-butyl bulk. The tert-butyl singlet itself supplies no vicinal H–H coupling information about its orientation.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-dioxane-chairs.png]
{title=Preferred and alternative dioxane chairs, proton assignments and the O-inside fused-ring preference}
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