Resolving Agents and Resolutions in Organic Chemistry

Prediction of the efficiency of diastereoisomer separation on the basis of the behaviour of enantiomeric mixtures

The composition of a crystalline diastereoisomer (ee_DIA) is determined either by the eutectic composition of the racemic compound (ee_EuRac) or that of resolving agent (ee_EuRes) and the higher ee value has the more dominant effect.

Direct crystallization of enantiomers and dissociable diastereomers

Molecules whose mirror images cannot be superimposed on each other are identified as being chiral, and a very important branch of separation science has developed around the resolution of compounds having pharmaceutical interest. At one time, scientists associated the phenomenon of optical rotation with the presence of carbon atoms bound to four different molecular fragments, and these asymmetrically substituted carbon atoms became known as "asymmetric carbons." Continued work showed that compounds incapable of rotating the plane of polarized light, but which were known to contain at least one asymmetric carbon atom, could be separated into chemically identical "optical isomers" that now exhibited the phenomenon of optical rotation. Over time, it became clear that optical activity could exist in compounds having no asymmetric atoms, and that other compounds existed that contained two or more asymmetric carbons, but still could not be rendered optically active. These findings necessitated a return to the proposal of Pasteur, who held that optical activity is a consequence of molecular dissymmetry. In other words, a molecule superimposable with its mirror image cannot be optically active, and any molecule not superimposable with its mirror image will exhibit optical activity.

Resolution of ephedrine in supercritical CO(2): A novel technique for the separation of chiral drugs

Racemic ephedrine has been resolved by diastereomeric salt formation with mandelic acid using supercritical CO(2) as precipitating agent. Crystallizations were performed using the Solution Enhanced Dispersion by Supercritical Fluids (SEDS) technique. Temperature was varied between 35 and 75 degrees C, and pressures ranged from 100 to 350 bar. Resolution, determined by chiral capillary electrophoresis, is described as a function of temperature and density of the supercritical fluid. A comparison of SEDS-produced material with a conventional resolution method shows that SEDS-crystallized material exhibits identical properties to conventionally crystallized material.