Diversity of plant nitrogenous compounds: recent results from biodiversity

Natural products from plants continue to provide original structures, both synthetic challenges for organic chemists and possible leads for the discovery of new drugs. The biosynthetic pathways by which plants produce these molecules are often other challenges. By a survey of some results obtained in the laboratory, this presentation will deal with the chemistry of plant original molecules, sometimes formed by unique biosynthetic pathways.

By contrast with the ubiquity of proteins in plants tissues, peptides are rarely found. However, from the seeds of several Annona (Annonaceae), as well as from the latex of african Jatropha (Euphorbiaceae), cyclopeptides with 6 to 10 proteinogenic amino acid residues have been isolated, sequenced and their 3D structures characterised.

The Daphniphyllum alkaloids elaborated by trees of the genus Daphniphyllum (Daphniphyllaceae, a small monogeneric asiatic family) form a unique class of highly complex and structurally diversified alkaloids, with polycyclic skeletons. They have attracted great interest for their intriguing biosynthetic pathway, which leads to more than 100 alkaloids isolated from the 10 species phytochemically studied by several groups. These alkaloids are presumably originated from an amino derivative of squalene formed at early step of the biosynthetic pathway and followed by multiple cyclisations.

The Myrioneuron genus (Rubiaceae) yielded several new alkaloid skeletons, based on the 9-substituted decahydroquinoline motif. Thus, we have isolated several new tricyclic oxazines, tricyclic derivatives of cis-decahydroquinoline for which the N-in and N-out forms have been separately characterized by X-ray crystallography, tetracyclic and hexacyclic alkaloids. Some of these structures were confirmed by their asymmetric total synthesis.

Recents results will be discussed in term of structure, synthesis or hemisynthesis, plausible biosynthetic pathways and bioactivities.