Accessing Cyclobutane Polymers: Overcoming Synthetic Challenges via Efficient Continuous Flow [2 + 2] Photopolymerization

We report an improved and efficient method to prepare well-defined, structurally complex truxinate cyclobutane polymers via a thioxanthone sensitized solution state [2 + 2] photopolymerization. Monomers with varying electron density and structure polymerize in good to excellent yields to afford a library of 42 polyesters. Monomers with internal olefin separation distances of greater than 5 Å undergo polymerization via intermolecular [2 + 2] photocycloaddition readily, as opposed to the intramolecular [2 + 2] photocycloaddition observed in monomers with olefins in closer proximity. Use of a continuous flow reactor decreases reaction time, increases polymer molecular weight, and decreases dispersity compared to batch reactions. Furthermore, under continuous flow, polymerization is readily scalable beyond what is possible with batch reactions. This advancement ushers truxinate cyclobutane-based polyesters, which have been historically limited to a few examples and only research scale quantities, to the forefront of development as new materials for potential use across industry sectors.

Sigma Receptor Ligands Are Potent Antiprion Compounds that Act Independently of Sigma Receptor Binding

Prion diseases are invariably fatal neurodegenerative diseases of humans and other animals for which there are no effective treatment options. Previous work from our laboratory identified phenethylpiperidines as a novel class of anti-prion compounds. While working to identify the molecular target(s) of these molecules, we unexpectedly discovered ten novel antiprion compounds based on their known ability to bind to the sigma receptors, σ1R and σ2R, which are currently being tested as therapeutic or diagnostic targets for cancer and neuropsychiatric disorders. Surprisingly, however, knockout of the respective genes encoding σ1R and σ2R (Sigmar1 and Tmem97) in prion-infected N2a cells did not alter the antiprion activity of these compounds, demonstrating that these receptors are not the direct targets responsible for the antiprion effects of their ligands. Further investigation of the most potent molecules established that they are efficacious against multiple prion strains and protect against downstream prion-mediated synaptotoxicity. While the precise details of the mechanism of action of these molecules remain to be determined, the present work forms the basis for further investigation of these compounds in preclinical studies. Given the therapeutic utility of several of the tested compounds, including rimcazole and haloperidol for neuropsychiatric conditions, (+)-pentazocine for neuropathic pain, and the ongoing clinical trials of SA 4503 and ANAVEX2-73 for ischemic stroke and Alzheimer's disease, respectively, this work has immediate implications for the treatment of human prion disease.

Synthesis of Neocannabinoids Using Controlled Friedel-Crafts Reactions

A one-step transformation to produce 8,9-dihydrocannabidiol (H2CBD) and related "neocannabinoids" via controlled Friedel-Crafts reactions is reported. Experimental and computational studies probing the mechanism of neocannabinoid synthesis from cyclic allylic alcohol and substituted resorcinol reaction partners provide understanding of the kinetic and thermodynamic factors driving regioselectivity for the reaction. Herein, we present the reaction scope for neocannabinoid synthesis including the production of both normal and abnormal isomers under both kinetic and thermodynamic control. Discovery and optimization of this one-step protocol between various allylic alcohols and resorcinol derivatives are discussed and supported with density functional theory calculations.

Channeling macrophage polarization by rocaglates increases macrophage resistance to Mycobacterium tuberculosis

Macrophages contribute to host immunity and tissue homeostasis via alternative activation programs. M1-like macrophages control intracellular bacterial pathogens and tumor progression. In contrast, M2-like macrophages shape reparative microenvironments that can be conducive for pathogen survival or tumor growth. An imbalance of these macrophages phenotypes may perpetuate sites of chronic unresolved inflammation, such as infectious granulomas and solid tumors. We have found that plant-derived and synthetic rocaglates sensitize macrophages to low concentrations of the M1-inducing cytokine IFN-gamma and inhibit their responsiveness to IL-4, a prototypical activator of the M2-like phenotype. Treatment of primary macrophages with rocaglates enhanced phagosome-lysosome fusion and control of intracellular mycobacteria. Thus, rocaglates represent a novel class of immunomodulators that can direct macrophage polarization toward the M1-like phenotype in complex microenvironments associated with hypofunction of type 1 and/or hyperactivation of type 2 immunity, e.g., chronic bacterial infections, allergies, and, possibly, certain tumors.

Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

Herein, we report a unified approach to azepines by dearomative photochemical rearrangement of aromatic N-ylides. Deprotonation of quaternary aromatic salts with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or N,N,N',N'-tetramethylquanidine (TMG) under visible light irradiation provides mono- and polycyclic azepines in yields up to 98%. This ring-expansion presents a new mode of access to functionalized azepines from N-heteroarenes using two straightforward steps and simple starting materials.

Reversible PCET and Ambient Catalytic Oxidative Alcohol Dehydrogenation by {V=O} Perfluoropinacolate Complexes

A new air-stable catalyst for the oxidative dehydrogenation of benzylic alcohols under ambient conditions has been developed. The synthesis and characterization of this compound and the related monomeric and dimeric V(IV)- and V(V)-pin^F (pin^F = perfluoropinacolate) complexes are reported herein. Monomeric V(IV) complex (Me₄N)₂[V(O)(pin^F)₂] (1) and dimeric (μ-O)₂-bridged V(V) complex (Me₄N)₂[V₂(O)₂(μ-O)₂(pin^F)₂] (3a) are prepared in water under ambient conditions. Monomeric V(V) complex (Me₄N)[V(O)(pin^F)₂] (2) may be generated via chemical oxidation of 1 under an inert atmosphere, but dimerizes to 3a upon exposure to air. Complexes 1 and 2 display a perfectly reversible V^IV/V couple at 20 mV (vs Ag/AgNO₃), whereas a quasi-reversible V^IV/V couple at -865 mV is found for 3a. Stoichiometric reactions of 3a with both fluorenol and TEMPOH result in the formation of (Me₄N)₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4a), which contains two V(IV) centers that display antiferromagnetic coupling. In order to structurally characterize the dinuclear anion of 4a, {K(18C6)}⁺ countercations were employed, which formed stabilizing K···O interactions between the counterion and each terminal oxo moiety and H-bonding between the oxygen atoms of the crown ether and μ-OH bridges of the dimer, resulting in {K(18C6)}₂[V₂(O)₂(μ-OH)₂(pin^F)₂] (4b). The formal storage of H₂ in 4a is reversible and proton-coupled electron transfer (PCET) from crystals of 4a regenerates 3a upon exposure to air over the course of several days. Furthermore, the reaction of 3a (2%) under ambient conditions with excess fluorenol, cinnamyl alcohol, or benzyl alcohol resulted in the selective formation of fluorenone (82% conversion), cinnamaldehyde (40%), or benzaldehyde (7%), respectively, reproducing oxidative alcohol dehydrogenation (OAD) chemistry known for VO_x surfaces and demonstrating, in air, the thermodynamically challenging selective oxidation of alcohols to aldehydes/ketones.

Photoredox Generated Carbonyl Ylides Enable a Modular Approach to Aryltetralin, Dihydronaphthalene, and Arylnaphthalene Lignans

A one-pot synthesis of dihydronaphthalenes and arylnaphthalenes from epoxides and common dipolarophiles is described. The reaction proceeds through photoredox activation of epoxides to carbonyl ylides, which undergo concerted [3 + 2] dipolar cycloaddition with dipolarophiles to provide tetrahydrofurans or 2,5-dihydrofurans. In the same flask, acid promoted rearrangement affords densely functionalized dihydronaphthalenes and arylnaphthalenes, respectively, in an overall redox-neutral sequence of transformations. Succinct total synthesis (4-6 steps) of pycnanthulignene B and C and justicidin E are reported.

A sterically encumbered photoredox catalyst enables the unified synthesis of the classical lignan family of natural products

Herein, we detail a unified synthetic approach to the classical lignan family of natural products that hinges on divergence from a common intermediate that was strategically identified from nature's biosynthetic blueprints.

Herein, we detail a unified synthetic approach to the classical lignan family of natural products that hinges on divergence from a common intermediate that was strategically identified from nature's biosynthetic blueprints. Efforts toward accessing the common intermediate through a convergent and modular approach resulted in the discovery of a sterically encumbered photoredox catalyst that can selectively generate carbonyl ylides from electron-rich epoxides. These can undergo concerted [3 + 2] dipolar cycloadditions to afford tetrahydrofurans, which were advanced (2–4 steps) to at least one representative natural product or natural product scaffold within all six subtypes in classical lignans. The application of those synthetic blueprints to the synthesis of heterolignans bearing unnatural functionality was demonstrated, which establishes the potential of this strategy to accelerate structure–activity-relationship studies of these natural product frameworks and their rich biological activity.

One-pot synthesis of epoxides from benzyl alcohols and aldehydes

A one-pot synthesis of epoxides from commercially available benzyl alcohols and aldehydes is described. The reaction proceeds through in situ generation of sulfonium salts from benzyl alcohols and their subsequent deprotonation for use in Corey–Chaykovsky epoxidation of aldehydes. The generality of the method is exemplified by the synthesis of 34 epoxides that were made from an array of electronically and sterically varied alcohols and aldehydes.

A photochemical flow reactor for large scale syntheses of aglain and rocaglate natural product analogues

Herein, we report the development of continuous flow photoreactors for large scale ESIPT-mediated [3+2]-photocycloaddition of 2-(p-methoxyphenyl)-3-hydroxyflavone and cinnamate-derived dipolarophiles. These reactors can be efficiently numbered up to increase throughput two orders of magnitude greater than the corresponding batch reactions.

Identification of Anti-prion Compounds using a Novel Cellular Assay

Prion diseases are devastating neurodegenerative disorders with no known cure. One strategy for developing therapies for these diseases is to identify compounds that block conversion of the cellular form of the prion protein (PrP^C) into the infectious isoform (PrP^Sc). Most previous efforts to discover such molecules by high-throughput screening methods have utilized, as a read-out, a single kind of cellular assay system: neuroblastoma cells that are persistently infected with scrapie prions. Here, we describe the use of an alternative cellular assay based on suppressing the spontaneous cytotoxicity of a mutant form of PrP (Δ105-125). Using this assay, we screened 75,000 compounds, and identified a group of phenethyl piperidines (exemplified by LD7), which reduces the accumulation of PrP^Sc in infected neuroblastoma cells by >90% at low micromolar doses, and inhibits PrP^Sc-induced synaptotoxicity in hippocampal neurons. By analyzing the structure-activity relationships of 35 chemical derivatives, we defined the pharmacophore of LD7, and identified a more potent derivative. Active compounds do not alter total or cell-surface levels of PrP^C, and do not bind to recombinant PrP in surface plasmon resonance experiments, although at high concentrations they inhibit PrP^Sc-seeded conversion of recombinant PrP to a misfolded state in an in vitro reaction (RT-QuIC). This class of small molecules may provide valuable therapeutic leads, as well as chemical biological tools to identify cellular pathways underlying PrP^Sc metabolism and PrP^C function.

Development of a Potent and Selective HDAC8 Inhibitor

A novel, isoform-selective inhibitor of histone deacetylase 8 (HDAC8) has been discovered by the repurposing of a diverse compound collection. Medicinal chemistry optimization led to the identification of a highly potent (0.8 nM) and selective inhibitor of HDAC8.

Fine-tuning of macrophage activation using synthetic rocaglate derivatives

Drug-resistant bacteria represent a significant global threat. Given the dearth of new antibiotics, host-directed therapies (HDTs) are especially desirable. As IFN-gamma (IFNγ) plays a central role in host resistance to intracellular bacteria, including Mycobacterium tuberculosis, we searched for small molecules to augment the IFNγ response in macrophages. Using an interferon-inducible nuclear protein Ipr1 as a biomarker of macrophage activation, we performed a high-throughput screen and identified molecules that synergized with low concentration of IFNγ. Several active compounds belonged to the flavagline (rocaglate) family. In primary macrophages a subset of rocaglates 1) synergized with low concentrations of IFNγ in stimulating expression of a subset of IFN-inducible genes, including a key regulator of the IFNγ network, Irf1; 2) suppressed the expression of inducible nitric oxide synthase and type I IFN and 3) induced autophagy. These compounds may represent a basis for macrophage-directed therapies that fine-tune macrophage effector functions to combat intracellular pathogens and reduce inflammatory tissue damage. These therapies would be especially relevant to fighting drug-resistant pathogens, where improving host immunity may prove to be the ultimate resource.

[2+2] Photocycloaddition of Cinnamates in Flow and Development of a Thiourea Catalyst

Cyclobutanes derived from the dimerization of cinnamic acids are the core scaffolds of many molecules with potentially interesting biological activities. By utilizing a powerful flow photochemistry platform developed in our laboratory, we have evaluated the effects of flow on the dimerization of a range of cinnamate substrates. During the course of the study we also identified a bis(thiourea) catalyst that facilitates better reactivity and moderate diastereoselectivity in the reaction. Overall, we show that carrying out the reaction in flow in the presence of the catalyst affords consistent formation of predictable cyclobutane diastereomers.

Multidimensional reaction screening for photochemical transformations as a tool for discovering new chemotypes

We have developed an automated photochemical microfluidics platform that integrates a 1 kW high-pressure Hg vapor lamp and allows for analytical pulse flow or preparative continuous flow reactions. Herein, we will discuss the use of this platform toward the discovery of new chemotypes through multidimensional reaction screening. We will highlight the ability to discretely control wavelengths with optical filters, allowing for control of reaction outcomes.

Synthesis of a novel chemotype via sequential metal-catalyzed cycloisomerizations

Sequential cycloisomerizations of diynyl o-benzaldehyde substrates to access novel polycyclic cyclopropanes are reported. The reaction sequence involves initial Cu(I)-mediated cycloisomerization/nucleophilic addition to an isochromene followed by diastereoselective Pt(II)-catalyzed enyne cycloisomerization.

Synthesis of azaphilone-based chemical libraries

The synthesis of azaphilone scaffolds that have been further diversified by cross coupling acylation and amine addition is reported. Methodology development also led to novel modifications including C5 acetoxylation and condensations producing isoquinolin-6(7H) structures. Overall, the library synthesis afforded three azaphilone sublibraries, including vinylogous pyridones which project diversity elements in four sectors of the azaphilone core.

Remodelling of the natural product fumagillol employing a reaction discovery approach

In the search for new biologically active molecules, diversity-oriented synthetic strategies break through the limitation of traditional library synthesis by sampling new chemical space. Many natural products can be regarded as intriguing starting points for diversity-oriented synthesis, wherein stereochemically rich core structures may be reorganized into chemotypes that are distinctly different from the parent structure. Ideally, to be suited to library applications, such transformations should be general and involve few steps. With this objective in mind, the highly oxygenated natural product fumagillol has been successfully remodelled in several ways using a reaction-discovery-based approach. In reactions with amines, excellent regiocontrol in a bis-epoxide opening/cyclization sequence can be obtained by size-dependent interaction of an appropriate catalyst with the parent molecule, forming either perhydroisoindole or perhydroisoquinoline products. Perhydroisoindoles can be further remodelled by cascade processes to afford either morpholinone or bridged 4,1-benzoxazepine-containing structures.

Synthesis and reactivity of bicyclo[3.2.1]octanoid-derived cyclopropanes

Photochemical oxa-di-π-methane rearrangement of bicyclo[3.2.1]octanoid scaffolds affords multifunctional, donor-acceptor cyclopropanes. A related photochemical reaction of an iminium ether substrate uncovered an unprecedented aza-di-π-methane rearrangement of a β,γ-unsaturated iminium. Donor-acceptor cyclopropanes have been evaluated as substrates for reactions generating several new chemotypes.

Catalytic enantioselective alkylative dearomatization-annulation: total synthesis and absolute configuration assignment of hyperibone K

The asymmetric total synthesis of the polyprenylated acylphloroglucinol hyperibone K has been achieved using an enantioselective alkylative dearomatization-annulation process. NMR and computational studies were employed to probe the mode of action of a chiral phase-transfer (ion pair) catalyst.

Multidimensional Screening and Methodology Development for Condensations Involving Complex 1,2-Diketones

Multidimensional reaction screening employing complex 1,2-cycloheptanediones is described. The studies have enabled the discovery of regioselective, Lewis acid-mediated condensations with substituted ureas and a diastereoselective hydrogenation process which proceeds via an interesting allylpalladium hydride isomerization.

Tandem processes identified from reaction screening: nucleophilic addition to aryl N-phosphinylimines employing La(III)-TFAA activation

Reaction screening of nucleophilic reaction partners for addition to N-diphenylphosphinylimines employing lanthanum(III) triflate as a catalyst and trifluoroacetic anhydride (TFAA) as an activator is reported. A number of tandem processes leading to novel chemotypes including aza-Prins/intramolecular Friedel-Crafts annulations have been identified, and both reaction scope and mechanism further investigated.

A time-resolved fluorescence-resonance energy transfer assay for identifying inhibitors of hepatitis C virus core dimerization

Binding of hepatitis C virus (HCV) RNA to core, the capsid protein, results in the formation of the nucleocapsid, the first step in the assembly of the viral particle. A novel assay was developed to discover small molecule inhibitors of core dimerization. This assay is based on time-resolved fluorescence resonance energy transfer (TR-FRET) between anti-tag antibodies labeled with either europium cryptate (Eu) or allophycocyanin (XL-665). The N-terminal 106-residue portion of core protein (core106) was tagged with either glutathione-S-transferase (GST) or a Flag peptide. Tag-free core106 was selected as the reference inhibitor. The assay was used to screen the library of pharmacologically active compounds (LOPAC) consisting of 1,280 compounds and a 2,240-compound library from the Center for Chemical Methodology and Library Development at Boston University (CMLD-BU). Ten of the 28 hits from the primary TR-FRET run were confirmed in a secondary amplified luminescent proximity homogeneous assay (ALPHA screen). One hit was further characterized by dose-response analysis yielding an IC(50) of 9.3 microM. This 513 Da compound was shown to inhibit HCV production in cultured hepatoma cells.

Development of an automated microfluidic reaction platform for multidimensional screening: reaction discovery employing bicyclo[3.2.1]octanoid scaffolds

An automated, silicon-based microreactor system has been developed for rapid, low-volume, multidimensional reaction screening. Use of the microfluidic platform to identify transformations of densely functionalized bicyclo[3.2.1]octanoid scaffolds will be described.

Reaction discovery employing macrocycles: transannular cyclizations of macrocyclic bis-lactams

Macrocyclic bis-lactams have been synthesized by cyclodimerization of homoallylic amino esters employing a Zr(IV)-catalyzed ester-amide exchange protocol. Base-mediated transannular cyclizations have been identified to access both bicyclic [5-11] and tricyclic [5-8-5] frameworks in good yield and diastereoselectivity. Preliminary mechanistic studies support an olefin isomerization-intramolecular conjugate addition pathway.

Identification of novel epoxide inhibitors of hepatitis C virus replication using a high-throughput screen

Using our high-throughput hepatitis C replicon assay to screen a library of over 8,000 novel diversity-oriented synthesis (DOS) compounds, we identified several novel compounds that regulate hepatitis C virus (HCV) replication, including two libraries of epoxides that inhibit HCV replication (best 50% effective concentration, < 0.5 microM). We then synthesized an analog of these compounds with optimized activity.

1,2,3,4-Tetrahydro-1,5-naphthyridines and related heterocyclic scaffolds: Exploration of suitable chemistry for library development

The chemistry of 1,2,3,4-tetrahydro-1,5-naphthyridines and 2,3,4,5-tetrahydro-1H-pyrido[3,2-b]azepines has been explored with the goal of discovering reactions at N1 suitable for library development. Epoxide openings, palladium-catalyzed N-arylations, DEPBT-promoted acylations, and urea formation through the reaction with isocyanates were all successful. The epoxide opening chemistry using homochiral epichlorohydrin, with epoxide reclosure and a second nucleophilic opening led to the preparation of a small 24-membered library.

Generation of Oxamic Acid Libraries: Antimalarials and Inhibitors of Plasmodium falciparum Lactate Dehydrogenase

Lactate dehydrogenase (LDH) is a key enzyme in the glycolytic pathway of Plasmodium falciparum (pf) and has several unique amino acids, related to other LDHs, at the active site, making it an attractive target for antimalarial agents. Oxamate, a competitive inhibitor, shows high substrate affinity for pfLDH. This class of compounds has been viewed as potential antimalarial agents. Thus, we have developed an effective automated synthetic strategy for the rapid synthesis of oxamic acid and ester libraries to screen for potential lead inhibitors. One hundred sixty-seven oxamic acids were synthesized using a "catch and release" method with overall yields of 20-70%. Most of the compounds synthesized had some inhibitory effects, but compounds 5 and 6 were the most active against both chloroquine- and mefloquine-resistant strains with IC50 values of 15.4 and 9.41 microM and 20.4 and 8.40 microM, respectively. Some oxamic acids showed activities against pfLDH and mammalian LDH (mLDH) at the micromolar range. These oxamic acids selectively inhibited pfLDH 2-5 fold over mLDH. Oxamic acid 21 was the most active against pfLDH at IC50 = 14 and mLDH at IC50 = 25 microM, suggesting that oxamic acid derivatives are potential inhibitors of pfLDH and that further study is required to develop selective inhibitors of pfLDH over mLDH.

Discovery of Chemical Reactions through Multidimensional Screening

Multidimensional reaction screening of ortho-alkynyl benzaldehydes with a variety of catalysts and reaction partners was conducted in an effort to identify new chemical reactions. Reactions affording unique products were selected for investigation of preliminary scope and limitations.

Convergent Synthesis of Complex Diketopiperazines Derived from Pipecolic Acid Scaffolds and Parallel Screening against GPCR Targets

A convergent approach to highly functionalized diketopiperazines (DKPs) using enantioenriched pipecolic acids is described. Scandium triflate-catalyzed [4 + 2] aza-annulation was employed to produce stereochemically well-defined building blocks. A resin "catch and release" strategy was devised to convert annulation products to pipecolic acid monomers. Complex diketopiperazines were efficiently assembled utilizing one-pot cyclodimerization of pipecolic acids. Massively parallel screening of the complex DKPs against a panel of molecular targets identified novel ligands for a number of G-protein-coupled receptors (GPCRs).

Chemical library synthesis using convergent approaches

New strategies are continuously being developed in combinatorial and diversity-oriented synthesis. Convergent design in target-oriented synthesis is commonly employed for construction of complex molecules. In this mini-review, we describe examples of convergent library design and synthesis as well as potential areas for future development.

Synthesis of a Library of Complex Macrodiolides Employing Cyclodimerization of Hydroxy Esters

The synthesis of complex macrodiolides involving microwave-accelerated transesterification of chiral, nonracemic, hydroxy esters is described. Methodology development studies indicate that both microwave power and reaction temperature play an important role in the efficiency of cyclodimerizations. Hydroxy ester monomer pairs were evaluated using an analytical rehearsal leading to the preparation of a 127-member library of highly diverse and stereochemically well-defined macrodiolides. Preliminary assays identified a novel macrodiolide antagonist of the kappa opioid receptor.