Expansion of the Stereochemical Space of Triterpenes by Mining Noncanonical Oxidosqualene Cyclases Across the Diversity of Green Plants.

Triterpenoids and steroids are structurally complex polycyclic natural products with potent biological functions, for example, as hormones. In all eukaryotes, the carbon skeletons of these compounds are generated by oxidosqualene cyclases, which carry out a polycyclization cascade to generate four or five rings with up to nine stereogenic centers in a targeted manner. The tight stereochemical control of this cascade reaction severely limits the stereochemical space accessible by known oxidosqualene cyclases.

Chemical tools for unpicking plant specialised metabolic pathways.

Elucidating the biochemical pathways of specialised metabolites in plants is key to enable or improve their sustainable biotechnological production. Chemical tools can greatly facilitate the discovery of biosynthetic genes and enzymes. Here, we summarise transdisciplinary approaches where methods from chemistry and chemical biology helped to overcome key challenges of pathway elucidation.

Biosynthesis of the allelopathic alkaloid gramine in barley by a cryptic oxidative rearrangement.

The defensive alkaloid gramine not only protects barley and other grasses from insects but also negatively affects their palatability to ruminants. The key gene for gramine formation has remained elusive, hampering breeding initiatives. In this work, we report that a gene encoding cytochrome P450 monooxygenase CYP76M57, which we name AMI synthase (AMIS), enables the production of gramine in , , and .

Post-Cyclization Skeletal Rearrangements in Plant Triterpenoid Biosynthesis by a Pair of Branchpoint Isomerases.

Triterpenoids possess potent biological activities, but their polycyclic skeletons are challenging to synthesize. The skeletal diversity of triterpenoids in plants is generated by oxidosqualene cyclases based on epoxide-triggered cationic rearrangement cascades. Normally, triterpenoid skeletons then remain unaltered during subsequent tailoring steps.

Identification of early quassinoid biosynthesis in the invasive tree of heaven () confirms evolutionary origin from protolimonoids.

The tree of heaven, (MILL.) SWINGLE, is a globally invasive plant known to secrete allelopathic metabolites called quassinoids. Quassinoids are highly modified triterpenoids.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry.

We report the synthesis of vanadium(V) oxo complex with a pincer-type dianionic mesoionic carbene (MIC) ligand and the general formula [VOCl(L)]. A comparison of the structural (SC-XRD), electronic (UV-vis), and electrochemical (cyclic voltammetry) properties of with the benzimidazolinylidene congener (general formula [VOCl(L)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium(V) but also for the vanadium(IV) oxidation state by isolating the reduced vanadium(IV) complexes and ([Co(Cp*)] = decamethylcobaltocenium).