Q56. - Organic Chemistry

Question

Question: Although ibuprofen is sold as a racemic mixture, only the S enantiomer acts as an analgesic. In the body, however, some of the R enantiomer is converted to the S isomer by tautomerization to an enol and then protonation to regenerate the carbonyl compound. Write a stepwise mechanism for this isomerization.

Step-by-Step Solution

Answer

Answer

 

Steps in the mechanism of isomerization

 

 

Step 1

Step 2

Step 3

Step 1: Chiral Drug

The analgesic drug ibuprofen is chiral and exists as a racemic mixture of two non-superimposable mirror image enantiomers (+)-(S)-ibuprofen and (-)-(R)-ibuprofen.

 

S-enantiomer is physiologically active and R-enantiomer is inactive, i.e., S-enantiomer is the one responsible for its analgesic and anti-inflammatory properties.

 

Step 2: Stepwise mechanism for isomerization

 Step 1. An enol is generated from a enolizable carbonyl compound in the presence of an acid catalyst

                                                 Enol formation  

  Step 2. Base abstracts a proton from enol oxygen to form a carbanion. 

                                              Formation of carbanion  

Step 3. Carbanion abstracts a proton from HA to form (s)-isomer.  

                                                 Formation of S isomer  

During this mechanism inversion of the configuration takes place at the asymmetric center.