= Solution
The axial penalties depend on the ring position. An axial methyl at C2 encounters the axial <hydrogens> on C4 and C6. The short C–O links and dioxane geometry make these <1,3-Diaxial interactions> particularly unfavorable. In contrast, an axial substituent at C5 points toward ring oxygens O1 and O3 rather than toward axial C–H groups at those positions. Those <oxygen> sites present much less steric obstruction than axial <hydrogens> on <carbon>.
\b[Avoiding the expensive axial C2 methyl outweighs the cost of axial tert-butyl at C5.] Transferring cyclohexane tert-butyl preferences unchanged would therefore give the wrong answer. This is primarily a position-dependent steric comparison, not the usual anomeric preference for an axial electronegative substituent at an acetal <carbon>. Primary <NMR> work on substituted dioxanes documents this axial C5 tert-butyl conformer.<Carbon-13 magnetic resonance. Conformation in some tert-butyl-1,3-dioxanes by Alan J. Jones and others>[1971, pp. 4772–4777, discussion of cis-2-methyl-5-tert-butyl-1,3-dioxane]{ref}
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