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 (IC 50 : 1.15 ± 0.08 nM) than acarbose (IC 50 : 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 (IC 50 : 1.65 ± 0.01 to 8.98 ± 0.07 μM) than ascorbic acid (IC 50 : 10.06 ± 0.19 μM), and 8 revealed stronger DPPH scavenging activity (IC 50 : 7.61 ± 0.17 μM) than ascorbic acid (IC 50 : 13.66 ± 0.13 μM).