Stilbenoids synthesis

Scheme 71 Improved synthesis of (E)-stilbenoids by olefin CM on a solid phase [154]...

The synthesis of tetraarylsilanes via Wurtz cross-coupling of bromoarenes and SiCU is a short and efficient route to tetraarylsilanes. Four conjugated units like biphenyl or naphthalene can be attached to the central silicon atom in high yield, but in the case of donor-substituted stilbenoid oligomers, this method gave only poor results [12]. For the preparation of tetrahedral silanes with phenylenevinylene chromophores, the convergent step was not performed on the central silicon atom but on a tetra- -tolylsilane (lb) functionalized with four phosphonic esters. The advantage of this approach is the additional extension of the n-system in the final step. 

The work on stilbenoids was reviewed by Gorham in 1995 [4]. Herein, we review the recent progress in the studies of stilbenoids with respect to their structure, distribution, extraction and isolation, technologies used in structure identification, synthesis and biosynthesis, and the last but not the least, bioactivities. The coverage of the new structures is from 1994 (especially those that have not been covered by Gorham [4]) to June 2006 (the references in 2006 may not be collected thoroughly as some of them might have not been included in the databases yet). 

Recently, microwave-assisted synthesis of eighteen 2-(alkyloxyaryl)-lH-benzimidazole derivatives 20 related to the natural stilbenoid family has been... 

The biosynthesis of the stilbenoids, including 1, has been previously reviewed. Briefly, the synthesis of 1 is dependent upon a single key enzyme known as stilbene synthase or resveratrol synthase as part of a mixed phenylpropanoid-polyketide pathway [2,3,4,5,6] (Fig. (1)). Stilbene synthase catalyzes the formation of 1 through the condensation of one p-coumaroyl CoA and three malonyl CoA molecules, both of which are ubiquitous intermediary plant metabolites. 

Whilst the above results demonstrate the clear potential of stilbenoid compounds for the photochemical synthesis of large aromatic systems, the scope has not yet been exhausted. Indeed, the ready synthetic availability of the stilbene precursors, together with the tolerance to a large selection of substituents and mild reaction conditions, make this approach particularly attractive for - and complementary to - other cyclization strategies such as the Lewis acid-mediated SchoU reaction, which is compatible with only a few classes of functional groups (vide supra). 

Photochemistry and mobility of the stilbenoid dendrimers [all-( )-l,3,5-tris[2-(3,4,5-tridodecyloxyphenyl)ethenyl]benzene] and [all-( )-l,3,5-tris(2- 3,5-bis[2-(3,4,5-trido-decyloxyphenyl)ethenyl]phenyl ethenyl)benzene] in their neat phases were synthesized and investigated [129]. Selectively deuterated, dodecyloxy-substituted stilbenoid dendrimers of the first and second generations (Figure 1.21) were synthesized by a convergent synthesis, using the Wittig-Homer reaction. Molecules deuterated at the... 

Synthesis of a number of antifungal stilbenoids can be induced by elicitation with fungal preparations or other factors such as UV light. A family of phytoalexins from mulberries (Moms spp., Moraceae) possess stilbene structures (Fig. 10.9) (Coxon, 1982). The moracins were isolated from shoots of Moms alba infected with Fusarium solani f. sp. mori and were not present in detectable quantities in uninfected tissue. Two additional compounds, oxyresveratrol (10) and 4 -prenyloxyresveratrol (Fig. 10.4), were isolated from fungus-infected xylem extracts of mulberry. Although oxyresveratrol (10) (Fig. 10.4) occurs in heartwood of mulberry, this compound is formed in the sapwood as a phytoalexin. Two similar compounds, broussonin A and B (14, 15), are found in the shoots of paper mulberry (Broussonetia papyrifera, Moraceae) infected with the same fungus (Coxon, 1982 Kuc, 1992). 

A breakthrough in the synthesis of stilbenoid derivatives as precursors of tetrathiahehcenes is the general and versatile method, based on a stereo-specific Suzuki-type coupling, shown in Scheme 5 (2005SYL1137). 

The method consists of the one-step synthesis of (Z)-l,2-(bis-benzo-dithienyl)ethenes 10 using double Suzuki coupling between stereochemi-caUy defined diboronic acid esters 11 and 2-iodo-benzodithiophene 12. The (Z)-alkenes thus obtained can be easily and efficiently photochemicaUy cyclized to the corresponding substituted tetrathia[7]helicenes. This methodology can also be applied to the synthesis of stilbenoid derivatives with two different heterocyclic systems such as the phosphole 13 (Figure 3) (2014CEJ12373). 

The authors applied this catalyst to a short and efficient synthesis of e t-corsi-furan A, the unnatural (-l-)-enantiomer of a metabolite derived from stilbenoid precursors (Scheme 3.11) [30]. They found that the hydrogenation of electron-deficient benzofurans was achieved under very mild conditions, while the presence of electron-donating groups in the benzofuran required harsher reaction conditions to achieve full conversion to the 2,3-dihydrobenzofuran. Furthermore, the study demonstrated that the oxygen atom of the benzofuran is crucial for getting high levels of enantioselectivity as the hydrogenation of 3-substituted indene leads to very low enantioselectivity (16% ee). 

These workers took this strategy one step forward with the synthesis of unsymmetrical hydroxylated stilbenoids, which are biologically important (see Figure 1.14). 

Jeong and coworkers applied a solid-supported CM reaction in the two-step synthesis of (F)-stilbenoids 22. Cross metathesis of substituted styrenes in solution resulted in three products (two homodimers and one heterodimer 22, see also Scheme 6.1) in an approximate 1 1 1 ratio. However, via immobilization of 20, the stilbenoids 21 were prepared and cleaved from the solid support in high yields (54—81%) as exclusive heterodimers 22 (Scheme 6.4), and with excellent E double bond selectivity (>99 1). Several solid supports... 

---