Metal-free directed sp2-C-H borylation.

Organoboron reagents are important synthetic intermediates that have a key role in the construction of natural products, pharmaceuticals and organic materials1. The discovery of simpler, milder and more efficient approaches to organoborons can open additional routes to diverse substances2-5. Here we show a general method for the directed C-H borylation of arenes and heteroarenes without the use of metal catalysts. C7- and C4-borylated indoles are produced by a mild approach that is compatible with a broad range of functional groups. The mechanism, which is established by density functional theory calculations, involves BBr3 acting as both a reagent and a catalyst. The potential utility of this strategy is highlighted by the downstream transformation of the formed boron species into natural products and drug scaffolds.

Enzyme-catalysed [6+4] cycloadditions in the biosynthesis of natural products.

Pericyclic reactions are powerful transformations for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. Their role in biosynthesis is increasingly apparent, and mechanisms by which pericyclases can catalyse reactions are of major interest1. [4+2] cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis2 for the formation of six-membered rings and are now well-established in biosynthesis3-6. [6+4] and other 'higher-order' cycloadditions were predicted7 in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring system8,9. However, although enzyme-catalysed [6+4] cycloadditions have been proposed10-12, they have not been proven to occur. Here we demonstrate a group of enzymes that catalyse a pericyclic [6+4] cycloaddition, which is a crucial step in the biosynthesis of streptoseomycin-type natural products. This type of pericyclase catalyses [6+4] and [4+2] cycloadditions through a single ambimodal transition state, which is consistent with previous proposals11,12. The [6+4] product is transformed to a less stable [4+2] adduct via a facile Cope rearrangement, and the [4+2] adduct is converted into the natural product enzymatically. Crystal structures of three pericyclases, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis establish the mechanism of this transformation. This work shows how enzymes are able to catalyse concerted pericyclic reactions involving ambimodal transition states.

Catalytic Enantioselective Alkyne Addition to Nitrones Enabled by Tunable Axially Chiral Imidazole-Based P,N-Ligands.

Although catalytic enantioselective alkyne addition is an established method for the synthesis of chiral propargylic alcohols and amines, addition to nitrones presents unique challenges, and no general chiral catalyst system has been developed. In this manuscript, we report the first Cu-catalyzed enantioselective alkyne addition to nitrones utilizing tunable axially chiral imidazole-based P,N-ligands. Our approach effectively overcomes difficulties in both reactivity and selectivity, resulting in a simple Cu-catalyzed protocol. The reaction accommodates a wide range of nitrones and alkynes, enabling the streamlined synthesis of chiral propargyl N-hydroxylamines via the enantioselective C-C bond formation. A diverse array of optically active nitrogen-containing compounds, including chiral hydroxylamines, can be accessed directly through facile transformations of the reaction products.

Cascade Photoreaction in Crystals: A Phase Change Caused by a Dewar Benzene Quantum Chain Triggers a Topochemical [2 + 2] Photodimerization.

Crystals undergoing tandem reactions where the first transformation enables the second one are rare. Using photoreactive Dewar benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic diacid (DB) as a hydrogen-bonding template for the [2π+2π] photodimerization of trans-4,4'-bipyridyl-ethenes (BPE), we obtained crystals DB-BPE-NT with a DB:BPE = 2:1 stoichiometry with double bonds at a nonreactive distance of 5.957 Å. Exposure to UV light resulted in valence bond isomerization to Hückel benzene 3,4,5,6-tetramethyl-1,2-dicarboxylic acid (HB) by a quantum chain reaction that triggered the sought-after topochemical [2π+2π] photodimerization reaction. Notably, crystals HB:BPE-T with the Hückel benzene isomer and BPE adopted a 2:2 stoichiometry and displayed an efficient topochemically templated photodimerization reaction.

Comparison of Cyclic and Linear PEG Conjugates.

Bioconjugation of polymers to proteins is a method to impart improved stability and pharmacokinetic properties to biologic systems. However, the precise effects of polymer architecture on the resulting bioconjugates are not well understood. Particularly, cyclic polymers are known to possess unique features such as a decreased hydrodynamic radius when compared to their linear counterparts of the same molecular weight, but have not yet been studied. Here, we report the first bioconjugation of a cyclic polymer, poly(ethylene glycol) (PEG), to a model protein, T4 lysozyme, containing a single engineered cysteine residue (V131C). We compare the stability and activity of this conjugate with those of a linear PEG-T4 lysozyme analogue of similar molecular weight. Furthermore, we used molecular dynamics (MD) simulations to determine the behavior of the polymer-protein conjugates in solution. We introduce cyclic polymer-protein conjugates as potential candidates for the improvement of biologic therapeutics.

An Evaluation of the Performance, Patient Acceptability, and Feasibility of a Point-of-Care HIV-Syphilis Assay in an Urban Emergency Department.

BACKGROUND: Point-of-care (POC) tests for sexually transmitted infections (STIs) permit delivery of results during the patient's emergency department (ED) encounter. We evaluated performance, patient acceptability, and feasibility of a new duplex POC test, Chembio Dual Path Platform HIV-Syphilis Assay, in an urban ED setting. METHODS: Convenience sampling approach prioritizing those considered at increased risk for an STI and/or with a history of HIV. For the performance evaluation, participants were tested for HIV/syphilis with the Chembio POC assay and the reference laboratory tests; sensitivity and specificity were determined. For the patient acceptability evaluation, participants completed pre- and post-user surveys. For the feasibility evaluation, ED clinical technicians completed a survey evaluating their perceptions regarding feasibility of use of this POC test. RESULTS: A total of 327 patients were consented and enrolled. The diagnostic sensitivity and specificity of the Chembio POC assay for HIV were 96.5% (95% confidence interval [CI], 90.1%-99.3%) and 99.6% (95% CI, 97.7%-100.0%), respectively, and for syphilis, the values were 93.9% (95% CI, 85.0%-98.3%) and 99.6% (95% CI, 97.9%-100.0%), respectively. Regarding patient acceptability, 87% trusted the result, and 93% reported that they were more likely to seek treatment if they received a positive STI test result in the ED rather than after the ED visit. Regarding feasibility, 90% of the technicians reported that they would recommend using the test in EDs. CONCLUSIONS: The Chembio Dual Path Platform HIV-Syphilis POC Assay had excellent performance characteristics when evaluated in an ED population, as well as high perceived acceptability from patients, and feasibility for ED use from clinical technicians. The test may have utility for HIV-syphilis screening among high-risk ED patients.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes.

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

Cyclic Diaryl λ3-Bromanes as a Precursor for Regiodivergent Alkynylation Reactions.

Regiodivergent reactions are a fascinating tool to rapidly access molecular diversity while using identical coupling partners. We have developed a new approach for regiodivergent synthesis using the dual character of hypervalent bromines. In addition to the recently reported reactivity of hypervalent bromines as aryne precursors, the first transition metal-catalyzed reaction is reported. Accordingly, the development of these two complementary transformations allows for the alteration of regioselectivity to furnish both ortho- and meta-substituted alkynylation products. Mechanistic and computational studies show how these selectivities are controlled.

Chemiexcitation Acceleration of 1,2-Dioxetanes by Spiro-Fused Six-Member Rings with Electron-Withdrawing Motifs.

The chemiluminescent light-emission pathway of phenoxy-1,2-dioxetane luminophores attracts growing interest within the scientific community. Dioxetane probes undergoing rapid flash-type chemiexcitation exhibit higher detection sensitivity than those with a slow glow-type chemiexcitation rate. We discovered that dioxetanes fused to non-strained six-member rings, with hetero atoms or inductive electron-withdrawing groups, present both accelerated chemiexcitation rates and elevated chemical stability compared to dioxetanes fused to four-member strained rings. DFT computational simulations supported the chemiexcitation acceleration observed by spiro-fused six-member rings with inductive electron-withdrawing groups of dioxetanes. Specifically, a spiro-dioxetane with a six-member sulfone ring exhibited a chemiexcitation rate 293-fold faster than that of spiro-adamantyl-dioxetane. A turn-ON dioxetane probe for the detection of the enzyme ß-galactosidase, containing the six-member sulfone unit, exhibited a S/N value of 108 in LB cell growth medium. This probe demonstrated a substantial increase in detection sensitivity towards E. coli bacterial cells expressing ß-galactosidase, with an LOD value that is 44-fold more sensitive than that obtained by the adamantyl counterpart. The accelerated chemiexcitation and the elevated chemical stability presented by dioxetane containing a spiro-fused six-member ring with a sulfone inductive electron-withdrawing group, make it an ideal candidate for designing efficient turn-on chemiluminescent probes with exceptionally high detection sensitivity.

Catalytic Enantioselective Synthesis of Axially Chiral Imidazoles by Cation-Directed Desymmetrization.

Axially chiral five-membered heterobiaryls synthesized by enantioselective catalysis typically feature large ortho-substituents or a heteroatom in the chiral axis to maintain a stable configuration. Herein we report a cation-directed catalytic enantioselective desymmetrization method that enables rapid access to axially chiral imidazoles with the basic nitrogen at the ortho position and efficiently integrates π-stacking moieties to ensure a stable axial configuration for further applications. The process is operationally simple, is highly enantioselective, and can be performed on the gram scale. The majority of the products are obtained in >90% ee, but interestingly even those with only moderate ee can readily be enriched to near optical purity by selective racemate crystallization. Together with a mild phosphine oxide reduction method, axially chiral imidazoles such as StackPhos and its derivatives are readily prepared in high yield and excellent enantioselectivity on the gram scale. The method also enables the preparation of new chiral non-phosphine-bearing imidazoles.

Cascade Synthesis of Fluorinated Spiroheterocyclic Scaffolding for Peptidic Macrobicycles.

Octafluorocyclopentene (OFCP) engages linear, unprotected peptides in polysubstitution cascades that generate complex fluorinated polycycles. The reactions occur in a single flask at 0-25 °C and require no catalysts or heavy metals. OFCP can directly polycyclize linear sequences using native functionality, or fluorospiroheterocyclic intermediates can be intercepted with exogenous nucleophiles. The latter tactic generates molecular hybrids composed of peptides, sugars, lipids, and heterocyclic components. The platform can create stereoisomers of both single- and double-looped macrocycles. Calculations indicate that the latter can mimic diverse protein surface loops. Subsets of the molecules have low energy conformers that shield the polar surface area through intramolecular hydrogen bonding. A significant fraction of OFCP-derived macrocycles tested show moderate to high passive permeability in parallel artificial membrane permeability assays.

ortho-Selective Dearomative [2π + 2σ] Photocycloadditions of Bicyclic Aza-Arenes.

Dearomative photocycloadditions are valuable chemical transformations, serving as an efficient platform to create three-dimensional molecular complexity. However, the photolability of the original addition product especially within the context of ortho cycloadditions often causes undesired consecutive rearrangements, rendering these ortho cycloadducts elusive. Herein, we report an ortho-selective intermolecular photocycloaddition of bicyclic aza-arenes including (iso)quinolines, quinazolines, and quinoxalines by utilizing a strain-release approach. With bicyclo[1.1.0]butanes as coupling partners, this dearomative [2π + 2σ] cycloaddition enables the straightforward construction of C(sp3)-rich bicyclo[2.1.1]hexanes directly connected to N-heteroarenes. Photophysical experiments and DFT calculations revealed the origin of the [2π + 2σ] selectivity and indicate that, in addition to the originally proposed energy transfer or direct excitation pathways, a chain reaction mechanism is operative depending on the reaction conditions.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes.

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected ß-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

The Aldol-Tishchenko Reaction of Butanone, Cyclobutanone and a 3-Pentanone Derived Sulfinylimine and DFT Calculations of the Stereo-determining Step.

Herein, we present a highly diastereoselective method to furnish acyclic 3-amino-1,5-diol derivatives using a tandem double-aldol-Tishchenko protocol (dr up to >99 : 1) using a butanone derived sulfinylimine. In most cases only 1 diastereomer predominates, from a possible 16. The reaction is also regioselective. In addition, the highly challenging cyclobutanone and 3-pentanone derivatives are also amenable to a double-aldol-Tishchenko reaction, although the dr values are modest. Despite that, clean single diastereomers can be isolated, which should prove very useful in medicinal chemistry and other areas. Detailed DFT calculations support the observed stereoselectivities in all cases, providing a rationale for the excellent dr values in the butanone series and the moderate values for the 3-pentanone class.

Computational generation of an annotated gigalibrary of synthesizable, composite peptidic macrocycles.

Peptidomimetic macrocycles have the potential to regulate challenging therapeutic targets. Structures of this type having precise shapes and drug-like character are particularly coveted, but are relatively difficult to synthesize. Our laboratory has developed robust methods that integrate small-peptide units into designed scaffolds. These methods create macrocycles and embed condensed heterocycles to diversify outcomes and improve pharmacological properties. The hypothetical scope of the methodology is vast and far outpaces the capacity of our experimental format. We now describe a computational rendering of our methodology that creates an in silico three-dimensional library of composite peptidic macrocycles. Our open-source platform, CPMG (Composite Peptide Macrocycle Generator), has algorithmically generated a library of 2,020,794,198 macrocycles that can result from the multistep reaction sequences we have developed. Structures are generated based on predicted site reactivity and filtered on the basis of physical and three-dimensional properties to identify maximally diverse compounds for prioritization. For conformational analyses, we also introduce ConfBuster++, an RDKit port of the open-source software ConfBuster, which allows facile integration with CPMG and ready parallelization for better scalability. Our approach deeply probes ligand space accessible via our synthetic methodology and provides a resource for large-scale virtual screening.

Intramolecular Hydrogen Bonding Enables a Zwitterionic Mechanism for Macrocyclic Peptide Formation: Computational Mechanistic Studies of CyClick Chemistry.

Macrocyclic peptides have become increasingly important in the pharmaceutical industry. We present a detailed computational investigation of the reaction mechanism of the recently developed "CyClick" chemistry to selectively form imidazolidinone cyclic peptides from linear peptide aldehydes, without using catalysts or directing groups (Angew. Chem. Int. Ed. 2019, 58, 19073-19080). We conducted computational mechanistic to investigate the effects of intramolecular hydrogen bonds (IMHBs) in promoting a kinetically facile zwitterionic mechanism in "CyClick" of pentapeptide aldehyde AFGPA. Our DFT calculations highlighted the importance of IMHB in pre-organization of the resting state, stabilization of the zwitterion intermediate, and the control of the product stereoselectivity. Furthermore, we have also identified that the low ring strain energy promotes the "CyClick" of hexapeptide aldehyde AAGPFA to form a thermodynamically more stable 15+5 imidazolidinone cyclic peptide product. In contrast, large ring strain energy suppresses "CyClick" reactivity of tetra peptide aldehyde AFPA from forming the 9+5 imidazolidinone cyclic peptide product.

Molecular Dynamics Simulations Guide Chimeragenesis and Engineered Control of Chemoselectivity in Diketopiperazine Dimerases.

In the biosynthesis of the tryptophan-linked dimeric diketopiperazines (DKPs), cytochromes P450 selectively couple DKP monomers to generate a variety of intricate and isomeric frameworks. To determine the molecular basis for selectivity of these biocatalysts we obtained a high-resolution crystal structure of selective Csp2 -N bond forming dimerase, AspB. Overlay of the AspB structure onto C-C and C-N bond forming homolog NzeB revealed no significant structural variance to explain their divergent chemoselectivities. Molecular dynamics (MD) simulations identified a region of NzeB with increased conformational flexibility relative to AspB, and interchange of this region along with a single active site mutation led to a variant that catalyzes exclusive C-N bond formation. MD simulations also suggest that intermolecular C-C or C-N bond formation results from a change in mechanism, supported experimentally through use of a substrate mimic.

An Exception to the Carothers Equation Caused by the Accelerated Chain Extension in a Pd/Ag Cocatalyzed Cross Dehydrogenative Coupling Polymerization.

The Carothers equation is often used to predict the utility of a small molecule reaction in a polymerization. In this study, we present the mechanistic study of Pd/Ag cocatalyzed cross dehydrogenative coupling (CDC) polymerization to synthesize a donor-acceptor (D-A) polymer of 3,3'-dihexyl-2,2'-bithiophene and 2,2',3,3',5,5',6,6'-octafluorobiphenyl, which go counter to the Carothers equation. It is uncovered that the second chain extension cross-coupling proceeds much more efficiently than the first cross-coupling and the homocoupling side reaction (at least 1 order of magnitude faster) leading to unexpectedly low homocoupling defects and high molecular weight polymers. Kinetic analyses show that C-H bond activation is rate-determining in the first cross-coupling but not in the second cross-coupling. Based on DFT calculations, the high cross-coupling rate in the second cross-coupling was ascribed to the strong Pd-thiophene interaction in the Pd-mediated C-H bond activation transition state, which decreases the energy barrier of the Pd-mediated C-H bond activation. These results have implications beyond polymerizations and can be used to ease the synthesis of a wide range of molecules where C-H bond activation may be the limiting factor.

A Pyridine Dearomatization Approach to the Matrine-Type Lupin Alkaloids.

(+)-Matrine and (+)-isomatrine are tetracyclic alkaloids isolated from the plant Sophora flavescens, the roots of which are used in traditional Chinese medicine. Biosynthetically, these alkaloids are proposed to derive from three molecules of (-)-lysine via the intermediacy of the unstable cyclic imine Δ1-piperidine. Inspired by the biosynthesis, a new dearomative annulation reaction has been developed that leverages pyridine as a stable surrogate for Δ1-piperidine. In this key transformation, two molecules of pyridine are joined with a molecule of glutaryl chloride to give the complete tetracyclic framework of the matrine alkaloids in a single step. Using this dearomative annulation, isomatrine is synthesized in four steps from inexpensive commercially available chemicals. Isomatrine then serves as the precursor to additional lupin alkaloids, including matrine, allomatrine, isosophoridine, and sophoridine.

Diastereoselective Radical Aminoacylation of Olefins through N-Heterocyclic Carbene Catalysis.

There have been significant advancements in radical-mediated reactions through covalent-based organocatalysis. Here, we present the generation of iminyl and amidyl radicals via N-heterocyclic carbene (NHC) catalysis, enabling diastereoselective aminoacylation of trisubstituted alkenes. Different from photoredox catalysis, single electron transfer from the deprotonated Breslow intermediate to O-aryl hydroxylamine generates an NHC-bound ketyl radical, which undergoes diastereocontrolled cross-coupling with the prochiral C-centered radical. This operationally simple method provides a straightforward access to a variety of pyrroline and oxazolidinone heterocycles with vicinal stereocenters (77 examples, up to >19:1 d.r.). Electrochemical studies of the acyl thiazolium salts support our reaction design and highlight the reducing ability of Breslow-type derivatives. A detailed computational analysis of this organocatalytic system suggests that radical-radical coupling is the rate-determining step, in which π-π stacking interaction between the radical intermediates subtly controls the diastereoselectivity.