Two new glycosides from the leaves of Ligustrum robustum and their antioxidant activities and inhibitory effects on α-glucosidase and α-amylase

The leaves of Ligustrum robustum (Roxb.) Blume have been used as Ku-Ding-Cha, a Chinese tea with functions of clearing heat and removing toxin, in Southwest China for nearly 2000 years. From the leaves of L. robustum, two new glycosides, [2-(4-hydroxyphenyl)-ethyl]-[3-O-α-L-rhamnopyranosyl-(1 → 4)-α-L-rhamnopyranosyl]-(6-O-trans-p-coumaroyl)-O-β-D-mannopyranoside (ligurobustoside R, 1) and benzyl-(3-O-α-L-rhamnopyranosyl)-(4-O-trans-p-coumaroyl)-O-β-D-mannopyranoside (ligurobustoside S, 2), together with 12 known compounds (3–14), were isolated and identified. The α-glucosidase and α-amylase inhibitory activities of 1–13 were evaluated and 13 showed stronger α-glucosidase inhibitory activity (IC50: 1.15 ± 0.08 nM) than acarbose (IC50: 5.50 ± 0.11 nM). The antioxidant activities of 1–13 were also evaluated by ABTS and DPPH assays. Compounds 1–8, 10, 11 and 13 exhibited more potent ABTS•+ scavenging activity (IC50: 1.65 ± 0.01 to 8.98 ± 0.07 μM) than ascorbic acid (IC50: 10.06 ± 0.19 μM), and 8 revealed stronger DPPH scavenging activity (IC50: 7.61 ± 0.17 μM) than ascorbic acid (IC50: 13.66 ± 0.13 μM).