On the chemistry of the resveratrol diastereomers

The (E)- and (Z)-diastereomers of resveratrol were investigated with respect to their photochemical and thermal diastereomerization reactions. The free enthalpy difference between the two diastereomers was estimated to be in the order of common stilbenes, with the (E)-diastereomer more stable by about 11-14 kJ mol(-1). The Arrhenius activation barrier of about 280 kJ mol(-1) was found to be quite high and implies that thermal equilibration cannot account for the (Z)-diastereomer found in nature. A preparative access to the (Z)-diastereomer by photodiastereomerization is described. The H-1 and C-13 NMR spectra of the two diastereomers were assigned and their absorption spectra and fluorescence quantum yields of the neutral and monodeprotonated species were determined.