Solution

ID: past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2025/paper-1/b4/b/v/solution

Each isomer has one favourable anomeric interaction. A has the favourable axial exocyclic oxygen in the upper-ring chair; B has it in the lower-ring chair. The NMR-constrained conformations therefore do not provide a doubly anomeric versus non-anomeric comparison.
Both can keep important carbon substituents equatorial and each obtains one anomeric stabilization. Appreciable amounts of both are therefore a reasonable prediction, rather than a strong preference based solely on counting anomeric interactions. The environments of the two ring oxygens are not identical, so this argument does not determine an exact one-to-one equilibrium ratio; quantitative preference would require further energetic information.

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