Q:

I have a question from Taber's 2006 homework 9, question #3. I was

wondering if it were possible to form the alkene using PPh3 and the

aldehyde instead of the way he completed it. Also, why does the O

group invert when the MgBr group adds to the epoxide?

A:

good questions.  My additions to the Taber answer are in blue in the graphic

I think you are asking if it would be possible to use a Wittig reaction, instead of elimination of Br, to form the alkene.  The issue is that you need to make the trans-alkene, and Wittig reaction gives the cis-alkene.  mCPBA would be used to put in the epoxide.

There was only inversion on the left side.  I redrew the structure (see pasted picture) to show that it is simply a bond rotation, and not inversion, on the right side.

-JMF