The lomaiviticins are dimeric genotoxic bacterial metabolites that contain unusual diazocyclopentadiene functional groups and 2-4 deoxyglycoside residues. Because only 6 of 19 carbon atoms in the monomeric aglycon unit are proton-attached, their structure determination by NMR spectroscopic analysis is non-trivial. Prior structure elucidation efforts established that the two halves of the lomaiviticins are joined by a single carbon-carbon bond appended to an oxidized cyclohexenone ring. This ring was believed to comprise a 4,5-dihydroxycyclohex-2-en-1-one. The bridging bond was positioned at C6. This structure proposal has not been tested because none of the lomaiviticins have been prepared by total chemical synthesis or (to the best of our knowledge) successfully analyzed by X-ray crystallography. Here we disclose microED studies which establish that (-)-lomaiviticin C contains a 4,6-dihydroxy-cyclohex-2-en-1-one residue, that the bridging carbon-carbon bond is located at C5, and that the orientation of the cyclohexenone ring and configuration of the secondary glycoside are reversed, relative to their original assignment. High-field (800 MHz) NMR analysis supports the revised assignment and suggests earlier efforts were misled by a fortuitous combination of a nearzero 3 JH4,H5 coupling constant and a 4-bond HMBC correlation that was interpreted as a 3-bond coupling. DFT calculations of the expected 13 C chemical shifts of the original and revised structures of the aglycon and (-)-lomaiviticin B provide further robust support for the structure revision. Because the interconversion of lomaiviticins A, B, and C has been demonstrated, these findings apply to each isolate. These studies clarify the structures of this family of metabolites and underscore the power of microED analysis in natural products structure determination.

Here we apply microcrystal electron diffraction (MicroED) to the structural determination of transition-metal complexes. We find that the simultaneous use of 300 keV electrons, very low electron doses, and an ultrasensitive camera allows for the collection of data without cryogenic cooling of the stage. This technique reveals the first crystal structures of the classic zirconocene hydride, colloquially known as “Schwartz’s reagent”, a novel Pd(II) complex not amenable to solution-state NMR or X-ray crystallography, and five other paramagnetic and diamagnetic transition-metal complexes.

More than 60% of pharmaceuticals are related to natural products (NPs), chemicals produced by living organisms. 1 Hence, new methods that accelerate natural product discovery are poised to profoundly impact human health. Of the many challenges that remain in natural product discovery, none are as pervasive as structural elucidation, as determination of the molecular structure of a newly discovered natural product can take months, years, or in some cases be altogether unachievable. This challenge can be fueled by lack of sufficient material for spectroscopic analysis, or difficulties in sourcing the producing organism. 2 Even in cases where the analyte is abundant, its physical properties, including molecular structure, can prevent unambiguous structural determination. 3,4 Here we report the use of microcrystal electron diffraction (MicroED), 5 an emerging cryogenic electron microscopy (CryoEM) technique, in combination with genome mining 6 to address these challenges. As proof-of-principle, we apply these techniques to fischerin (1), an orphan NP isolated more than 30 years ago, with potent

4-Hydroxy-2-pyridone alkaloids have attracted attention for synthetic and biosynthetic studies due to their broad biological activities and structural diversity. Here, we elucidated the pathway and chemical logic of (−)-sambutoxin (1) biosynthesis. In particular, we uncovered the enzymatic origin of the tetrahydropyran moiety and showed that the p-hydroxyphenyl group is installed via a late-stage, P450-catalyzed oxidation of the phenylalanine-derived side chain rather than via a direct incorporation of tyrosine.

Few nucleoside-derived natural products have been identified from animals, despite the ubiquity of nucleosides in living organisms. Here, we use a combination of synthesis and the emerging electron microscopy technique microcrystal electron diffraction to determine the structures of several N 3 -(β-glucopyranosyl)uric acid derivatives in Caenorhabditis elegans. These noncanonical gluconucleosides further integrate an ascaroside moiety, for which we present a shortened synthetic route. The production of a phosphorylated gluconucleoside is influenced by evolutionarily conserved insulin signaling.