New Phenolic Compounds from Eucalyptus tereticornis with Antioxidant Activity

Fourteen phenolic compounds, comprising four sesquilignans (1‒4), five flavonoids (5‒9), two phenylethanol glycosides (10 and 11) together with three acylphloroglucinols (12‒14), were isolated from Eucalyptus tereticornis. Their structures were elucidated through spectroscopic data analyses and electronic circular dichroism (ECD) calculations. Notably, tereticornisin A (1), phloretin 2'-O-(6-deoxy-β-d-glucopyranoside) (6), phloretin 2'-O-(6-deoxy-β-d-galactopyranoside) (7), and 2-(4-hydroxyphenyl)ethyl-[6-O-(Z)-feruloyl]-β-d-glucopyranoside (10) were new compounds. In addition, phloretin-2'-O-β-d-xylopyranoside (5) was isolated and definitively assigned for the first time, despite its structure having been previously speculated based on HPLC-UV-MS/MS analysis. Eight of the isolates displayed inhibitory activity against the ABTS free radicals with IC₅₀ values that ranged from 8.07 ± 0.10 to 14.41 ± 0.30 µM, comparable to the positive control (IC₅₀ 18.12 ± 0.21 µM). 12‒14 exhibited no antibacterial activity on methicillin-resistant Staphylococcus aureus (MRSA) strains Newman, USA300, and Mu50, and vancomycin-resistant Enterococcus faecium (VREfm) strain HS-0649.