Q17. - Organic Chemistry

Question

Question: The analgesic naproxen can be prepared by a stepwise reaction sequence from ester A. Using enolate alkylation in one step, what reagents are needed to convert A to naproxen? Write the structure of each intermediate. Explain why a racemic product is formed.

Step-by-Step Solution

Answer

Answer

 

The reagents needed to convert A to naproxen are shown below along with the intermediates formed:-

A racemic mixture is formed because after the formation of an enolate, methylation can take place from any position resulting in two stereoisomers which are mirror images to each other.

Step 1: Racemization

Racemization is a process of conversion of an optically active compound into a racemic (optically inactive) form in the presence of heat or by any chemical method. 

Step 2: Formation of a racemic mixture

A racemic mixture is formed when the intermediate formed is an enolate and via methylation, it can attack any methyl cation from both the top and bottom of the plane. The formed structures are stereoisomers and that is interrelated to each other by a mirror-image relationship. 

Representation of the formation of a racemic mixture

Step 2: Formation of naproxen

The first two steps involve the formation of an enolate followed by methylation with methyl iodide. The last step involved is the ester hydrolysis in an acidic medium to obtain the desired product.

Representation of the formation of naproxen

Representation of the formation of a racemic mixture