Chapter 21, Problem 21.12P

Interpretation Introduction

Interpretation:

Synthesis of the given reaction is to be shown.

Concept introduction:

$\text{OH-}$ group on the starting material is not a good leaving group and must be converted to a good leaving group, followed by an $\text{E2}$ reaction. To convert this $\text{OH-}$ group to a good leaving group, we can undergo sulfonation by $\text{TsCl}$. Sulfonation of an alcohol occurs with retention of configuration at the $\text{C-OH}$ carbon. As shown in 21-11:

However, the target here has opposite configuration about $\text{C=C}$ bond that does in 21-11. Thus, the substrate from which $\text{E2}$ product is produced must have the opposite configuration at the carbon bonded to the leaving group. This substrate can be produced by treating the starting material with ${\text{PBr}}_{\text{3}}$ instead of $\text{TsCl}$. ${\text{PBr}}_{\text{3}}$ bromination of an alcohol converts to a poor ${\text{OH}}^{\text{-}}$ leaving group into a good ${\text{Br}}^{\text{-}}$ leaving group via inversion of configuration at the $\text{C-OH}$ carbon.