Oplopane Sesquiterpenes from Ligularia knorringiana and Their Anti-Complementary Activity

Three new oplopane sesquiterpenes, knorringianalarins D - F (1 - 3, respectively), and five known analogues (4 - 8, respectively), were isolated from the roots and rhizomes of Ligularia knorringiana. The structures of three new compounds were identified as 4-acetoxy-11,12-epoxy-2-hydroxy-3-(2-methylbutyryloxy)-9-(4-methylsenecioyloxy)oplop-10(14)-ene (1), 3,4-diacetoxy-9-(4-acetoxy-4-methylsenecioyloxy)-11,12-epoxy-8-(2-methylbutyryloxy)oplop-10(14)-ene (2), and (1R,5R,6R,7R,9R)-5,9,11-trihydroxy-4,15-dinoroplop-10(14)-en-3-one (3) based on spectroscopic methods including 1D- and 2D-NMR, mass spectrometry, and CD spectroscopy techniques. All compounds were evaluated for their anti-complementary activity on the classical pathway of the complement system in vitro. Among which, three oplopane sesquiterpenes (3, 7, and 8) exhibited better anti-complementary effects with CH50 values ranging from 0.33 to 0.89 mm, which are plausible candidates for developing potent anti-complementary agents.