The inhibitory activity of 37 medicinal plant extracts against aldose reductase (AR) activity was evaluated. The most potent AR inhibitory activity was found in the MeOH extract of the leaves of Tussilago farfara (Compositae). Enzyme assay-guided fractionation of the extract led to the isolation of a new flavonoid glycoside, kaempferol 3-O-[3,4-O-(isopropylidene)-α-L-arabinopyranoside] (1), along with 15 known compounds (2–16), of which 3, 5, 13, 15, and 16 were isolated from T. farfara for the first time. The structures of 1–16 were elucidated based on MS and NMR data. Dicaffeoylquinic acid derivatives (7–12) showed potent AR inhibitory activity with IC50 values ranging from 0.58 to 5.38 μM, whereas flavonoid glycosides 1, 3, 5, and 6 showed weak inhibitory activity with IC50 values of 13.9, 15.1, 13.3, and 14.1 μM, respectively.
