# Would I use PBr3 or HBr in this synthesis?

In the above reaction, I know that I have to use either $\ce{HBr}$ or $\ce{PBr3}$ and then $\ce{NaCN}$, DMF, $\ce{H+}$ and $\ce{H2O}$. However, I'm just confused on when to use $\ce{HBr}$ or $\ce{PBr3}$. How is the stereochemistry controlled?

• Try to think in terms of desirable configuration. It seems to me that neither would work as desired. – permeakra Mar 5 '18 at 6:31
• Who says you have to use PBr3? – Waylander Mar 5 '18 at 10:11