C17-Labdane diterpenoid alkaloids bearing a rare skeleton with anti-inflammatory and anti-oxidant activities from Forsythia suspensa

Two undescribed C₁₇-Labdane diterpenoid alkaloids, named forsylinfenines A and B (1-2), attributable to a rare 4,4,10,13-tetramethyl-1(2),3(4),5(10),6(7)-octahydrobenzo[f]quinolin skeleton, along with three known β-carboline-type alkaloids (3-5), were isolated. The chemical structures including absolute configurations of two undescribed compounds were established by means of integrated spectroscopic techniques and electronic circular dichroism (ECD) calculations. In addition, a plausible biosynthetic pathway for the formation of compounds 1 and 2 was proposed. In vitro, five alkaloids (1-5), especially two undescribed alkaloids with rare skeleton (1-2), exhibited significant anti-inflammatory activities due to inhibiting the release of TNF-α, IL-6, and IL-1β, as well as effective anti-oxidant activities owing to preventing the production of ROS in the LPS-induced RAW264.7 cells.