Isolation and purification of pure, active enantiomers is important in the development of active pharmaceutical ingredients and drug products. However, recovering the desired optical isomer from a racemic mixture means half of the chemical produced — the undesired isomer — will be wasted, unless it can be recycled in some manner. Now, researchers from the Tokyo University of Science (Japan; www.tus.ac.jp) have developed a system that, at least for one important class of compounds — chiral sulfoxides — is able to produce the desired isomer with high efficiency. This is done by a two-step process, as described in a recent issue of the Journal of Organic Chemistry.
In the first step (diagram), the desired isomer is separated from the racemic mixture in a high-performance liquid chromatography (HPLC) column containing a chiral stationary phase. The undesired enantiomer then passes through a photoreactor — a glass tube containing an immobilized photocatalyst (2,4,6-triphenylpyrrylium tetrafluoroborate) and transparent glass beads. Irradiation with blue light (402 nm) from a light-emitting diode (LED) causes a rapid photoracemization of the undesired isomer back into a racemic mixture, which can then be recycled back to the HPLC purification column. The cycle can be repeated until the yield of the desired compound is as high as desired.
![chiral compounds](https://www.chemengonline.com/wp-content/uploads/2023/07/screen-shot-2023-06-13-at-122111-pm.png)
Source: Tokyo University of Science
The researchers demonstrated the technology for synthesizing four enantiometrically pure chiral alkyl aryl sulfoxides, achieving optical purities of 98–99%. High yields (>80%) for the desired compound are achieved after 4 to 6 cycles.