Phlomomelanosides A-E, Five New Iridoid Glycosides From the Whole Plant of Phlomoides melanantha and Their Biological Activities.

Phlomoides melanantha, a perennial herb belonging to the genus Phlomoides within the Lamiaceae family, has not been previously studied for its chemical constituents. In this research, a phytochemical investigation on the whole plant of P. melanantha led to the isolation of eleven glycosides, including five novel iridoid glycosides named phlomomelanosides A-E (1-5) and six known analogues (6-11). The structures and relative configurations of these isolated compounds were determined using high-resolution mass spectrometry and detailed 1D and 2D nuclear magnetic resonance techniques. Additionally, the absolute configurations of 1, 4, and 5 were determined by comparing experimental and calculated electronic circular dichroism spectra. The antioxidant properties of all isolated compounds were evaluated using the 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay. Compound 4 exhibited moderate scavenging activity, with an IC50 value of 107.32 ± 4.29 µM. Furthermore, all compounds were evaluated for their ability to inhibit α-glucosidase, but none demonstrated significant inhibitory effects. This work expands the known chemical diversity of iridoid glycosides in the Phlomoides genus and represents the first report on the chemical constituents and biological activities of P. melanantha, providing a foundation for future studies on its potential medicinal applications.