Chapter 17, Problem 17.22P

Interpretation Introduction

Interpretation:

The synthesis of the target molecule starting with benzaldehyde is to be determined.

Concept introduction:

Synthesis may be designed by thinking in the reverse direction, from the product to the starting compound in a method called retrosynthesis. Retrosynthesis involves starting with the product and determining a suitable precursor from which it can be synthesized without considering specific reactions. The precursor must be a simpler molecule, which may or may not have a close structural relation to the target molecule. This process is repeated until an easily available (or easily prepared) precursor is reached. Each of these steps is called a transform.

Aldehydes react with alkyl lithium $\text{(R-Li)}$ to produce an alcohol on an acid workup. ${\text{R}}^{\text{-}}$ From alkyl lithium acts as a strong nucleophile, and the carbonyl carbon of the aldehyde acts as an electrophile.

Alcohols undergo a substitution of the hydroxyl group by bromine when treated with ${\text{PBr}}_{\text{3}}$. The alcohol acts as a nucleophile and attacks ${\text{PBr}}_{\text{3}}$ to displace a bromide ion and form a phosphorus ester. In the second step, the bromide ion acts as a nucleophile in an ${\text{S}}_{\text{N}}\text{2}$ reaction to replace the phosphorus ester group to form alkyl bromide.

The Wittig reagent synthesis reaction leads to the formation of a $\text{C-PPh3}$ bond from the halogenated compound. This reaction is carried out in the presence of $\text{PPh3}$ and a strong base such as $\text{Bu-Li}$.

This Wittig reagent undergoes Wittig reaction in the presence of a ketone resulting in the formation of a $\text{C=C}$ bond.