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Optical Isomerism In Thalidomide


 

Thalidomide has just one chiral atom and so exists as two enantiomers. The diagram to the right shows the molecule without hydrogens. Notice that two of the groups attached to the chiral centre are part of the same ring structure. They are classified as two different groups. since moving around from the chiral centre the order of atoms is different each way. It is said the chiral atom has two different views around the ring.

 

Two Images To Show the Enantiomers Of Thalidomide

S isomer of Thalidomide R isomer of thalidomide The only difference is the positioning of the functional groups, yet it was believed this affected much more than the optical activity of the compound.
(Click on images for 3D molfiles).
'S' Optical isomer 'R' Optical Isomer Reminder of Naming

The Reality of Optical Isomerism In Thalidomide

Laboratory tests after the thalidomide disaster showed that in some animals the 'R' enantiomer was tetragenic but the 'S' isomer was an effective sedative. It is now known that even when a stereo selective sample of thalidomide (only one of the optical isomers) is created, if administered pH in the body, can cause racemizing. The means that both enantiomers are formed in a roughly equal mix in the blood. So, even if a drug of only the 'S' isomer had been created, the disaster would not have been averted.