Stereochemical outcomes in opening of vinyl epoxides


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> Explain the outcomes of the following reactions. In each case the major product is shown.
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> ![](https://i.stack.imgur.com/iJLrz.png)
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Firstly, I don't understand why the chloride always attacks on the right hand side of the epoxide. Also, regarding the stereochemistry for (ii), I assume the chlorine can open the epoxide as it is antiperiplanar to the left hand side of the epoxide?




While @orthocresol was busy finding the Carreira paper, I was busy constructing my own thoughts on the stereochemical consequences of the reactions of **1** (iii) and 5 (ii). Yes, it is an issue of conformation. It is far easier to rationalize the results after the fact. To predict the most reactive conformation of epoxide **1** the Newman projection **2**, viewed along the red bond of **1**, places the two carbon chains *anti* to one another. Intermediate **3** undergoes straightforward SN2 displacement at the allylic site.   
  

In the case of epoxide **5** (ii), the two alkyl chains maintain the *anti* positions in conformation **6** but now the carbon-bound chlorine acts as a neighboring group in an intramolecular SN2 displacement to form intermediate **7**. This species suffers a second SN2 displacement to afford **8** with retention of stereochemistry at the reacting site. Compound (i) follows the same pathway as **1** (iii). The role of water is to cleave the silylether.  
  

[![enter image description here](https://i.stack.imgur.com/Y0xOL.jpg)](https://i.stack.imgur.com/Y0xOL.jpg) 


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