Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 1 / B4 / b / iii / 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 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, those roles reverse.

Thus the oxygen placements at the shared spiro carbon are 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.