Unusual chemical transformations of natural flavonoids

Any new data on chemical properties of natural flavonoids seem to be interesting in many aspects. They are, for instance, i. activity of carbonyl group in flavanonole molecules (like taxifolin) in reactions of nucleophylic substitution, ii. the tautomerization of flavanonoles that leads to endiol forms, and iii. the sensitivity of flavonoles (like quercetin) to nucleophylic attack on C2-atom of ring B, including bases catalyzed hydrolysis. First group of reactions (i) includes the formation of MO-TMS derivatives prior their GC analysis that was recommended for some dihydroflavones (e.g., naringenin [1]), that means the possibility of similar reactions with other NH₂-compounds. The tautomerization (ii) was proposed first for the explanation of formation of alphitonin from taxifolin at hydrothermolysis [2]. However, no direct evidences of endiol existence are known up to present, namely its fixing in the form of TMS-derivatives like it is known for isoflavones [3]. The irreversible instability of quercetin in basic media is confirmed by the results of its stepwise UV-spectrophotometric titration. The registration of UV spectra of this compounds in the pH range 4→10 indicates the natural changes; a part of them can be explained by ionization ArOH ↔ ArO^- [pKa ≈ 7.1 (7-OH)]. Surprisingly, the following restoration of initial pH-value 4 gives not H-form of quercetin, but another compound (λ_max 294 nm, no absorbance at 350–400 nm). Possible explanations of this phenomenon are discussed.

Acknowledgements: DIOD Co., Moscow, Russia.

References: 1. http://www.upsc.se/RTI.pdf. 2. Ohmura, W.O. et al. (2002), Holzforshung 56:493–497. 3. Joannou, G.E. (2000), Tetrahedron Lett. 41: 7925–7928.