|Hammond et al. in 1965 introduced asymmetric reactions involving a chiral photosensitizer (DOI). Racemic trans-diphenylcyclopropane was subjected to non-polarized UV radiation in presence of a chiral amide converting it to the achiral (meso) cis isomer up to 40%. The optical rotation of the remaining trans isomer was found to increase from 0 to 28° (7% optical yield). This effect is attributed to the formation of an excimer complex allowed transfer of chirality from the sensitizer to the reaction product.|
Optical yields were improved by Inoue et al. in 1989(DOI) and Schuster et al. in 1990 ( DOI)
Inoue photoisomerized cis-cyclooctene to the trans isomer (chiral) with 12% optical purity with a photosensitizer based on chiral borneol. Schuster's system is based on a Diels-Alder reaction between beta-methylstyrene and cyclohexadiene and a complex Binaphthalene sensitizer.