Proanthocyanidins rearrangement

Foo, L.Y. et al., Proanthocyanidins from Lotuspedunculatus, Phytochemistry, 45, 1689, 1997. Steynberg, P.J. et al.. Acid-catalyzed rearrangements of flavan-4-phloroglucinol derivatives to novel 6-hydroxyphenyl-6a,llb-dihydro-677-[l]benzofuro[2,3-c]chromenes and hydroxyphenyl-3,2 -spirobi[dihydro[l]benzofurans], J. Chem. Soc., Perkin Trans. 1, 2395, 1997. 

Precipitation studies at comparatively high concentrations have shown that proanthocyanidins form complexes with poly(L-proline) as well as with poly(vinylpyrrolidone) (9,W). Figure 2 depicts the repeat units of poly(vinylpyrrolidone y and poly(L-proline). The L-prolyl unit contains one fewer methylene and has the remaining atoms rearranged so that the amide and five-membered ring become parts of the backbone of the chain. The results reported in the proceeding section might lead to the expectation that addition of poly(L-proline) to an aqueous solution of one of the fluorophores would produce 1/1(0) > 1. This expectation is not realized. Poly(L-proline) with M = 60000 does not produce I/l(0) > 1. Instead it quenches the fluorescence. For example, the values of I/l(0) for 3.6 X 10 5 M (+)-catechin in water are 0.75 and 0.41, respectively. 

Base-Catalyzed Pyran Ring Rearrangement of Oligomeric Proanthocyanidins... 

An explanation for the formation of 2jR,3jR-(2,3-c/5)-proanthocyanidins from the 2i ,3S-(2,3-/ra/Z5 )-flavan-3,4-diols could lie in a tautomeric rearrangement of quinone methide intermediates to flav-3-en-3-ols, which could then be stereospecifically converted back to either 2,3-trans or 2,3-cis quinone methides (145). Chemical evidence supporting this thesis has been obtained by the formation of diarylprqpanone derivatives from the reaction of polymeric procyanidins with phenylmethanethiol under alkaline conditions (223). Enzymes controlling the quinone methide to flav-3-en-3-ol rearrangements rather than C-3 inversion of dihydroflavonols may be involved. In either case, evidence continues to mount that the flavan-3,4-diols are indeed central intermediates in the biogenesis of proanthocyanidins and that this conversion is under enzymic (genetic) control (219, 341, 342). 

Until recently, little attention has been given to the reactions of polymeric proanthocyanidins under alkaline conditions. It was known that solution of polymeric procyanidins in base caused a marked decrease in the reactivity of these polymers with aldehyde and that both carbonyl and relatively acidic functions were generated analogous to the rearrangement of catechin to catechinic acid (152, 335). 

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