Guaianolide Sesquiterpene Lactones from Centaurothamnus maximus

Centaurothamnus maximus (family Asteraceae), is a leafy shrub indigenous to the southwestern Arabian Peninsula. With a paucity of phytochemical data on this species, we set out to chemically characterize the plant. From the aerial parts, two newly identified guaianolides were isolated: 3 beta-hydroxy-4 alpha(acetoxy)-4 beta(hydroxymethyl)-8 alpha-(4-hydroxy methacrylate)-1 alpha H,5 alpha H, 6 alpha H-gual-10(14),11(13)-dien-6,12-olide (1) and 15-descarboxy picrolide A (2). Seven previously reported compounds were also isolated: 3 beta, 4 alpha, 8 alpha-trihydroxy-4-(hydroxymethyl)-l alpha H, 5 alpha H, 6 beta H, 7 alpha H-guai-10(14),11(13)-dien-6,12-olide (3), chlorohyssopifolin B (4), cynaropikrin (5), hydroxyjanerin (6), chlorojanerin (7), isorhamnetin (8), and quercetagetin-3,6-dimethyl ether-4'-O-beta-d-pyranoglucoside (9). Chemical structures were elucidated using spectroscopic techniques, including High Resolution Fast Atom Bombardment Mass Spectrometry (HR-FAB-MS), 1D NMR; H-1, C-13 NMR, Distortionless Enhancement by Polarization Transfer (DEPT), and 2D NMR (H-1-H-1 COSY, HMQC, HMBC) analyses. In addition, a biosynthetic pathway for compounds 1-9 is proposed. The chemotaxonomic significance of the reported sesquiterpenoids and flavonoids considering reports from other Centaurea species is examined.

Automated summary

In this study, a leafy shrub indigenous to southwestern Arabian Peninsula, Centaurothamnus maximus, was chemically characterized. Two new guaianolides were isolated from the plant, along with seven previously reported compounds. The chemical structures were elucidated using various spectroscopic techniques, and a biosynthetic pathway for the compounds was proposed. The study also examined the chemotaxonomic significance of the reported sesquiterpenoids and flavonoids in comparison to other Centaurea species.