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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.
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.
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