Five New Iridoids From the Leaves of Viburnum macrocephalum f. macrocephalum and Their Free Radical Scavenging Activity.

An investigation of the chemical constituents of Viburnum macrocephalum f. macrocephalum leaves (VMML) resulted in the purification of five previously unreported iridoids, viburetins D-H (1-5), along with one known analogue (6). Compounds 1-5 were identified by combined spectroscopic and spectrometric techniques, including 1D/2D NMR, HRESIMS, and ECD. Antioxidant assays revealed that compounds 2, 5, and 6 exhibited DPPH (IC50: 36.58-53.90 µM) and ABTS (IC50: 25.96-45.80 µM) free radical scavenging activity, whereas compound 1 was appreciably active in the ABTS assay (IC50 = 58.47 µM). Notably, compound 6 was the most potent antioxidant, showing greater activity than the positive control Trolox in both assays (DPPH: IC50 = 36.58 vs. 41.12 µM; ABTS: IC50 = 25.96 vs. 29.09 µM). These findings have enriched the chemical diversity of iridoids from VMML and provided preliminary evidence for their free radical scavenging activity.