Study of the structure and stereochemistry of flavonoid O-rhamnosides with the aid of PMR spectroscopy

The PMR method has been used in the study of the structure and stereochemistry of a number of flavonoid rhamnosides. The systematic study of the PMR spectra of the native compounds and their TMS ethers and complete acetates has permitted the size of the oxide ring, the conformation of the rhamnose residue, and, in combination with the results of polarimetric analysis, the configuration of the glycosidic bond to be determined. Information on the size of the oxide ring is given by — in addition to, the J_{2, 3} proton coupling constant — the signals of the protons at C-5 in the spectra of the full acetates. In the diagnostic region of 3.2–4.0 ppm, only the proton H-5 resonates in the rhamnopyranosides, while for the rhamnofuranosides a different structure of the H-4 signal must be expected. It has been shown that all the compounds studied (more than 20 specimens) are α-L-rhamnopyranosides and have the¹C₄ conformation of the sugar residue. In view of this, the hypothesis has been expressed that the existence of natural flavonoid rhamnosides in the furanoside form is improbable.