Helenanolide-type sesquiterpene lactones. Part 2. The molecular conformations of arnifolin and some related helenanolides as determined by X-ray crystallographic and NMR spectroscopic analyses

Discovery of a non-stereoselective cytochrome P450 catalyzing either 8α- or 8β-hydroxylation of germacrene A acid from the Chinese medicinal plant, Inula hupehensis

Sesquiterpene lactones (STLs) are C15 terpenoid natural products with α-methylene γ-lactone moiety. A large proportion of STLs in Asteraceae species is derived from the central precursor germacrene A acid (GAA). Formation of the lactone rings depends on the regio-(C6 or C8) and stereoselective (α- or β-)hydroxylations of GAA, producing STLs with four distinct stereo-configurations (12,6α-, 12,6β-, 12,8α-, and 12,8β-olide derivatives of GAA) in nature. Curiously, two configurations of STLs (C12,8α and C12,8β) are simultaneously present in the Chinese medicinal plant, Inula hupehensis. However, how these related yet distinct STL stereo-isomers are co-synthesized in I. hupehensis remains unknown. Here, we describe the functional identification of the I. hupehensis cytochrome P450 (CYP71BL6) that can catalyze the hydroxylation of GAA in either 8α- or 8β-configuration, resulting in the synthesis of both 8α- and 8β-hydroxyl GAAs. Of these two products, only 8α-hydroxyl GAA spontaneously lactonizes to the C12,8α-STL while the 8β-hydroxyl GAA remains stable without lactonization. Chemical structures of the C12,8α-STL, named inunolide, and 8β-hydroxyl GAA were fully elucidated by nuclear magnetic resonance analysis and mass spectrometry. The CYP71BL6 displays 63-66% amino acid identity to the previously reported CYP71BL1/2 catalyzing GAA 6α- or 8β-hydroxylation, indicating CYP71BL6 shares the same evolutionary lineage with other stereoselective cytochrome P450s, but catalyzes hydroxylation in a non-stereoselective manner. We observed that the CYP71BL6 transcript abundance correlates closely to the accumulation of C12,8-STLs in I. hupehensis. The identification of CYP71BL6 provides an insight into the biosynthesis of STLs in Asteraceae.

Crystal structure of pseudoguainolide

The lactone ring in the title molecule, C15H22O3(systematic name: 3,4a,8-trimethyldodecahydroazuleno[6,5-b]furan-2,5-dione), assumes an envelope conformation with the methine C atom adjacent to the the methine C atom carrying the methyl substituent being the flap atom. The other five-membered ring adopts a twisted conformation with the twist being about the methine–methylene C—C bond. The seven-membered ring is based on a twisted boat conformation. No specific interactions are noted in the the crystal packing.

Natural and synthetic α-methylenelactones and α-methylenelactams with anticancer potential

α-Methylene-γ- and δ-lactones, as well as α-methylene-γ- and δ-lactams, are plant-derived compounds often used in traditional medicine for the treatment of inflammatory diseases. In recent years, the anticancer properties of these compounds and the molecular mechanisms of their action have been studied extensively. In the search for modern anticancer drugs, various synthetic analogs of α-methylene-γ- and δ-lactones and lactams have been synthesized and tested for their cytotoxic activity. In this review, we give a brief description of the occurrence and biological activity of such compounds isolated from plants and their diverse synthetic analogs.

What made sesquiterpene lactones reach cancer clinical trials?

Sesquiterpene lactones (SLs) are plant-derived compounds often used in traditional medicine against inflammation and cancer. This review focuses on the chemical and biological properties of SLs that lead to enhanced anticancer and anti-inflammatory effects. The chemical properties comprise alkylating center reactivity, lipophilicity, and molecular geometry and electronic features. SLs in clinical trials are artemisinin, thapsigargin and parthenolide and many of their synthetic derivatives. These drugs are selective toward tumor and cancer stem cells by targeting specific signaling pathways, which make them lead compounds in cancer clinical trials.

Helenanolide-type sesquiterpene lactones--III. Rates and stereochemistry in the reaction of helenalin and related helenanolides with sulfhydryl containing biomolecules

The reactivity of the two potential Michael addition sites of the helenanolide-type sesquiterpene lactone helenalin towards the physiological thiols glutathione (GSH) and cysteine (cys) in aqueous solution was investigated by 1H NMR spectroscopic experiments. In the presence of one molar equivalent of GSH, the reaction was shown to occur with high regio- and stereoselectivity at the beta-position of C-2 in the cyclopentenone ring. Addition to the exocyclic methylene group at the lactone ring was found to occur in the presence, of GSH in molar ratios over 1:1, but proceeded at a rate 10 times smaller than at C-2 leading to the 2 beta,13(11 beta)-bis-glutathionyl adduct. In contrast, addition of free cys highly favoured the exocyclic methylene group. Addition of GSH to the cyclopentenone of 11 alpha, 13-dihydrohelenalin (plenolin) showed the same characteristics as observed with helenalin while 2 alpha-acetoxy-2,3-dihydro-4 beta H-helenalin (chamissonolide) did not form an adduct when incubated with an equimolar amount of GSH. Explanations for the observed differences in reactivity of the two potential reaction sites based on MO computations are given and implications for the biological activity of this type of sesquiterpene lactones are discussed.