Profisetinidins

The profisetinidins have been studied intensively by Roux s school at The University of the Orange Free State, Bloemfontein, South Africa. The heartwoods of Acacia mearnsii and Colophospermum mopane contain a series of 2R isomers and these compounds have all been synthesized (Tables 7.6.9 and 7.6.10). The commercially important quebracho tannins from Schinopsis and Rhus contain a series of 2S isomers and these compounds have also been synthesized recently. Fifteen dimers, nine trimers, and nine tetramers have now been thoroughly de- 

Refers to absolute configuration of the parent flavan-3,4-diol and flavan-3-ol 

---

The profisetinidins are the most important proanthocyanidins of commerce, making up the major constituents of wattle and quebracho tannins. New entries are collated in Table 11.8. 

The structures of the di- and trimeric profisetinidins from Pithecellobium dulce (Guamii-chil) were rigorously corroborated via synthesis.The synthetic approach was additionally motivated by the precariousness of unequivocally differentiating between 2,3-cis-3,4-trans-and 2,3-c7.s-3,4-c7.s-confugurations of the chain-extension units on the basis of H NMR coupling constants.Furthermore, the powerful nuclear Overhauser effect (NOE) method for differentiating between 2,4-cis- and 2,4-tra i -substitution is less useful at the di- and trimeric levels due to the adverse effects of dynamic rotational isomerism about the interflavanyl bond(s) on NMR spectra at ambient temperatures. 

Steynberg, J.P. et al.. Oligomeric fiavanoids. Part 4. Base-catalyzed conversions of (—)-fisetinidol-(+)-catechin profisetinidins with 2,3-trflwj-3,4-cw-flavan-3-ol constituent units, J. Chem. Soc., Perkin Trans. 1, 1331, 1988. 

Bonnet, S.L. et al., Structure and synthesis of phlobatannins related to the (4(3,6 4a,8)-bis-fiseti-nidol-catechin profisetinidin trifiavanoid. Phytochemistry, 43, 215, 1996. 

Saunders, C.M. et al., Controlled biometric synthesis of profisetinidin trifiavanoids related phlobatannins, Tetrahedron, 52, 6003, 1996. 

R2 = R2= OH. Prodelphinidins. Resorcinol-pattern A-ring proanthocyanidins R1 = R2 = H. Profisetinidins. 

Procyanidin oligomers [(4a) n = 0 to 10] apparently possess mainly linear conformations, compared with the proven angularity of profisetinidin and prorobinetinidin analogues (2) and (3). 

Probably mainly as the result of a more uniform mass distribution and with considerable emphasis on lower mass units, prorobinetinidins and profisetinidins (1) to (3) occur in mixtures that are more readily water-soluble than those of procyanidins (4a) and prodelphinidins (4b). However, oxidative effects as the result of weathering of barks may also play a role in reducing the solubility of the latter group. 

The profisetinidins are the most important proanthocyanidins of commerce, constituting the bulk of wattle Acacia meamsii) and quebracho Schinopsis species) tannins. Their natural occurrence, general synthesis protocols, and chemical transformations, especially under basic conditions, were comprehensively reviewed in Ferreira et al.l Ferreira and Li," Porter, Hemingway, and Ferreira and Bekker. ... 

The prorobinetinidins also feature prominently in wattle bark extract and hence complement the profisetinidins in this important industrial resource. Similar to the profisetinidins, the natural occurrence, synthesis, and typical chemical transformations were comprehensively reviewed. ... 

Scheme 1. Flavan-4-ols (1), flavan-3,4-diols (2) or 4-benzylsulfanyl analogues, and flavan-3-ols (5) as precursors to profisetinidin (R =H) or procyanidin (R =OH) oligomers with (2R,35 ) constituent flavanyl units...

Synthesis of the First Profisetinidins with Epiflsetinidol Constituent Units... 

The elegance of this simple biomimetic approach to the synthesis of proanthocyanidin oligomers was demonstrated during synthesis of the mixed profisetinidin trimers (22) and (23), i.e. analogues possessing different ABC and GHI chain extender units. Triflavanoid (22) with its fisetinidol ABC and epifisetinidol GHI units was formed by acid-catalyzed reaction of fisetinidol-(4a->8)-catechin (24) (57) and epifise-... 

Scheme 2. Synthesis of dimeric profisetinidins (16)-(19) with epifisetinidol chain...

Scheme 6. Proposed route to the cleavage of the interflavanyl bond and of the C-ring in profisetinidins e.g. (24) and permethylaryl ether (68)...

---