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 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.