Structural and Functional Analysis of Bacterial Sulfonosphingolipids and Rosette-Inducing Factor 2 (RIF-2) by Mass Spectrometry-Guided Isolation and Total Synthesis.

We have analyzed the abundance of bacterial sulfonosphingolipids, including rosette-inducing factors (RIFs), in seven bacterial prey strains by using high-resolution tandem mass spectrometry (HRMS2 ) and molecular networking (MN) within the Global Natural Product Social Molecular Networking (GNPS) web platform. Six sulfonosphingolipids resembling RIFs were isolated and their structures were elucidated based on comparative MS and NMR studies. Here, we also report the first total synthesis of two RIF-2 diastereomers and one congener in 15 and eight synthetic steps, respectively. For the total synthesis of RIF-2 congeners, we employed a decarboxylative cross-coupling reaction to synthesize the necessary branched α-hydroxy fatty acids, and the Garner-aldehyde approach to generate the capnine base carrying three stereogenic centers. Bioactivity studies in the choanoflagellate Salpingoeca rosetta revealed that the rosette inducing activity of RIFs is inhibited dose dependently by the co-occurring sulfonosphingolipid sulfobacins D and F and that activity of RIFs is specific for isolates obtained from Algoriphagus.

Metabolic Pathway Rerouting in Paraburkholderia rhizoxinica Evolved Long-Overlooked Derivatives of Coenzyme F420.

Coenzyme F420 is a specialized redox cofactor with a negative redox potential. It supports biochemical processes like methanogenesis, degradation of xenobiotics, and the biosynthesis of antibiotics. Although well-studied in methanogenic archaea and actinobacteria, not much is known about F420 in Gram-negative bacteria. Genome sequencing revealed F420 biosynthetic genes in the Gram-negative, endofungal bacterium Paraburkholderia rhizoxinica, a symbiont of phytopathogenic fungi. Fluorescence microscopy, high-resolution LC-MS, and structure elucidation by NMR demonstrated that the encoded pathway is active and yields unexpected derivatives of F420 (3PG-F420). Further analyses of a biogas-producing microbial community showed that these derivatives are more widespread in nature. Genetic and biochemical studies of their biosynthesis established that a specificity switch in the guanylyltransferase CofC reprogrammed the pathway to start from 3-phospho-d-glycerate, suggesting a rerouting event during the evolution of F420 biosynthesis. Furthermore, the cofactor activity of 3PG-F420 was validated, thus opening up perspectives for its use in biocatalysis. The 3PG-F420 biosynthetic gene cluster is fully functional in Escherichia coli, enabling convenient production of the cofactor by fermentation.

Stereoselective synthesis of unnatural (2S,3S)-6-hydroxy-4-sphingenine-containing sphingolipids.

6-Hydroxy-(4E)-sphingenine-containing sphingolipids are found in mammalian and bacterial membranes and have multiple intra- and intercellular functions. Most sphingolipids contain a (2S,3R)-2-amino-1,3-diol core structure, but only limited examples of unnatural (2S,3S)-2-amino-1,3-diol derivates have so far been reported. Using an underexplored hydrozirconation-transmetalation reaction and an unusual three-step-one-pot deprotection sequence, we were able to synthesize several unnatural (2S,3S)-6-hydroxy-(4E)-sphingenine-containing sphingolipids in only three (protected) or four (deprotected) consecutive steps, respectively, including a fluoresence-labeled derivative suitable for future biological studies.

Expanding the Rubterolone Family: Intrinsic Reactivity and Directed Diversification of PKS-derived Pyrans.

We characterized two key biosynthetic intermediates of the intriguing rubterolone family (tropolone alkaloids) that contain a highly reactive pyran moiety (in equilibrium with the hydrolyzed 1,5-dione form) and undergo spontaneous pyridine formation in the presence of primary amines. We exploited the intrinsic reactivity of the pyran moiety and isolated several new rubterolone derivatives, two of which contain a unique thiazolidine moiety. Three rubterolone derivatives were chemically modified with fluorescence and biotin tags using peptide coupling and click reaction. Overall, eight derivatives were fully characterized by HRMS/MS and 1D and 2D NMR spectroscopy and their antimicrobial, cytotoxic, anti-inflammatory and antiparasitic activities evaluated.

Isolation, Biosynthesis and Chemical Modifications of Rubterolones A-F: Rare Tropolone Alkaloids from Actinomadura sp. 5-2.

The discovery of six new, highly substituted tropolone alkaloids, rubterolones A-F, from Actinomadura sp. 5-2, isolated from the gut of the fungus-growing termite Macrotermes natalensis is reported. Rubterolones were identified by using fungus-bacteria challenge assays and a HRMS-based dereplication strategy, and characterised by NMR and HRMS analyses and by X-ray crystallography. Feeding experiments and subsequent chemical derivatisation led to a first library of rubterolone derivatives (A-L). Genome sequencing and comparative analyses revealed their putative biosynthetic pathway, which was supported by feeding experiments. This study highlights how gut microbes can present a prolific source of secondary metabolites.

Tuning ligand effects and probing the inner-workings of bond activation steps: generation of ruthenium complexes with tailor-made properties.

Activating chemical bonds through external triggers and understanding the underlying mechanism are at the heart of developing molecules with catalytic and switchable functions. Thermal, photochemical, and electrochemical bond activation pathways are useful for many chemical reactions. In this Article, a series of Ru(II) complexes containing a bidentate and a tripodal ligand were synthesized. Starting from all-pyridine complex 1(2+), the pyridines were stepwise substituted with "click" triazoles (2(2+)-7(2+)). Whereas the thermo- and photoreactivity of 1(2+) are due to steric repulsion within the equatorial plane of the complex, 3(2+)-6(2+) are reactive because of triazoles in axial positions, and 4(2+) shows unprecedented photoreactivity. Complexes that feature neither steric interactions nor axial triazoles (2(2+) and 7(2+)) do not show any reactivity. Furthermore, a redox-triggered conversion mechanism was discovered in 1(2+), 3(2+), and 4(2+). We show here ligand design principles required to convert a completely inert molecule to a reactive one and vice versa, and provide mechanistic insights into their functioning. The results presented here will likely have consequences for developing a future generation of catalysts, sensors, and molecular switches.

Trifluoromethanesulfonic acid catalyzed friedel-Crafts alkylations of 1,2,4-trimethoxybenzene with aldehydes or benzylic alcohols.

Trifluoromethanesulfonic acid in acetonitrile was found to efficiently catalyze Friedel-Crafts alkylations of 1,2,4-trimethoxybenzene with a variety of simple or functionalized aldehydes to provide di- or triarylmethanes in high yields. The operationally simple protocol allowed a short synthesis of the phenylpropanoid natural product (-)-tatarinoid C establishing its absolute configuration. Under the developed reaction conditions a benzylic alcohol instead of an aldehyde also underwent reactions with 1,2,4-trimethoxybenzene and other nucleophiles to afford unsymmetrically substituted compounds.