New phenolic derivatives and bioactive constituents from Castanopsis lamontii Hance: enzyme inhibitory activities and structure-activity insights

Phytochemical investigation of the mature leaves of Castanopsis lamontii, an underexplored traditional folk medicine, led to the isolation of two new phenolic derivatives, lamontin B (1) and 6-O-(5-O-methylgalloyl)-D-glucopyranose (2) and eleven known compounds. Their structures were elucidated through extensive spectroscopic analysis. Bioactivity assays revealed that kaempferol (10), quercetin (11), and castanopsinin E (13) exhibited potent α-glucosidase inhibitory activities (IC50: 0.03–0.15 mM), which were significantly more active than the positive control acarbose (IC50: 1.5 mM). Furthermore, isodehydrodigallic acid (4), 10, and 11 displayed significant tyrosinase inhibition (IC50: 0.32–0.61 mM), surpassing kojic acid (IC50: 0.83 mM). Structure-activity relationship analysis suggested that the C-28 glycosylation in triterpenoid ellagitannins and the B-ring catechol moiety in flavonoids are pivotal for their α-glucosidase inhibition.