Stable Chloro- and Bromoxenate Cage Anions; [X₃(XeO₃)₃]^3– and [X₄(XeO₃)₄]^4– (X = Cl or Br)

The number of isolable compounds which contain different noble-gas–element bonds is limited for xenon and even more so for krypton. Examples of Xe–Cl bonds are rare, and prior to this work, no Xe–Br bonded compound had been isolated in macroscopic quantities. The syntheses, isolation, and characterization of the first compounds to contain Xe–Br bonds and their chlorine analogues are described in the present work. The reactions of XeO₃ with [N­(CH₃)₄]Br and [N­(C₂H₅)₄]­Br have provided two bromo­xenate salts, [N­(C₂H₅)₄]₃[Br₃(XeO₃)₃] and [N(CH₃)₄]₄[Br₄(XeO₃)₄], in which the cage anions have Xe–Br bond lengths that range from 3.0838(3) to 3.3181(8) Å. The isostructural chloroxenate anions (Xe–Cl bond lengths, 2.9316(2) to 3.101(4) Å) were synthesized by analogy with their bromine analogues. The bromo- and chloroxenate salts are stable in the atmosphere at room temperature and were characterized in the solid state by Raman spectroscopy and low-temperature single-crystal X-ray diffraction, and in the gas phase by quantum-chemical calculations. They are the only known examples of cage anions that contain a noble-gas element. The Xe–Br and Xe–Cl bonds are very weakly covalent and can be viewed as σ-hole interactions, similar to those encountered in halogen bonding. However, the halogen atoms in these cases are valence electron lone pair donors, and the σ*_Xe–O orbitals are lone pair acceptors

Metal-Free Aryl Cross-Coupling Directed by Traceless Linkers

The metal-free, highly selective synthesis of biaryls poses a major challenge in organic synthesis. We report the scope and mechanism of a promising new approach to (hetero)biaryls by the photochemical fusion of aryl substituents tethered to a traceless linker (photosplicing). Interrogating photosplicing with varying reaction conditions and comparison of diverse synthetic probes (40 examples, including a suite of heterocycles) showed that the reaction has a surprisingly broad scope and involves neither metals nor radicals. Quantum chemical calculations revealed that the C–C bond is formed by an intramolecular photochemical process that involves an excited singlet state and the traverse of a five-membered transition state, thus warranting consistent ipso‑ipso‑coupling fidelity. These results demonstrate that photosplicing is a unique aryl cross-coupling method in the excited state that can be applied to synthesize a broad range of biaryls. </div

Non-canonical two-step biosynthesis of anti-oomycete indole alkaloids in Kickxellales

Abstract Background Fungi are prolific producers of bioactive small molecules of pharmaceutical or agricultural interest. The secondary metabolism of higher fungi (Dikarya) has been well-investigated which led to > 39,000 described compounds. However, natural product researchers scarcely drew attention to early-diverging fungi (Mucoro- and Zoopagomycota) as they are considered to rarely produce secondary metabolites. Indeed, only 15 compounds have as yet been isolated from the entire phylum of the Zoopagomycota. Results Here, we showcase eight species of the order Kickxellales (phylum Zoopagomycota) as potent producers of the indole-3-acetic acid (IAA)-derived compounds lindolins A and B. The compounds are produced both under laboratory conditions and in the natural soil habitat suggesting a specialized ecological function. Indeed, lindolin A is a selective agent against plant-pathogenic oomycetes such as Phytophthora sp. Lindolin biosynthesis was reconstituted in vitro and relies on the activity of two enzymes of dissimilar evolutionary origin: Whilst the IAA–CoA ligase LinA has evolved from fungal 4-coumaryl-CoA synthetases, the subsequently acting IAA-CoA:anthranilate N-indole-3-acetyltransferase LinB is a unique enzyme across all kingdoms of life. Conclusions This is the first report on bioactive secondary metabolites in the subphylum Kickxellomycotina and the first evidence for a non-clustered, two-step biosynthetic route of secondary metabolites in early-diverging fungi. Thus, the generally accepted “gene cluster hypothesis” for natural products needs to be reconsidered for early diverging fungi