New Stilbene Dimer Compounds from the Fruits of Hypericum patulum and Their Antioxidant and Anti-α-Glucosidase Activities

Three previously undescribed phenolic dimers, including a pair of enantiomeric stilbene dimers (1a/1b) and one optically pure stilbene dimer (2), were separated from the fruits of the Miao medicine Hypericum patulum Thunb. Structural clarification and assignment of absolute configurations were achieved through 1D/2D NMR spectroscopy, optical rotation measurements, and analysis of experimental electronic circular dichroism (ECD) spectra. Their antioxidant and anti-α-glucosidase activities were evaluated. The new stilbene dimers exhibited potent antioxidant activity in both DPPH and ABTS free radical scavenging assays. Moreover, (−)-caesalstilbene C (1b) demonstrated the highest anti-DPPH and anti-ABTS activities among all isolated compounds. Compounds 1–2 also showed significant α-glucosidase-inhibitory activity, comparable to the positive control acarbose. These findings provide a foundation for developing novel antioxidants or α-glucosidase inhibitors as lead compounds in the search for new antidiabetic agents.