Hop bitter acids from the pistillate flower of Humulus lupulus L. and their anti-inflammatory and anti-dengue virus activities

Nine undescribed hop bitter acids, humulupulones A–I (1–9), along with three known ones (10–12) were isolated from the pistillate flower of Humulus lupulus L. (hops). Structurally, compounds 1 and 2 possess an unprecedented nitrogen-containing skeleton in α-acids, and this is the first time that nitrogen has replaced oxygen at the same position, thereby forming a new skeleton consisting of a fused α-acid and a dimethyl-substituted pyridine. Compound 3 is a rare pyridine-2,4-dione derivative simultaneously incorporating prenyl and isopropyl moieties. Compounds 7–10 were separated into four pairs of enantiomers by chiral HPLC separation. Their structures were unambiguously established by comprehensive spectroscopic analysis, X-ray crystallography, and ECD calculations. Moreover, compounds 2, 4, and 7 exhibited significant anti-inflammatory activities in the CuSO₄-induced zebrafish inflammation model. Compounds 4, 5, 7–10 and 12 possessed significant anti-dengue virus (DENV) activities at the viral adsorption and entry stages with IC₅₀ values ranging from 0.29 μM to 70.09 μM, while compounds 7, 10, and 11 exerted anti-DENV activities at the viral replication stage with IC₅₀ values of 71.47 ± 7.30, 19.36 ± 6.77, and 16.18 ± 3.49 μM, respectively.