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Chapter 23: Q56. (page 924)

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.

Short Answer

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Answer

Steps in the mechanism of isomerization

Step 1

Step 2

Step 3

Step by step solution

01

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.

02

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.

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