Biosynthesis of GMGT lipids by a radical SAM enzyme associated with anaerobic archaea and oxygen-deficient environments.

Archaea possess characteristic membrane-spanning lipids that are thought to contribute to the adaptation to extreme environments. However, the biosynthesis of these lipids is poorly understood. Here, we identify a radical S-adenosyl-L-methionine (SAM) enzyme that synthesizes glycerol monoalkyl glycerol tetraethers (GMGTs). The enzyme, which we name GMGT synthase (Gms), catalyzes the formation of a C(sp3)-C(sp3) linkage between the two isoprenoid chains of glycerol dialkyl glycerol tetraethers (GDGTs). This conclusion is supported by heterologous expression of gene gms from a GMGT-producing species in a methanogen, as well as demonstration of in vitro activity using purified Gms enzyme. Additionally, we show that genes encoding putative Gms homologs are present in obligate anaerobic archaea and in metagenomes obtained from oxygen-deficient environments, and appear to be absent in metagenomes from oxic settings.

Total Synthesis of (+)-Discorhabdin V.

The discorhabdin natural products are a large subset of pyrroloiminoquinone alkaloids with a myriad of biological activities. Despite garnering much synthetic attention, few members have thus far been completed, particularly those featuring a bridging carbon-nitrogen bond that is found in numerous discorhabdins, including discorhabdin V. Herein we report the first total synthesis and full stereochemical assignment of (+)-discorhabdin V. To access the pyrroloiminoquinone we developed a convergent N-alkylation/oxidative aminocyclization/bromination cascade that joins two key components, which are both made on multigram scale. An intramolecular Heck reaction then forms the quaternary carbon center in an intermediate containing the carbon-nitrogen bridge, and a reductive N,O-acetal cyclization sequence introduces the final piperidine ring. Furthermore, we have established the relative configuration of (+)-discorhabdin V through experimental NOESY data and DP4 NMR probability calculations. The absolute configuration of the natural product has also been determined by circular dichroism and the use of an amino acid derived chiral starting material. Our work represents one of only two reports of a total synthesis of a nitrogen-bridged discorhabdin and paves the way for future biological evaluation of such compounds.

A Metal-Free Cyclobutadiene Reagent for Intermolecular [4 + 2] Cycloadditions.

Cyclobutadiene is a highly reactive antiaromatic hydrocarbon that has fascinated chemists for over 60 years. However, its preparation and uses in chemical synthesis are sparing, in part due to its lengthy synthesis that generates hazardous byproducts including excess heavy metals. Herein, we report a scalable, metal-free cyclobutadiene reagent, diethyldiazabicyclohexene dicarboxylate, and explore its intermolecular [4 + 2] cycloaddition with various electron-deficient alkenes. We also demonstrate its utility in a three-step synthesis of dipiperamide G and a diverse array of product derivatizations including bromocyclobutadiene.

Aqueous Amine-Tolerant [2+2] Photocycloadditions of Unactivated Olefins.

The Kochi-Salomon reaction is the only photochemical [2+2] cycloaddition capable of combining two electronically unactivated olefins into a cyclobutane. Yet, the reaction has remained largely unexplored and suffers many drawbacks, most notably an intolerance to Lewis/Brønsted basic amines and amides. Since these groups are ubiquitous in biologically active pharmaceuticals, an amine-tolerant Kochi-Salomon reaction would greatly facilitate rapid exploration of novel drug scaffolds. Herein, we disclose a transformation that is run in water with the most widely available Cu(II) salts and mineral acids. Furthermore, we apply this methodology to synthesize a variety of amine-containing cyclobutanes, including known and novel pharmacological analogues.

Conversion of Aryl Azides to Aminopyridines.

A longstanding challenge in fundamental functional group interconversion has been the direct transformation of benzene into pyridine via nitrogen insertion and carbon deletion. Herein, we report a protocol for the transformation of aryl azides, easily accessible from their corresponding anilines, to 2-aminopyridines using blue light and oxygen. Mechanistic studies corroborate that the arene to pyridine conversion is achieved by nitrogen insertion into the benzene ring followed by oxidative carbon extrusion.

Synthesis and Characterization of the Potential Energetic Propellant Plasticizer 3-Nitratoethyl-N-nitramino-5-nitratomethyl Isoxazole.

The synthesis of 3-(nitratoethyl-N-nitramino)-5-(nitratomethyl) isoxazole (C6 H7 N5 O9 , 1) is presented, and its energetic properties were ascertained and analyzed for energetic applications potential. 1 was found to be a solid without melting behavior, begins to decompose at 140 °C, and has a thermal onset decomposition temperature of 171.5 °C. 1 was synthesized in 5 steps from glyoxylic acid, and was found to exhibit acceptable sensitivities to impact, friction, and electrostatic discharge. The presence of the nitratoethyl nitramino (NENA) moiety, coupled with the high density (1.71 g cm-3 ) and superior calculated specific impulse (247.6 s) over the commonly employed gun propellant n-butyl NENA (density=1.22 g cm-3 , specific impulse=221 s), makes 1 a potential energetic plasticizer for next generation gun and rocket propellants. In addition, a modified procedure for the synthesis of dibromoformaldoxime (DBFO) was developed to provide this material in respectable yields on one mole scale. The safety considerations of DBFO are also highlighted, in which this compound sublimes, and must be handled with care, as it will cause burns upon contact with the skin.

Cannabinoid receptor 1 antagonist genistein attenuates marijuana-induced vascular inflammation.

Epidemiological studies reveal that marijuana increases the risk of cardiovascular disease (CVD); however, little is known about the mechanism. Δ9-tetrahydrocannabinol (Δ9-THC), the psychoactive component of marijuana, binds to cannabinoid receptor 1 (CB1/CNR1) in the vasculature and is implicated in CVD. A UK Biobank analysis found that cannabis was an risk factor for CVD. We found that marijuana smoking activated inflammatory cytokines implicated in CVD. In silico virtual screening identified genistein, a soybean isoflavone, as a putative CB1 antagonist. Human-induced pluripotent stem cell-derived endothelial cells were used to model Δ9-THC-induced inflammation and oxidative stress via NF-κB signaling. Knockdown of the CB1 receptor with siRNA, CRISPR interference, and genistein attenuated the effects of Δ9-THC. In mice, genistein blocked Δ9-THC-induced endothelial dysfunction in wire myograph, reduced atherosclerotic plaque, and had minimal penetration of the central nervous system. Genistein is a CB1 antagonist that attenuates Δ9-THC-induced atherosclerosis.

Enantioselective Cyclobutenylation of Olefins Using N-Sulfonyl-1,2,3-Triazoles as Vicinal Dicarbene Equivalents.

Cyclobutenes are highly useful synthetic intermediates as well as important motifs in bioactive small molecules. Herein, we report a regio-, chemo-, and enantioselective synthesis of cyclobutenes from olefins using N-sulfonyl-1,2,3-triazoles as vicinal dicarbene equivalents or alkyne [2 + 2] cycloaddition surrogates. Terminal and cis-olefins can be transformed into enantioenriched cyclopropanes via rhodium catalysis. Then, in one pot, treatment of these intermediates with tosyl hydrazide and base effects diazo formation followed by rhodium-catalyzed ring expansion to yield enantioenriched cyclobutenes. These cyclobutenes can be transformed into highly substituted, enantioenriched cyclobutanes, including structures relevant to natural product scaffolds.

Automatic discovery of photoisomerization mechanisms with nanosecond machine learning photodynamics simulations.

Photochemical reactions are widely used by academic and industrial researchers to construct complex molecular architectures via mechanisms that often require harsh reaction conditions. Photodynamics simulations provide time-resolved snapshots of molecular excited-state structures required to understand and predict reactivities and chemoselectivities. Molecular excited-states are often nearly degenerate and require computationally intensive multiconfigurational quantum mechanical methods, especially at conical intersections. Non-adiabatic molecular dynamics require thousands of these computations per trajectory, which limits simulations to ∼1 picosecond for most organic photochemical reactions. Westermayr et al. recently introduced a neural-network-based method to accelerate the predictions of electronic properties and pushed the simulation limit to 1 ns for the model system, methylenimmonium cation (CH2NH2 +). We have adapted this methodology to develop the Python-based, Python Rapid Artificial Intelligence Ab Initio Molecular Dynamics (PyRAI2MD) software for the cis-trans isomerization of trans-hexafluoro-2-butene and the 4π-electrocyclic ring-closing of a norbornyl hexacyclodiene. We performed a 10 ns simulation for trans-hexafluoro-2-butene in just 2 days. The same simulation would take approximately 58 years with traditional multiconfigurational photodynamics simulations. We generated training data by combining Wigner sampling, geometrical interpolations, and short-time quantum chemical trajectories to adaptively sample sparse data regions along reaction coordinates. The final data set of the cis-trans isomerization and the 4π-electrocyclic ring-closing model has 6207 and 6267 data points, respectively. The training errors in energy using feedforward neural networks achieved chemical accuracy (0.023-0.032 eV). The neural network photodynamics simulations of trans-hexafluoro-2-butene agree with the quantum chemical calculations showing the formation of the cis-product and reactive carbene intermediate. The neural network trajectories of the norbornyl cyclohexadiene corroborate the low-yielding syn-product, which was absent in the quantum chemical trajectories, and revealed subsequent thermal reactions in 1 ns.

Enantioselective Total Synthesis of the Archaeal Lipid Parallel GDGT-0 (Isocaldarchaeol)*.

Archaeal glycerol dibiphytanyl glycerol tetraethers (GDGT) are some of the most unusual membrane lipids identified in nature. These amphiphiles are the major constituents of the membranes of numerous Archaea, some of which are extremophilic organisms. Due to their unique structures, there has been significant interest in studying both the biophysical properties and the biosynthesis of these molecules. However, these studies have thus far been hampered by limited access to chemically pure samples. Herein, we report a concise and stereoselective synthesis of the archaeal tetraether lipid parallel GDGT-0 and the synthesis and self-assembly of derivatives bearing different polar groups.

Mechanochemical synthesis of an elusive fluorinated polyacetylene.

Polymer mechanochemistry has traditionally been employed to study the effects of mechanical force on chemical bonds within a polymer backbone or to generate force-responsive materials. It is under-exploited for the scalable synthesis of wholly new materials by chemically transforming the polymers, especially products inaccessible by other means. Here we utilize polymer mechanochemistry to synthesize a fluorinated polyacetylene, a long-sought-after air-stable polyacetylene that has eluded synthesis by conventional means. We construct the monomer in four chemical steps on gram scale, which involves a rapid incorporation of fluorine atoms in an exotic photochemical cascade whose mechanism and exquisite stereoselectivity were informed by computation. After polymerization, force activation by ultrasonication produces a gold-coloured, semiconducting fluoropolymer. This work demonstrates that polymer mechanochemistry is a valuable synthetic tool for accessing materials on a preparative scale.

Halogenation-Dependent Effects of the Chlorosulfolipids of Ochromonas danica on Lipid Bilayers.

The chlorosulfolipids are amphiphilic natural products with stereochemically complex patterns of chlorination and sulfation. Despite their role in toxic shellfish poisoning, potential pharmacological activities, and unknown biological roles, they remain understudied due to the difficulties in purifying them from natural sources. The structure of these molecules, with a charged sulfate group in the middle of the hydrophobic chain, appears incompatible with the conventional lipid bilayer structure. Questions about chlorosulfolipids remain unanswered partly due to the unavailability of structural analogues with which to conduct structure-function studies. We approach this problem by combining enantioselective total synthesis and membrane biophysics. Using a combination of Langmuir pressure-area isotherms of lipid monolayers, fluorescence imaging of vesicles, mass spectrometry imaging, natural product isolation, small-angle X-ray scattering, and cryogenic electron microscopy, we show that danicalipin A (1) likely inserts into lipid bilayers in the headgroup region and alters their structure and phase behavior. Specifically, danicalipin A (1) thins the bilayer and fluidizes it, allowing even saturated lipid to form fluid bilayers. Lipid monolayers show similar fluidizing upon insertion of danicalipin A (1). Furthermore, we show that the halogenation of the molecule is critical for its membrane activity, likely due to sterically controlled conformational changes. Synthetic unchlorinated and monochlorinated analogues do not thin and fluidize lipid bilayers to the same extent as the natural product. Overall, this study sheds light on how amphiphilic small molecules interact with lipid bilayers and the importance of stereochemistry and halogenation for this interaction.

Bicyclohexene-peri-naphthalenes: Scalable Synthesis, Diverse Functionalization, Efficient Polymerization, and Facile Mechanoactivation of Their Polymers.

Pursuing polymers that can transform from a nonconjugated to a conjugated state under mechanical stress to significantly change their properties, we developed a new generation of ladder-type mechanophore monomers, bicyclo[2.2.0]hex-5-ene-peri-naphthalene (BCH-Naph), that can be directly and efficiently polymerized by ring-opening metathesis polymerization (ROMP). BCH-Naphs can be synthesized in multigram quantities and functionalized with a wide range of electron-rich and electron-poor substituents, allowing tuning of the optoelectronic and physical properties of mechanically generated conjugated polymers. Efficient ROMP of BCH-Naphs yielded ultrahigh molecular weight polymechanophores with controlled MWs and low dispersity. The resulting poly(BCH-Naph)s can be mechanically activated into conjugated polymers using ultrasonication, grinding, and even simple stirring of the dilute solutions, leading to changes in absorption and fluorescence. Poly(BCH-Naph)s represent an attractive polymechanophore system to explore multifaceted mechanical response in solution and solid states, owing to the synthetic scalability, functional diversity, efficient polymerization, and facile mechanoactivation.

The cascade unzipping of ladderane reveals dynamic effects in mechanochemistry.

Force can induce remarkable non-destructive transformations along a polymer, but we have a limited understanding of the energy transduction and product distribution in tandem mechanochemical reactions. Ladderanes consist of multiple fused cyclobutane rings and have recently been used as monomeric motifs to develop polymers that drastically change their properties in response to force. Here we show that [4]-ladderane always exhibits 'all-or-none' cascade mechanoactivations and the same stereochemical distribution of the generated dienes under various conditions and within different polymer backbones. Transition state theory fails to capture the reaction kinetics and explain the observed stereochemical distributions. Ab initio steered molecular dynamics reveals unique non-equilibrium dynamic effects: energy transduction from the first cycloreversion substantially accelerates the second cycloreversion, and bifurcation on the force-modified potential energy surface leads to the product distributions. Our findings illustrate the rich chemistry in closely coupled multi-mechanophores and an exciting potential for effective energy transduction in tandem mechanochemical reactions.

Enantioselective Synthesis of Azamerone.

A concise and selective synthesis of the dichlorinated meroterpenoid azamerone is described. The paucity of tactics for the synthesis of natural-product-relevant chiral organochlorides motivated the development of unique strategies for accessing these motifs in enantioenriched forms. The route features a novel enantioselective chloroetherification reaction, a Pd-catalyzed cross-coupling between a quinone diazide and a boronic hemiester, and a late-stage tetrazine [4+2]-cycloaddition/oxidation cascade.

A Convenient C-H Functionalization Platform for Pyrroloiminoquinone Alkaloid Synthesis.

Pyrroloiminoquinone alkaloids represent a structurally intriguing class of natural products that display an array of useful biological properties. Here, we present a versatile and scalable platform for the synthesis of this diverse family - and in particular the antitumor discorhabdins - built upon sequential selective C-H functionalization of tryptamine. The utility of this strategy is showcased through short formal syntheses of damirones A-C, makaluvamines D and I, and discorhadbin E. Additionally, we describe efforts to develop the first catalytic asymmetric entry to the discorhabdin subclass.

Catalytic Regio- and Enantioselective Haloazidation of Allylic Alcohols.

Herein we report a highly regio- and stereoselective haloazidation of allylic alcohols. This enantioselective reaction uses readily available materials and can be performed on a variety of alkyl-substituted alkenes and can incorporate either bromine or chlorine as the electrophilic halogen component. Both halide and azido groups of the resulting products can be transformed into valuable building blocks with complete stereospecificity. The first example of an enantioselective 1,4-haloazidation of a conjugated diene is reported as well as its application to a concise synthesis of an aza-sugar.

Synthesis and Mechanochemical Activation of Ladderene-Norbornene Block Copolymers.

We have recently reported a polymechanophore system, polyladderene (PLDE), which dramatically transforms into polyacetylene (PA) upon mechanical stimulation. Herein, we optimized conditions to synthesize unprecedented block copolymers (BCPs) containing a force-responsive block by sequential ring-opening metathesis polymerization of different norbornenes and bromoladderene. Successful extension from PLDE to other blocks required careful timing and low temperatures to preserve the reactivity of the PLDE-appended catalyst. The PLDE-containing BCPs were sonochemically activated into visually soluble PA with a maximum absorption λ ≥ 600 nm and unique absorption patterns corresponding to noncontinuous activation of ladderene units. Access to polymechanophore BCPs paves the way for new stress-responsive materials with solution and solid state self-assembly behaviors and incorporation of polymechanophores into other materials.

Ladderane phospholipids form a densely packed membrane with normal hydrazine and anomalously low proton/hydroxide permeability.

Ladderane lipids are unique to anaerobic ammonium-oxidizing (anammox) bacteria and are enriched in the membrane of the anammoxosome, an organelle thought to compartmentalize the anammox process, which involves the toxic intermediate hydrazine (N2H4). Due to the slow growth rate of anammox bacteria and difficulty of isolating pure ladderane lipids, experimental evidence of the biological function of ladderanes is lacking. We have synthesized two natural and one unnatural ladderane phosphatidylcholine lipids and compared their thermotropic properties in self-assembled bilayers to distinguish between [3]- and [5]-ladderane function. We developed a hydrazine transmembrane diffusion assay using a water-soluble derivative of a hydrazine sensor and determined that ladderane membranes are as permeable to hydrazine as straight-chain lipid bilayers. However, pH equilibration across ladderane membranes occurs 5-10 times more slowly than across straight-chain lipid membranes. Langmuir monolayer analysis and the rates of fluorescence recovery after photobleaching suggest that dense ladderane packing may preclude formation of proton/hydroxide-conducting water wires. These data support the hypothesis that ladderanes prevent the breakdown of the proton motive force rather than blocking hydrazine transmembrane diffusion in anammox bacteria.