Stilbene pathway

Efforts to increase the production of trans-R for commercial apphcations have focused on heterologous expression of the phenylpropanoid and stilbene pathways in S. cerevisiae and E. coli, although as the stilbene pathway does not exist in yeast or bacteria, the entire functional pathway needs to be introduced. Thus, when in both systems, 4cl from N. tabacum and sts from V. vinifera were expressed, 16 and 6 mg trans-R were produced in E. coli and yeast cells, respectively. However, the... 

Rajanikanth and Ravindranath44 have recently published a deoxygenation reaction for sulphoxides that uses metallic lithium in refluxing dimethoxyethane. Dialkyl and alkyl phenyl sulphoxides were reduced cleanly in yields around 70%, even if sterically hindered, but benzyl sulphoxides gave mixtures of products. For example, benzyl phenyl sulphoxide gave frans-stilbene (33%), benzyl phenyl sulphide (20%) and diphenyl disulphide (47%). These products can be rationalized by reaction pathways such as in equation (17) ... 

As the mechanism, a radical and a cationic pathway are conceivable (Eq. 31). The stereochemical results with rac- or mcjo-1,2-diphenyl succinic acid, both yield only trans-stilbene [321], and the formation of a tricyclic lactone 51 in the decarboxylation of norbornene dicarboxylic acid 50 (Eq. 32) [309] support a cation (path b, Eq. 31) rather than a biradical as intermediate (path a). 

Spivack J, TK Leib, JH Lobos (1994) Novel pathway for bacterial metabolism of bisphenol A. Rearrangements and stilbene cleavage in bisphenol A metabolism. J Biol Chem 269 7323-7329. 

The EfZ ratio of stilbenes obtained in the Rh2(OAc)4-catalyzed reaction was independent of catalyst concentration in the range given in Table 22 357). This fact differs from the copper-catalyzed decomposition of ethyl diazoacetate, where the ratio diethyl fumarate diethyl maleate was found to depend on the concentration of the catalyst, requiring two competing mechanistic pathways to be taken into account 365), The preference for the Z-stilbene upon C ClO -or rhodium-catalyzed decomposition of aryldiazomethanes may be explained by the mechanism given in Scheme 39. Nucleophilic attack of the diazoalkane at the presumed metal carbene leads to two epimeric diazonium intermediates 385, the sterically less encumbered of which yields the Z-stilbene after C/C rotation 357,358). Thus, steric effects, favoring 385a over 385 b, ultimately cause the preferred formation of the thermodynamically less stable cis-stilbene. 

For stilbene bromination, a markedly non-linear structure-reactivity relationship is observed (Fig. 5). Detailed analysis of the kinetic effects of two substituents, X and Y, on each aromatic ring shows that the three pathways leading to the C+ and carbocations and to the bromonium ion can... 

Fig. 5 Reactivity-structure relationship for the bromination of monosubstituted stilbenes (data from Ruasse and Dubois, 1972). The curvature shows the X-dependence of the competition between carbocation and bromonium ion pathways.

The three-pathway bromination of stilbenes can interestingly be compared with the dehydration of 1,2-diarylethanols (Noyce et al, 1968), which unambiguously takes place through two - and 0-aryl carbocations. The ratios of the two reaction constants, p /pp, are very similar (Table 14), despite large differences in solvents and in the nature of the encounter complexes formed in the step preceeding the ionization. 

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

An informative set of calculations was carried out by Brandt et al, coupled to experimental studies that demonstrated first-order dependence of the turnover rate on both catalyst and H2, and zero-order dependence on alkene (a-methyl-(E)-stilbene) concentration [71]. The incentive for this investigation was the absence of any characterized advanced intermediates on the catalytic pathway. As a result of the computation, a catalytic cycle (for ethene) was proposed in which H2 addition to iridium was followed by alkene coordination and migratory insertion. The critical difference in this study was the proposal that a second molecule of H2 is involved that facilitates formation of the Ir alkylhydride intermediate. In addition, the reductive elimination of R-H and re-addition of H2 are concerted. This postulate was subsequently challenged. For hydrogenation of styrene by the standard Pfaltz catalyst, ES-MS analysis of the intermediates formed at different stages in the catalytic cycle revealed only Ir(I) and Ir(III) species, supporting a cycle (at least under low-pressure conditions in the gas... 

J. F. Letard, R. Lapouyade, and W. Rettig, Relaxation pathways in photoexcited electron-rich stilbenes (D-D stilbenes) as compared to D-A stilbenes, Chem. Phys. Lett. 222, 209-216 (1994). 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

In cattle feces, 64% of the total residues was identified as diethylstilbestrol, 23% as 3-(p-hydroxyphenyl)-2-hexene-4-one, and less than 1% as 4 -hydroxypro-piophenone (43). The identification of 4 -hydroxypropiophenone as a metabolite of diethylstilbestrol implies that dienestrol is formed through an epoxide-diol pathway and that these metabolites show electrophilic reactivity (45). These observations have to be seen in connection with the mutagenic and carcinogenic activity of diethylstilbestrol and possibly also the other stilbene estrogens. 

---