Flavonoids from Taraxacum mongolicum

Isoetin, 5,7,2′,4′,5′-pentahydroxyflavone, is a rare, structurally simple natural product belonging to the flavone sub-group of flavonoids. The first reports on naturally occurring isoetin derivatives were published in the 1970s though methoxy-derivatives with the same substitution pattern had already been synthesized a decade earlier. A glucoside of isoetin was first discovered in the genus Isoetes (Lycopodiopsida). In the forty years following the discovery of the new naturally occurring flavonoid aglycone, only a limited number of reports on isoetin and its derivatives have been published. Simple, i.e. non-methyl-ether derivatives of isoetin have been found in the Isoetaceae, Asteraceae, Ranunculaceae, Rosaceae, and Rubiaceae families; while methyl ethers and their derivatives have been found in the Lycopodiaceae, Asteraceae, Cucurbitaceae, Fabaceae, and Pedaliaceae. A total of 14 non-methyl-ether-derivatives (including isoetin) and the same number of methyl ether derivatives have been described, some methyl derivatives only as synthetic compounds, others even only as virtual compounds generated for in silico studies. The published NMR data of isoetin and its derivatives as well as chemosystematic studies using isoetin derivatives as markers are compiled and critically assessed. Moreover, the papers dealing with bioactivities of isoetin and its derivatives are summarized.