Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 3 / Section G / 49 / c / Solution

The oxime is reduced at its C=N bond rather than cleaving N–O: hydride addition to the imine carbon followed by protonation gives an N-substituted hydroxylamine. In a chair-like model of the cis-fused ring system, the fused framework and methyl group make the two faces inequivalent. The favoured hydride trajectory accesses the face that places the newly installed C–H bond behind the original drawing and the retained C–NHOH bond in front, giving D. It avoids the more congested trajectory into the folded bicyclic framework; a flat hexagon comparison alone misses this facial distinction.

The existing methyl and junction centres are retained. The selectivity is therefore diastereoselective substrate control, not enantioselectivity from sodium cyanoborohydride, which is achiral. For a racemic starting bicycle the same relative stereochemical preference gives a racemic pair of D enantiomers.