Stilbene has a double-bond which can assume two differing geometrical configurations, a trans-form and a cis-form, but which are nowadays often re-designated (E)- and (Z)- respectively although the two terminologies are not directly interchangeable, but are in this specific case.
Of those, (E)-Stilbene, the straighter of the two, is the most energetically favourable configuration (lowest energy), it is fully planar and thus retains aromaticity between the electrons on the two benzene rings and the connecting link (resonance hybridisation). (E)-Stilbene and its derivatives are also used in the manufacture of synthetic oestrogens as well as possessing intrinsic oestrogenic activity themselves. It is thought that (E)-Stilbene forms liquid crystals, which is not possible with the non-planar (Z)-Stilbene form.
(Z)-Stilbene, on the other hand, is twisted out of plane by the forces between the hydrogen atoms on opposite benzene rings (steric hindrance - and is therefore in a higher energy state). The slight twist causes it to lose much of its aromatic character (the pi-orbitals do not overlap as well and then it cannot share electrons between the benzene rings). It is not very common and almost unknown in the natural world. The huge electronic differences between the two geometric isomers affect the stacking of the molecules and hence the melting point, with the weaker (Z)-Stilbene having a much lower melting point of, 6C as opposed to 125C for the (E)-isomer, a remarkably large difference (most cis/trans isomers have much lower melting point differentials than this).
However, the cis form of
Phenanthrene has a larger temperature differential between the two isomers, m.p. 101C for the cis-form Phenanthrene against 218C for the trans-form Anthracene. However, in the case of Phenanthrene, it retains planarity and thus its full aromaticity. Note the similarity between Phenanthrene and (Z)-Stilbene caused by the addition of just another bond between the two rings (and the loss of two hydrogen atoms). (Neither Anthracene nor Phenanthrene occur in Yucca, they are just shown for comparison).
Stilbene and many derivatives of it are found in many plants, Yucca being just one. Stilbene is used as one of the saturable gain mediums in dye-lasers. The two forms can inter-transform into one another under the influence of light, a process called photoisomerisation.
Stilbene, and some other linear-chain dimers of Stilbene, are used as the active saturable gain mediums in tuneable dye-lasers. This makes use of Stilbene and its dimers to fluoresce under UV illumination rather than for its ability to transform into the more energetic (Z)-isomer since the relaxation time for a conformational change back into the (E)-isomer would be far too long for any lasing effect to occur. It is also used in the manufacturing of dyes and optical brighteners, perhaps in washing powders.
Resveratrol too can exist in two geometrical isomers, the cis- (Z)-Resveratrol and the more stable trans- (E)-Resveratrol which is much more abundant. The two absorb at differing wavelengths in aqueous solution (λmax) 286nm versus 304nm respectively). The trans geometric isomer can also be transformed into the cis-form under the influence of UV light of 350nm wavelength (photoisomerisation).
Resveratrol is a stilbene derivative found in both grapes in red wine and which is believed to by a phytoalexin, which are produced by plants in response to and to repel fungal attack. Resveratrol is a phytoalexin, a compound produced naturally as a defence mechanism in plants under attack by a fungal or bacterial pathogen. However the Grapevine fungal pathogen Botrytis cinearea is able to oxidise Resveratrol into derivatives such as Pallidol (a dimer of Resveratrol) or δ-
Viniferin (a Resveratrol de-hydro dimer) which have reduced anti-fungal activity. δ-Viniferin is particularly interesting because it is a dimer between the trans- and the cis-forms of Resveratrol.
It is found in several other fruits but has particularly high concentrations in the skins of red
Grapes and in minuscule amounts survives the fermentation process in the production of red wine. Its numerous 'reported' health benefits (or detriments) are so far un-proven. It was first found in the very poisonous
White Hellebore (Veratrum album).
Yuccaols are novel spiro-structures which have various hydroxy or methoxy stilbenoids (similar to the Tetrahydroxymethoxystilbene shown above) linked by a lactone at a point to a residue probably derived from a flavone. Yuccaols contain two Phloroglucinol residues (leftmost and bottom right rings).
Larixinol, superficially similar to the Yuccaols but when inspected turns out to have many differences, is a compound previously identified in the Asian tree Dahurian Larch Larix gmelinii. Larixinol is a spiro-biflavonoid which contains two Phloroglucinol residues (central ring and bottom right ring). The flavonol component is possibly derived from
DiHydroKaempferol. It too is also a lactone and is found in the bark only of Yucca gloriosa.