α-Glucosidase Inhibitors from the Stems of Knema globularia .

Six new compounds, globunones A-F ( 1 - 6 ), and two new flavonoids ( 7 and 8 ) together with nine known compounds ( 9 - 17 ) were isolated from the stems of Knema globularia. The chemical structures of 1 - 8 were elucidated by an analysis of their NMR and high-resolution electrospray ionization mass spectrometry data as well as by comparison with literature values. The absolute configurations were determined using time-dependent density functional theory electronic circular dichroism (TD-DFT-ECD). Globunones A-E ( 1 - 5 ) represent the initial combined structures of a flavan-3-ol core and a 1,4-benzoquinone core. Globunone F ( 6 ) is the first flavanone-type compound bearing a 2-(2,4-dihydroxyphenyl)-2-oxoethyl group found to date in Nature. Compounds 1 - 3 and 6 - 17 were tested for their yeast α-glucosidase inhibitory activity. All compounds tested (except for 13 and 14 ) showed potent inhibition toward α-glucosidase with IC 50 values in the range 0.4-26.6 μM. Calodenin A ( 15 ) was the most active compound with an IC 50 value of 0.4 μM (the positive control, acarbose, IC 50 93.6 μM). A kinetic analysis of 15 revealed that it is a noncompetitive inhibitor with a K i value of 3.4 μM.