Flavonol Glycosides with α-Glucosidase Inhibitory Activities and New Flavone C-Diosides from the Leaves of Machilus konishii

Seventeen flavonoids, five of which are flavone C-diosides, 1 -5, were isolated from the BuOH- and AcOEt-soluble fractions of the leaf extract of Machilus konishii. Among 1 -5, apigenin 6-C-beta-D-xylopyranosyl-2 ''-O-beta-D-glucopyranoside (2), apigenin 8-C-alpha-L-arabinopyranosyl-2 ''-O-beta-D-glucopyranoside (4), and apigenin 8-C-beta-D-xylopyranosyl-2 ''-O-beta-D-glucopyranoside (5) are new. Both 4 and 5 are present as rotamer pairs. The structures of the new compounds were elucidated on the basis of NMR-spectroscopic analyses and MS data. In addition, the H-1- and C-13-NMR data of apigenin 6-C-alpha-L-arabinopyranosyl-2 ''-O-beta-D-glucopyranoside (3) were assigned for the first time. The isolated compounds were assayed against alpha-glucosidase (type IV from Bacillus stearothermophilus). Kaempferol 3-O-(2-beta-D-apiofuranosyl)-alpha-L-rhamnopyranoside (12) was found to possess the best inhibitory activity with an IC50 value of 29.3 mu m.