Tasmannioside A, a New Eudesmane Sesquiterpene Glycoside, and Antioxidant Phenolic Derivatives from Tasmannia lanceolata

Tasmannia lanceolata (pepperberry) is a pungent plant rich in polyphenols with considerable potential as a natural antioxidant source. In this study, a phytochemical investigation of T. lanceolata extract was performed using column chromatography over RP-C18, silica gel, and Sephadex LH-20. Eight compounds were isolated, and their structures were elucidated by extensive spectroscopic analyses (1D and 2D NMR, HR-ESI-MS, UV, IR, CD, and ORD). Compound 1 was identified as a new eudesmane-type sesquiterpene glycoside, designated tasmannioside A, which exhibited α-glucosidase inhibitory activity with an inhibition rate of 21.63 ± 1.57% at 0.172 mM. The known compounds comprised one organic acid, shikimic acid (2); three phenolic acids, protocatechuic acid (3), methyl chlorogenate (4), and ethyl chlorogenate (7); one flavonoid, catechin-(5,6-bc)-4α/4β-(3,4-dihydroxyphenyl)-dihydro-2(3H)-pyranone (5); one phenylpropanoid glycoside, michehedyoside D (6); and one sesquiterpenoid, oplodiol (8). In the DPPH radical scavenging assay, compounds 2–7 demonstrated antioxidant activity, with the scavenging ratios of 0.04, 6.96, 4.57, 5.80, 1.97, and 3.15, respectively. Structure–activity relationship analysis revealed that the catechol moiety is critical for potent radical scavenging. These findings enrich the phytochemical diversity of T. lanceolata and provide preliminary in vitro evidence supporting further investigation of its phenolic constituents as natural antioxidants. However, in vivo efficacy, bioavailability, and long-term safety remain to be established.