= Solution
Use the upper-ring chair placing $H_a$ or $H_c$ axial. The aryl ester <substituent> on that <carbon> is then equatorial. Use a lower-ring chair placing $H_b$ or $H_d$ axial, with the attached <side chain> equatorial. The spiro configuration distinguishes the two structures: in \b[A], the lower-ring <oxygen> bond is axial with respect to the upper ring, while the upper-ring <oxygen> bond is equatorial with respect to the lower ring. In \b[B], those roles reverse.
Thus the <oxygen> placements at the shared spiro <carbon> are \b[axial/equatorial for A and equatorial/axial for B]. Side-chain conformations are unnecessary, but changing the lower chair without tracking its <substituent> positions would change this conclusion.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-1-spiroacetal-chairs.png]
{title=Chair assignments for the two spiroacetals: axial reporter hydrogens and one axial exocyclic oxygen in each isomer}
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