Proton-assisted switching of reaction pathways of stilbene analogues brought by direct irradiation

Determination of the pKa Values of trans-Resveratrol, a Triphenolic Stilbene, by Singular Value Decomposition. Comparison with Theory

Several independent determinations of the pK_a values of trans-resveratrol in water have led to conflicting results. Singular value decomposition analysis of UV absorption spectra of trans-resveratrol (t-Resv) in N₂-outgased aqueous solutions buffered to pH values in the 7.0-13.6 range yielded the UV spectra of the three anionic forms and the corresponding pK_a values: pK_a1 = 9.1₆, pK_a2 = 9.7₇, and pK_a3 = 10.5₅ in very good agreement with calculated theoretical values. The analysis of the absorption spectra guided the assignment of the fluorescence spectrum of each anionic form. With the resolved spectra on hand, we applied the Förster equation to estimate pK_a* values of 2.5 and 0, respectively, for the p- and m-OH substituents of t-Resv in S₁. Theory supports a proposed mechanism for the reaction of t-Resv anions with O₂.

Switching the Mallory Reaction to Synthesis of Naphthalenes, Benzannulated Heterocycles, and Their Derivatives

This review analyzes the new life of a long-known reaction, the photocyclization of diarylethenes, which became a classical tool for the synthesis of phenanthrenes and their heterocyclic analogues (Mallory reaction). It has been shown in recent years that certain diarylethenes undergo photorearrangement to naphthalenes, benzannulated heterocycles, or related products with bicyclic unit. Herein, I analyze how the Mallory reaction path can be altered to obtain bicyclic rather than tricyclic products. The mechanistic aspects and scope of the reaction are discussed.

Stabilising fleeting intermediates of stilbene photocyclization with amino-borane functionalisation: the rare isolation of persistent dihydrophenanthrenes and their [1,5] H-shift isomers

Amino-borane functionalization is the key for isolating rare air-stable dihydrophenanthrene intermediates in stilbene photocyclization.

The key intermediate, 4a,4b-dihydrophenanthrene (DPH), involved in the photocyclization of stilbene and derivatives is known to be unstable, and is therefore poorly characterized/understood. We have found that functionalising stilbenes with NMe₂ and BMes₂ groups can greatly enhance the stability of 4a,4b-DPHs, allowing quantitative isolation and full characterization of these rare species. Furthermore, we discovered that the new amino-borane decorated 4a,4b-DPHs can undergo thermal [1,5] H sigmatropic shift, forming isomers 4a,10a-DPHs. Both 4a,4b-DHPs and 4a,10a-DHPs are stable towards air and moisture, while only the former were found to undergo oxidative dehydrogenation upon irradiation at 365 nm under air, yielding brightly blue/green fluorescent NMe₂ and BMes₂ functionalised phenanthrene analogues. Control studies established that the trans-Mes₂B–Ph–NMe₂ unit is responsible for the stability of these isolated 4a,4b-DHPs and their [1,5]-H shift isomers.

Theoretical and experimental exploration of the photochemistry of resveratrol: beyond the simple double bond isomerization

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has been detected in various studies although its nature was unknown. In this work, we aim to provide a thorough description of the photochemical course of this reaction through experimental and computational approaches working in a synergetic association.

Photochemical generation of a new, highly fluorescent compound from non-fluorescent resveratrol

UV irradiation of trans-resveratrol leads to its photochemical transformation to a new, highly fluorescent compound, whose chemical structure was unambiguously identified. The new compound has large values of fluorescence quantum yield, Stokes' shift, and two-photon absorption cross section, which make it suitable for bio-imaging, multi-color labeling, and two-photon microscopy.

Antifungal activity of resveratrol against Botrytis cinerea is improved using 2-furyl derivatives

The antifungal effect of three furyl compounds closely related to resveratrol, (E)-3,4,5-trimethoxy-β-(2-furyl)-styrene (1), (E)-4-methoxy-β-(2-furyl)-styrene (2) and (E)-3,5-dimethoxy-β-(2-furyl)-styrene (3) against Botrytis cinerea was analyzed. The inhibitory effect, at 100 µg ml(-1) of compounds 1, 2, 3 and resveratrol on conidia germination, was determined to be about 70%, while at the same concentration pterostilbene (a dimethoxyl derivative of resveratrol) produced complete inhibition. The title compounds were more fungitoxic towards in vitro mycelial growth than resveratrol and pterostilbene. Compound 3 was the most active and a potential explanation of this feature is given using density functional theory (DFT) calculations on the demethoxylation/demethylation process. Compound 3 was further evaluated for its effects on laccase production, oxygen consumption and membrane integrity of B. cinerea. An increase of the laccase activity was observed in the presence of compound 3 and, using Sytox Green nucleic acid stain, it was demonstrated that this compound altered B. cinerea membrane. Finally, compound 3 partially affected conidia respiration.