NMR Solution Structure Study of the Representative Component Hydroxysafflor Yellow A and Other Quinochalcone C‑Glycosides from Carthamus tinctorius

Hydroxysafflor yellow A (HSYA), a representative component of Carthamus tinctorius, has attracted much attention because of its remarkable cardiovascular activities. Its structure was originally reported in 1993 and has been widely cited to date. In our experiments, its solution structure was studied using NMR techniques in different solvents, including DMSO-d₆, pyridine-d₅, and CD₃OH. The results indicate that the structure of HSYA is different than the previously described 1b, with 3-enol-1,7-diketo form. The structure has two keto–enol tautomers (2a and 2b), and 2a, with the 1-enol-3,7-diketo form, is the preferred tautomer. On the basis of this finding, other published quinochalcone C-glycoside structures were revised. Furthermore, a trend in the ¹³C NMR data of the (E)-olefinic carbons of quinochalcone C-glycosides is summarized, and a hypothesis is proposed for the relationship between the features of the molecular structure and the preferred keto–enol tautomer.