Four new iridoid glucosides 1–4, named blumeosides A–D, were isolated from the methanolic stem-bark extract of Fagraea blumei G. DON. (Loganiaceae). They were accompanied by the benzyl-alcohol derivative di-O-methylcrenatin (5) and the flavone C-glucoside swertisin (6). The structures of 1–4 were established by spectroscopic methods, including FAB-MS, and 1H- and 13C-NMR, and by alkaline hydrolysis. Blumeosides A (1) and C (3) are 10-O-(2,5-dihydroxytercphthalo) adoxosidic acid and 10-O-(2-hydroxyterephthalo)adoxosidic acid, respectively. In blumeosides B (4) and D (2), both carboxylic groups of the terephthalic-acid moiety are esterified by adoxosidic-acid units, Blumeosides A–D (1–4) inhibited bleaching of crocin induced by alkoxyl radicals. Blumeosides A (1) and D (2) also demonstrated scavenging properties towards the 2,2-diphenyl-1-picryl-hvdrazvl (CDPPH) radical in TLC autographic and spectrophotometric assays.

Iridoid Glucosides with Free Radical Scavenging Properties from Fagraea blumei

The (seco)iridoids and their derivatives, the monoterpenoid indole alkaloids (MIAs), form two large families of plant-derived bioactive compounds with a wide spectrum of high-value pharmacological and insect-repellent activities. Vinblastine and vincristine, MIAs used as anticancer drugs, are produced by Catharanthus roseus in extremely low levels, leading to high market prices and poor availability. Their biotechnological production is hampered by the fragmentary knowledge of their biosynthesis. Here we report the discovery of the last four missing steps of the (seco)iridoid biosynthesis pathway. Expression of the eight genes encoding this pathway, together with two genes boosting precursor formation and two downstream alkaloid biosynthesis genes, in an alternative plant host, allows the heterologous production of the complex MIA strictosidine. This confirms the functionality of all enzymes of the pathway and highlights their utility for synthetic biology programmes towards a sustainable biotechnological production of valuable (seco)iridoids and alkaloids with pharmaceutical and agricultural applications.

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The seco-iridoid pathway from Catharanthus roseus

Chemotaxonomy of the Oleaceae: iridoids as taxonomic markers

Chemotaxonomy of Plantago. Iridoid glucosides and caffeoyl phenylethanoid glycosides.

The monoterpene glycoside secologanin is a key intermediate in the biosynthesis of most indole, cinchona, ipecacuanha, and pyrroloquinoline alkaloids, as well as of simple monoterpene alkaloids. More than a thousand alkaloids are formed from secologanin in vivo; this represents almost a quarter of this large group of natural products. It is also the parent compound of the secoiridoids. Many of the compounds derived from secologanin display a high degree of biological activity and are employed as pharmaceuticals, e.g., the dimeric indole alkaloid leurocristine (vincristine) which is used very successfully in the treatment of acute leukemia. A knowledge of the biosynthesis and biological reactions of secologanin provides a sound basis for the biosynthesis-orientated classification of numerous natural products and the taxonomy of many plants. Secologanin and structurally related substances can be synthesized in a few steps by stereocontrolled photochemical and thermal cycloadditions. Its biomimetic reaction with amines and amino acids yields other natural products and compounds of pharmacological interest.

Søren Rosendal

Papers

13C and1H NMR spectroscopy as a tool in the configurational analysis of iridoid glucosides

Biosynthesis of the iridoid glucoside cornin in Verbena officinalis