Cyclization of Stilbene and its Derivatives

Gilbert, A. (2004) Cyclization of stilbene and its derivatives, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and... 

The intramolecular coupling reaction does not occur when diphenylethylenes, that is, stilbene and its methyl derivatives, are electrolyzed under the same conditions (Stuart and Ohnesorge 1971). This difference in the anodic behavior of these substances was attributed to the low stability of the cation-radicals of stilbene and its methyl derivatives in comparison to the cation-radicals of tetraphenylethylene. The participation of the cation-radicals in the cyclization of tetraphenylethylene has been unequivocally proved (Svanholm et al. 1974, Steckhan 1977). 

In special cases, other photochemical reactions can totally suppress an acyl group migration.76,117 The primary photochemical process of 195 is fast equilibration with trans-isomer 196. The crr-isomer 195 then cyclizes in 65% yield to dehydroaporphane (197) in a similar way as does stilbene and its derivatives.118 7>a j-isomer 196 cyclizes in 10-21% yield to dehydroproto-berberine (198).76 The latter reaction is analogous to the already discussed cyclization 110 -> 112 and 134 -> 131 of aromatic carbonates and carbamates. 

The intramolecular coupling reaction between an aryldiazonium salt (III) with an arene subunit to form five- or six-membered ring by the influence of copper or an acid, is called the Pschorr cyclization reaction [85]. In historical examples the Pschorr cyclization was used in the synthesis of phenanthrene and its derivatives. For example, c -stilbene derivative 49 is diazotized to give 50, and subsequently treated with the activated copper to obtain phenanthrene (51) in 34% yield [86], Scheme 21. 

Fluorescence Spectra. frons-Stilbene and some derivatives show fluorescence emission in the range from 340 to 400 nm [187], the fluorescence maximum (lf) is sensitive to substituents (Table 10). m-Stilbenes, in contrast, exhibit no fluorescence in solution at ambient temperatures [95]. Only recently, the fluorescence excitation and emission spectra (A = 275 and 409 nm, respectively) of ris-stilbene in n-hexane at room temperature have been reported [240,241], A very short lifetime (<10ps) has been assumed for the excited cis singlet state in fluid solutions (Section A.5). At lower temperatures (below — 120°C) and in rigid media c/s-stilbene shows a broad emission around 450 nm [113, 176, 242 245]. It does not originate from the trans isomer or a cyclization product and is hence assigned to fluorescence [246], Increased viscosity of the solvent likewise results in an increase of the fluorescence quantum yield of the cis isomer ([Pg.39]

Addition. Resveratrol. The biosynthetic pathway to resveratrol diverges from the flavonoid pathway after the third malonyl-CoA condensation. Cyclization of the common polyketide intermediate catalyzed by resveratrol synthase yields the stilbene derivative resveratrol, whereas the same intermediate catalyzed by chal-cone synthase yields the common flavonoid precursor chalcone (Fig. 6.8). Resveratrol, well known as a functional food ingredient (e.g., grape skin, red wine) with strong antioxidant properties (cardiovascular protection), and its 3-glucopyranoside piceid turned out to be also present in the skin of tomato fruits, S. lycopersicum (Ragab et al. 2006). 

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