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.
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.