Cynasibirolide A, One New Humulanolide Sesquiterpene from Cynanchum acutum subsp. sibiricum

Abstract Cynasibirolide A ( 1 ), one new humulanolide sesquiterpene, together with four known analogs, asteriscanolide ( 2 ), (1 S ,8 S )‐8‐hydroxyhumula‐2 Z ,6 E ,9 E ‐trien‐1,12‐olide ( 3 ), (1 S ,7 R )‐8‐oxohumula‐2 Z ,9 E ‐dien‐1,12‐olide ( 4 ), and (+)‐6,7,9,10‐tetrahydroasteriscunolide ( 5 ) were isolated from the roots and rhizomes of Cynanchum acutum subsp. sibiricum . Their structures and configurations were elucidated by spectroscopic methods, including 2D‐NMR techniques, and the structure of 1 was confirmed by single‐crystal X‐ray diffraction. All compounds were evaluated for their anti‐complementary activity in vitro , and compound 3 exhibited anti‐complement effect with CH 50 value of 0.45 mM.