The overlooked rotational isomerism of C-glycosyl flavonoids

C-glycosyl flavonoids are important secondary plant metabolites with a wide range of biological activities. Rotational isomerism, arising from restricted bond rotation, has been observed on a portion of C-glycosyl flavonoids. NMR technique contributes most to the observation and research of this phenomenon. Signal duplication in NMR spectra may be the key characteristic of C-glycosyl flavonoids existing as rotamers. Bulky steric hindrance from the substituents at position 7 and sugar moieties are responsible for the restricted bond rotation. There are other influence factors including temperature, solvents, H-bonds and π-stacking, but these are of lesser importance. Difference exists between 8-C-glycosyl flavonoids and their 6-C-glycosyl isomers despite sharing the same flavonoid aglycone and sugar moiety. 8-C-glycosyl flavonoids are more likely to suffer from restricted rotation. The energy barriers between rotamers of C-glycosyl flavonoids seem not high enough for atropisomerism to be realized and the isolation of rotamers should be difficult.