Looking at the structure of the camphor molecule, you can see that, because of the conformational rigidity of the fused ring structure, the re and si faces of the carbonyl group are not equivalent - a...Looking at the structure of the camphor molecule, you can see that, because of the conformational rigidity of the fused ring structure, the re and si faces of the carbonyl group are not equivalent - approach by a nucleophile appears to be less hindered from the bottom (si) side than from the top (re) side, (it is easier to visualize this if you build a model).
