Asymmetric Synthesis

Many compounds exhibit optical isomerism. Two isomers exist that are identical in every way except that they are mirror images of each other. Naturally occurring compounds in plants and animals are often found as just one of the two isomers.

You will find more information about optical isomerism on the Catalysis site.

In some circumstances the isomers may have very different properties. For example the compound limonene tastes of oranges or lemons depending on the isomer! In the case of pharmaceutical products, one isomer may be therapeutic and the other may be either ineffective or even dangerous. The S-isomer of ibuprofen is about 100 times more effective as a painkiller than the R-isomer. The most tragic example was the use of thalidomide in the late 1950s and early 1960s to help prevent "morning sickness" during pregnancy. The S-isomer is an effective sedative, but unfortunately the R-isomer was responsible for birth defects.

 

chemical structures: thalidomide enantiomers
chemical structures: thalidomide enantiomers chemical structures: thalidomide enantiomers chemical structures: thalidomide enantiomers chemical structures: thalidomide enantiomers
chemical structures: thalidomide enantiomers
chemical structures: thalidomide enantiomers
One third of all drugs are sold as a single isomeric form, but it is often not possible to synthesise only one isomer. A mixture is made and the isomers are then separated, with the unwanted isomer sometimes being returned for racemisation. Details of how this is done with ibuprofen using enzyme catalysis can be found on the catalysis site

Much progress has already been made in synthesising one isomer of a pair. Some work being done at the University of Leeds, UK, is indicating that using scCO2 as the solvent for reactions may be another way of achieving this. They oxidised a derivative of cysteine (an amino acid), and found that whereas usual methods produced a 50/50 mixture of isomers, using scCO2 gave one isomer almost exclusively.

chemical equation
chemical equation chemical equation chemical equation
chemical equation chemical equation
chemical equation chemical equation chemical equation
chemical equation
chemical equation

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