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The phenyl group locks the conformation of epoxide since it stays equatorial, so there is only one conformation in each of the two isomers. The azide (N3-) nucleophile must attack the epoxide from behind in order to interact with the C-O σ* orbital. This means that the nucleophile and the epoxide oxygen will always be trans in the product. In the cases above, the ring opens out to give a diaxial chair conformation product. Although the two groups are axial, the chair conformation is stable.
Nucleophilic attack at the other carbon would give a diaxial twist boat conformation, which could twist to give a fully equatorial chair product, but the kinetic barrier to forming the high energy, diaxial twist boat intermediate is too high for this to happen.
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