Concerning the diastereomerization of stilbenoid hypericin derivatives

Experiments on a newly prepared (E)-configured omega-benzal-hypericin derivative using TLC and H-1 NMR together with quantum chemical calculations revealed that in stilbenoid hypericin derivatives photodiastereomerization between the (E)- and (Z)-diastereomers occurs in principle. However, due to its low diastereomerization quantum yield and photo and thermal equilibria, which reside mostly on the side of the (E)-diastereomer, this photoreaction is only of marginal importance to the photochemistry of stilbenoid hypericin derivatives. Thus, photodiastereomerization does not appreciably interfere with the photoreactions important for photodynamic therapy. This was demonstrated by comparing the sensitized bilirubin photodestruction of hypericin and the omega-benzal-hypericin derivative.