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As far as I understand sharpless epoxidation, you first need to rotate the molecule such that the vincyclic alcohol is in the upper right quadrant. Then, the epoxide forms on the top/bottom of the double bond, depending on the chirality of DET. (See image below, from masterorganicchemistry.com: Sharpless Epoxidation)

My question is that why do we need to rotate the molecule when forming the epoxide, because can't we just form the epoxide directly without rotation? And if we can't, then we still can rotate the molecule back to its starting orientation after a epoxidation, right? Pretty confused about this.

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  • $\begingroup$ You don't have to. That thing is just to help you, a human, figure out which face the epoxide forms on. The chemicals don't need to work it out that way, and of course they don't need to rotate the alkene into whatever position. $\endgroup$
    – orthocresol
    Commented Jul 29, 2017 at 23:08
  • $\begingroup$ @orthocresol How does rotating the molecule even help a human in determining the structure, if the epoxide is formed above/below the plane either way? $\endgroup$
    – Teoc
    Commented Jul 30, 2017 at 0:16
  • $\begingroup$ Rotation allows you to pick out a preferred orientation to discuss the molecule. Once the canonical orientation is achieved, then conventional descriptions such as up, down, left, right have unambiguous meaning. $\endgroup$
    – Zhe
    Commented Jul 30, 2017 at 1:36

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