Bioassay-Guided Isolation of Triterpenoids as α-Glucosidase Inhibitors from Cirsium setosum

Cirsium setosum (C. setosum) has a potential antihyperglycemic effect, but it is unclear what bioactive components play a key role. According to the α-glucosidase inhibition activity, three new taraxastane-type triterpenoids of 3β-hydroxy-30-hydroperoxy-20-taraxastene (1), 3β-hydroxy-22α-methoxy-20-taraxastene (2), and 30-nor-3β,22α-dihydroxy-20-taraxastene (3), as well as five known taraxastane triterpenoids of 3β,22-dihydroxy-20-taraxastene (4), 20-taraxastene-3,22-dione (5), 3β-acetoxy-20-taraxasten-22-one (6), 3β-hydroxy-20-taraxasten-22-one (7), and 30-nor-3β-hydroxy-20-taraxastene (8) were obtained from the petroleum ether-soluble portion of the ethanol extract from C. setosum. All chemical structures of the compounds were elucidated by spectroscopic data analysis and compared with literature data. Compounds 4–8 were identified for the first time from this plant, and compounds 1, 2, 4, and 7 exhibited more potent α-glucosidase inhibitory activity—with IC50 values of 18.34 ± 1.27, 26.98 ± 0.89, 17.49 ± 1.42, and 22.67 ± 0.25 μM, respectively—than acarbose did (positive control, IC50 42.52 ± 0.32 μM).