Defensive Sesquiterpenes from Senecio candidans and S. magellanicus, and Their StructureActivity Relationships

Revisiting the absolute chirality and polymorphism of (-)-Istanbulin A

The terpenoid (-)-Istanbulin A is a natural product isolated from Senecio filaginoides DC, one of the 270 species of Senecio (Asteraceae) which occurs in Argentina. The structure and absolute configuration of this compound [9a-hydroxy-3,4a,5-trimethyl-4a,6,7,8a,9,9a-hexahydro-4H,5H-naphtho[2,3-b]-furan-2,8-dione or (4S,5R,8R,10S)-1-oxo-8β-hydroxy-10βH-eremophil-7(11)-en-12,8β-olide, C₁₅H₂₀O₄] were determined by single-crystal X-ray diffraction studies. It proved to be a sesquiterpene lactone showing an eremophilanolide skeleton whose chirality is described as 4S,5R,8R,10S. Structural results were also in agreement with the one- and two-dimensional (1D and 2D) NMR and HR-ESI-MS data, and other complementary spectroscopic information. In addition, (-)-Istanbulin A is a polymorph of the previously reported form of (-)-Istanbulin A, form I; thus, the title compound is denoted form II or polymorph II. Structural data and a literature search allowed the chirality of Istanbulin A to be revisited. The antimicrobial and antifungal activities of (-)-Istanbulin A, form II, were evaluated in order to establish a reference for future comparisons and applications related to specific crystal forms of Istanbulins.

Insect Antifeedant Components of Senecio fistulosus var. fistulosus-Hualtata

From a bioactive methanolic extract of Senecio fistulosus, the antifeedant effects of the alkaloidal and non-alkaloidal fractions were tested against the insects Spodoptera littoralis, Myzus persicae and Rhopalosiphum padi, with the non-alkaloidal fraction being antifeedant. The phytochemical study of the non-alkaloidal fraction of S. fistulosus, resulted in the isolation of four compounds, two 9-oxo-furanoeremophilanes (1, 2), an eremophilanolide, 1β,10β-epoxy-6-acetoxy-8α-hydroxy-eremofil-7(11)-en-8β,12-olide (3) and a maaliol derivative (4). The alkaloidal fraction yielded two known pyrrolizidine alkaloids (5, 6). Compounds 1, 3 and 4 are new natural products. Furanoeremophilane 2 was a strong antifeedant against S. littoralis and maaliane 4 inhibited the settling of M. persicae.

Sesquiterpenes, flavonoids, shikimic acid derivatives and pyrrolizidine alkaloids from Senecio kingii Hook

Twenty-four compounds including eleven eremophilanolides (1-11), one eremophilane (13), five shikimic acid derivatives (14-18), six flavonoids (19-24), and the macrocyclic unsaturated pyrrolizidine alkaloid integerrimine (25) were isolated from Senecio kingii, an endemic species from the Magallanes Region (Chile). Compounds 3, 5, 6, 8-11 and 13-18 have not been previously reported as natural products. Their molecular structures were determined by NMR spectroscopic analysis and comparison with published NMR data. An X-ray-analysis of compound 3 has been performed. Their insecticidal and antifungal activities were tested, being compound 3 the strongest insect antifeedant. Compounds 6, 9 and 18 were moderate antifungals.

Eremophilane-type sesquiterpenoids with diverse skeletons from Ligularia sagitta

Five new highly oxygenated eremophilane-type sesquiterpenoids, possessing C19 (1 and 2), C15 (3 and 4), and C14 (8) skeletons, along with eight known eremophilenolides were obtained from the aerial parts of Ligularia sagitta. The absolute configuration of 1 was assigned by X-ray diffraction analysis and that of 3 by ECD spectroscopy. Compounds 1-10 were evaluated for their antibacterial activities against Staphyloccocus aureus, Bacillus subtilis, Escherichia coli, Bacillus cereus, and Erwinia carotovora. Compounds 4 and 5 displayed broad-spectrum inhibitory activity against these bacteria with MIC values of approximately 7.25 μg/mL, followed by 3 and 6 with MIC values in the range of 23.0-125.0 μg/mL. Compounds 3 and 8 showed mild activity against three human tumor cell lines (IC50 ≈ 13 μM). Preliminary structure-activity relationships for these eremophilenolides are reported.

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2012 is reviewed, and 471 references are cited.