New Flavoalkaloids with Potent α‑Glucosidase and Acetylcholinesterase Inhibitory Activities from Yunnan Black Tea ‘Jin-Ya’

As a subgroup of flavoalkaloids, N-ethyl-2-pyrrolidinone substituted flavan-3-ols are reported to possess various biological activities that may play important roles in the beneficial healthcare functions of tea. The HPLC and LC-MS analyses showed the existence of N-ethyl-2-pyrrolidinone substituted flavan-3-ols in ‘Jin-Ya’, which is a Yunnan black tea produced only from the buds of the tea plant Camellia sinensis var. assamica. Further phytochemical study on this precious black tea led to the identification of 8 flavoalkaloids, 1–8, along with 11 known flavan-3-ols (9–14) and flavonol glycosides (15–19). The new compounds, (−)-6-(5″S)-N-ethyl-2-pyrrolidinone-epiafzelechin (1), (−)-8-(5″R)-N-ethyl-2-pyrrolidinone-epiafzelechin-3-O-gallate (2a), and (−)-8-(5″S)-N-ethyl-2-pyrrolidinone-epiafzelechin-3-O-gallate (2b), were identified based on extensive spectroscopic analysis. Flavoalkaloids 2–6 showed inhibitory activity on α-glucosidase with IC50 values ranging from 2.09 to 8.47 μM, compared to those of quercetin and acarbose (IC50 = 6.87 and 228.9 μM, respectively). Moreover, compounds 2, 3, and 6 displayed an inhibitory effect on acetyl-cholinesterase with IC50 values of 14.23, 33.79, and 34.82 μM, respectively, compared to tacrine (IC50 = 0.223 μM).