= Solution
Number the <nitrogen> N1, the phenyl-bearing <carbon> C2, and then continue through C3–C6 around the piperidine. Call the solid-wedge <epoxide> face up; the <phenyl group> is down in both starting structures. The bulky <phenyl group> favors a pseudoequatorial orientation.
<epoxide> opening follows the \b[Fürst-Plattner preference for a chair-like, trans-diaxial opening trajectory]. Attack at the two <epoxide> <carbons> is not equivalent: the favored route evolves toward an ordinary chair, whereas the alternative demands a less favorable twist/boat-like distortion. Such conformational control of epoxy-2-arylpiperidine opening is supported by synthetic studies.<1,3-Allylic Strain as a Strategic Diversification Element for Constructing Libraries of Substituted 2-Arylpiperidines by Thomas C. Coombs and others>[Scheme 1, chair-like trans-diaxial epoxide opening; applied here with the free-NH conformer]{ref}
For the first <epoxide>, \b[azide attacks C5 from down], breaking the C5–O bond. The residual C4–O bond is retained, so work-up gives \b[C5–N3 down and C4–OH up], with the original C2–Ph down. For the second, down-face <epoxide>, \b[azide attacks C4 from up], leaving \b[C4–N3 up and C5–OH down]. These assignments are shown in the middle row of the original stereochemical map.
In each mechanism an azide terminal <nitrogen> attacks the <epoxide> <carbon>, the C–O bonding pair goes to <oxygen>, and <protonation> gives the <alcohol>. Each attacked center inverts, and the azide and OH are trans. The phenyl-bearing center is unchanged. Reversing the <epoxide> face changes which <carbon> has the favored axial attack trajectory; a steric rule based only on distance from phenyl misses that conformational effect.
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