Late-stage Intermediates Research Articles

Biomimetic total synthesis of the reported structure of (+)-selaginedorffone B.

The first enantioselective total synthesis of the reported structure of the structurally unique aromatic tetraterpenoid of anti-cancer potential, (+)-selaginedorffone B (2), has been accomplished from two modified abietane diterpenoids through an intermolecular Diels-Alder reaction between a bio-inspired diene 3 (HOMO counterpart) and dienophile 4 (corresponding LUMO counterpart) in a 23-step sequence, whereas the core framework of the monomeric abietane diterpenoid was constructed via alkyne-activated ene-cyclization. Computational analysis was conducted to reveal the intricate regio and diastereoselectivity of this novel Diels-Alder reaction, strengthening the experimental results. The absolute configuration of the synthesized molecule was validated through X-ray studies of late-stage intermediates as well as comprehensive 2D NMR analysis.

Dissecting the contribution of sterol structure to membrane phase separation in living cells.

The Bloch hypothesis states that the sterol biosynthesis pathway parallel's sterol structural evolution, where the product of each step should support optimized cellular and physiological functions when compared with its precursors. Here we explore this model in the yeast vacuole, which provides an in vivo system in which to explore the role of sterol structure to membrane phase separation. We compare the ability of sterol intermediates in the ergosterol biosynthesis pathway to support micron-sized phase separation by systematic genetic perturbations to the ergosterol biosynthesis pathway, driving the accumulation of different sterol intermediates in cells. Using fluorescence microscopy of stationary phase cells, we quantified changes in vacuole phase separation as a function of sterol species with the aid of the well-established vacuole associated protein marker Pho8-GFP. Interestingly, we observed a non-linear trend in the ability of sterol intermediates along the pathway to allow phase separation. Accumulation of early stage intermediates drove robust vacuole phase separation,while cells producing certain late stage intermediates exhibited little to no phase separation relative to the final product (ergosterol). Analogous experiments in model lipid membranes (giant unilamellar vesicles) corroborated our findings. These results suggest a more complex evolutionary trajectory for ergosterol biosynthesis, where minute structural differences in late stage intermediates do not predictably enhance membrane phase separation, contrary to the Bloch hypothesis. Thus, the structural features of ergosterol satisfy a fitness landscape in fungi where membrane phase separation is one of several functions lipid evolution selected for.

New synthesis of a late-stage tetracyclic key intermediate of lumateperone.

New approaches have been tested for the synthesis of lumateperone intermediates. As a result of these efforts, a novel synthesis of the late-stage tetracyclic key intermediate of lumateperone starting from the commercially available quinoxaline is described. The tetracyclic skeleton was constructed by the reaction of 1-trifluoroacetyl-4-aminoquinoxaline with ethyl 4-oxopiperidine-1-carboxylate in a Fischer indole synthesis. The inexpensive starting material, the efficient synthetic steps, and the avoidance of the borane-based reduction step provide a reasonable potential for scalability.

Development of an Enantioselective Synthesis of (-)-Euonyminol.

We detail the development of the first enantioselective synthetic route to euonyminol (1), the most heavily oxidized member of the dihydro-β-agarofuran sesquiterpenes and the nucleus of the macrocyclic alkaloids known as the cathedulins. Key steps in the synthetic sequence include a novel, formal oxyalkylation reaction of an allylic alcohol by [3 + 2] cycloaddition; a tandem lactonization-epoxide opening reaction to form the trans-C2-C3 vicinal diol residue; and a late-stage diastereoselective trimethylaluminum-mediated α-ketol rearrangement. We report an improved synthesis of the advanced unsaturated ketone intermediate 64 by means of a 6-endo-dig radical cyclization of the enyne 42. This strategy nearly doubled the yield through the intermediate steps in the synthesis and avoided a problematic inversion of stereochemistry required in the first-generation approach. Computational studies suggest that the mechanism of this transformation proceeds via a direct 6-endo-trig cyclization, although a competing 5-exo-trig cyclization, followed by a rearrangement, is also energetically viable. We also detail the challenges associated with manipulating the oxidation state of late-stage intermediates, which may inform efforts to access other derivatives such as 9-epi-euonyminol or 8-epi-euonyminol. Our successful synthetic strategy provides a foundation to synthesize the more complex cathedulins.

Can't touch this: Highly omniphobic coatings based on self-textured C6-fluoroponytailed polyvinylimidazolium monoliths

The quaternization of 1-vinylimidazole with 1H,1H,2H,2H-perfluorooctyl iodide, followed by anion metathesis reactions, allowed for the synthetic access of novel polymerizable fluorosurfactant salts with excellent yields, scalable from multigram up to bulk quantities on the kg scale. Subsequently, a series of doubly fluoroponytailed monomers as well as bridged bis-vinylimidazolium cross-linkers have been prepared via expedient thionation / S-alkylation. Thus, the resulting pool of these late stage intermediates offers great potential in task-specific surface tuning. Through polymerization in appropriate porogen mixtures combined with the micellar self-aggregation of the perfluoroalkyl moieties along with synergistic ionic interactions of the novel fluorosurfactants, the formation of nano-scalar monolithic structures is induced as evidenced by SEM imaging. According to contact angle measurements, the lotus-like textures hereby created drastically enhance oil and water repellency of the resulting polymer surface. Heeding the well-established cooperative concepts of chemical surface functionalization, accompanied by physical roughness, represents a proven strategy to impart high omniphobicity. However, the applicative maturity of systems based on nanophysics is frequently restricted by tedious and intricate patterning techniques as well as the environmental constraints of surface-active fluoroorganic compounds, on which high-performance systems indispensably rely.

Evolution of a Synthetic Strategy for Complex Polypyrrole Alkaloids: Total Syntheses of Curvulamine and Curindolizine.

Structurally unprecedented antibacterial alkaloids containing multiple electron-rich pyrrole units have recently been isolated from Curvularia sp. and Bipolaris maydis fungi. This article documents the evolution of a synthetic program aimed at accessing the flagship metabolites curvulamine and curindolizine which are presumably a dimer and trimer of a C10N biosynthetic building block, respectively. Starting with curvulamine, we detail several strategies to merge two simple, bioinspired fragments, which while ultimately unsuccessful, led us toward a pyrroloazepinone building block-based strategy and an improved synthesis of this 10π-aromatic heterocycle. A two-step annulation process was then designed to forge a conserved tetracyclic bis-pyrrole architecture and advanced into a variety of late-stage intermediates; unfortunately, however, a failed decarboxylation thwarted the total synthesis of curvulamine. By tailoring our annulation precursors, success was ultimately found through the use of a cyanohydrin nucleophile which enabled a 10-step total synthesis of curvulamine. Attempts were then made to realize a biomimetic coupling of curvulamine with an additional C10N fragment to arrive at curindolizine, the most complex family member. Although unproductive, we developed a 14-step total synthesis of this alkaloid through an abiotic coupling approach. Throughout this work, effort was made to harness and exploit the innate reactivity of the pyrrole nucleus, an objective which has uncovered many interesting findings in the chemistry of this reactive heterocycle.

Synthetic and Mechanistic Implications of Chlorine Photoelimination in Nickel/Photoredox C(sp3)-H Cross-Coupling.

In recent years, the development of light-driven reactions has contributed numerous advances in synthetic organic chemistry. A particularly active research area combines photoredox catalysis with nickel catalysis to accomplish otherwise inaccessible cross-coupling reactions. In these reactions, the photoredox catalyst absorbs light to generate an electronically excited charge-transfer state that can engage in electron or energy transfer with a substrate and the nickel catalyst. Our group questioned whether photoinduced activation of the nickel catalyst itself could also contribute new approaches to cross-coupling. Over the past 5 years, we have sought to advance this hypothesis for the development of a suite of mild and site-selective C(sp3)-H cross-coupling reactions with chloride-containing coupling partners via photoelimination of a Ni-Cl bond.On the basis of a report from the Nocera laboratory, we reasoned that photolysis of a Ni(III) aryl chloride species, generated by single-electron oxidation of a typical Ni(II) intermediate in cross-coupling, might allow for the catalytic generation of chlorine atoms. Combining this with the ability of Ni(II) to accept alkyl radicals, we hypothesized that photocatalytically generated chlorine atoms could mediate hydrogen atom transfer (HAT) with C(sp3)-H bonds to generate a substrate-derived alkyl radical that is captured by the Ni center in cross-coupling. A photoredox catalyst was envisioned to promote the necessary single-electron oxidation and reduction of the Ni catalyst to facilitate an overall redox-neutral process. Overall, this strategy would offer a visible-light-driven mechanism for chlorine radical formation enabled by the sequential capture of two photons.As an initial demonstration, we developed a Ni/photoredox-catalyzed α-oxy C(sp3)-H arylation of cyclic and acyclic ethers. This method was extended to a mild formylation of abundant and complex aryl chlorides through selective 2-functionalization of 1,3-dioxolane. Seeking to develop a suite of reactions that introduce carbon at all different oxidation states, we explored C(sp3)-H cross-coupling with trimethyl orthoformate, a common laboratory solvent. We found that trimethyl orthoformate serves as a source of methyl radical for a methylation reaction via β-scission from a tertiary radical generated upon chlorine-mediated HAT. Since chlorine radical is capable of abstracting unactivated C(sp3)-H bonds, our efforts have also been directed at cross-coupling with a range of feedstock chemicals, such as alkanes and toluenes, along with late-stage intermediates, using chloroformates as coupling partners. Overall, this platform enables access to valuable synthetic transformations with (hetero)aryl chlorides, which despite being the most ubiquitous and inexpensive aryl halide coupling partners, are rarely reactive in Ni/photoredox catalysis.Little is known about the photophysics and photochemistry of organometallic Ni complexes relevant to cross-coupling. We have conducted mechanistic investigations, including computational, spectroscopic, emission quenching, and stoichiometric oxidation studies, of Ni(II) aryl halide complexes common to Ni/photoredox reactions. These studies indicate that chlorine radical generation from excited Ni(III) is operative in the described C(sp3)-H functionalization methods. More generally, the studies illustrate that the photochemistry of cross-coupling catalysts cannot be ignored in metallaphotoredox reactions. We anticipate that further mechanistic understanding should facilitate new catalyst design and lead to the development of new synthetic methods.

Bio- and Chemocatalysis for the Synthesis of Late Stage SAR-Enabling Intermediates for ROMK Inhibitors and MK-7145 for the Treatment of Hypertension and Heart Failure

A synthetic strategy to provide two late-stage intermediates for the synthesis of diverse analogues of ROMK inhibitors for the treatment of hypertension and heart failure is described. Key transfor...

A Novel Tool to Investigate the Early and Late Stages of α-Synuclein Aggregation.

The accumulation of an inherently disordered protein α-synuclein (α-syn) aggregates in brain tissue play a pivotal role in the pathology and etiology of Parkinson's disease. Aggregation of α-syn has been found to be complex and heterogeneous, occurring through multitudes of early- and late-stage intermediates. Because of the inherent complexity and large dynamic range (between a few microseconds to several days under in vitro measurement conditions), it is difficult for the conventional biophysical and biochemical techniques to sample the entire time window of α-syn aggregation. Here, for the first time, we introduced the Z-scan technique as a novel tool to investigate different conformations formed in the early and late stage of temperature and mechanical stress-induced α-syn aggregation, in which different species showed its characteristic nonlinear characteristics. A power-dependent study was also performed to observe the changes in the protein nonlinearity. The perceived nonlinearity was accredited to the thermal-lensing effect. A switch in the sign of the refractive nonlinearity was observed for the first time as a signature of the late oligomeric conformation, a prime suspect that triggers cell death associated with neurodegeneration. We validate Z-scan results using a combination of different techniques, like thioflavin-T fluorescence assay, fluorescence correlation spectroscopy, Fourier-transform infrared spectroscopy, and atomic force microscopy. We believe that this simple, inexpensive, and sensitive method can have potential future applications in detecting/monitoring conformations in other essential peptides/proteins related to different neurodegenerative and other human diseases.

Backbone dihedral angles prediction servers for protein early-stage structure prediction

Abstract Three-dimensional protein structure prediction is an important task in science at the intersection of biology, chemistry, and informatics, and it is crucial for determining the protein function. In the two-stage protein folding model, based on an early- and late-stage intermediates, we propose to use state-of-the-art secondary structure prediction servers for backbone dihedral angles prediction and devise an early-stage structure. Early-stage structures are used as a starting point for protein folding simulations, and any errors in this stage affect the final predictions. We have shown that modern secondary structure prediction servers could increase the accuracy of early-stage predictions compared to previously reported models.

Development of an Effective Scalable Enantioselective Synthesis of the HIV-1 Entry Inhibitor BNM-III-170 as the Bis-Trifluoroacetate Salt.

We report here the development and optimization of a process synthesis for the HIV-1 entry inhibitor BNM-III-170 bis-TFA salt (1). The synthesis features a dynamic-kinetic resolution (DKR) to establish the initial stereogenicity. By taking advantage of significant sequence modifications of our first generation synthesis, inconjunction with the low solubility of late-stage intermediates, the overall efficiency of the synthesis has been significantly improved, now to proceed in an overall yield of 9.64% for the 16-steps, requiring only a single chromatographic separation.

An Isotopic Ratio Outlier Analysis Approach for Global Metabolomics of Biosynthetically Talented Actinomycetes.

Actinomycetes are powerhouses of natural product biosynthesis. Full realization of this biosynthetic potential requires approaches for recognizing novel metabolites and determining mediators of metabolite production. Herein, we develop an isotopic ratio outlier analysis (IROA) ultra-high performance liquid chromatography-mass spectrometry (UHPLC/MS) global metabolomics strategy for actinomycetes that facilitates recognition of novel metabolites and evaluation of production mediators. We demonstrate this approach by determining impacts of the iron chelator 2,2′-bipyridyl on the Nocardiopsis dassonvillei metabolome. Experimental and control cultures produced metabolites with isotopic carbon signatures that were distinct from corresponding “standard” culture metabolites, which were used as internal standards for LC/MS. This provided an isotopic MS peak pair for each metabolite, which revealed the number of carbon atoms and relative concentrations of metabolites and distinguished biosynthetic products from artifacts. Principal component analysis (PCA) and random forest (RF) differentiated bipyridyl-treated samples from controls. RF mean decrease accuracy (MDA) values supported perturbation of metabolites from multiple amino acid pathways and novel natural products. Evaluation of bipyridyl impacts on the nocazine/XR334 diketopiperazine (DKP) pathway revealed upregulation of amino acid precursors and downregulation of late stage intermediates and products. These results establish IROA as a tool in the actinomycete natural product chemistry arsenal and support broad metabolic consequences of bipyridyl.

Total Synthesis of Pentacyclic (-)-Ambiguine P Using Sequential Indole Functionalizations.

The first synthesis of a pentacyclic ambiguine (ambiguine P) is reported. The synthesis takes advantage of sequential alkylations of an indole core to rapidly construct the pentacyclic framework of the natural product. Key to the success of the synthesis was the use of a Nicholas reaction to alkylate at C2, crafting a fused seven-membered ring that is characteristic of the pentacyclic ambiguines, as well as the use of an amide-directed functionalization at C12 to set a requisite quaternary center. A versatile late-stage intermediate was prepared that may be applicable to the synthesis of the other pentacyclic ambiguines.

Natural Products in the "Marketplace": Interfacing Synthesis and Biology.

Drugs are discovered through the biological screening of collections of compounds, followed by optimization toward functional end points. The properties of screening collections are often balanced between diversity, physicochemical favorability, intrinsic complexity, and synthetic tractability (Huggins, D. J.; et al. ACS Chem. Biol. 2011, 6, 208; DOI: 10.1021/cb100420r ). Whereas natural product (NP) collections excel in the first three attributes, NPs suffer a disadvantage on the last point. Academic total synthesis research has worked to solve this problem by devising syntheses of NP leads, diversifying late-stage intermediates, or derivatizing the NP target. This work has led to the discovery of reaction mechanisms, the invention of new methods, and the development of FDA-approved drugs. Few drugs, however, are themselves NPs; instead, NP analogues predominate. Here we highlight past examples of NP analogue development and successful NP-derived drugs. More recently, chemists have explored how NP analogues alter the retrosynthetic analysis of complex scaffolds, merging structural design and synthetic design. This strategy maintains the intrinsic complexity of the NP but can alter the physicochemical properties of the scaffold, like core instability that renders the NP a poor chemotype. Focused libraries based on these syntheses may exclude the NP but maintain the molecular properties that distinguish NP space from synthetic space (Stratton, C. F.; et al. Bioorg. Med. Chem. Lett. 2015, 25, 4802; DOI: 10.1016/j.bmcl.2015.07.014 ), properties that have statistical advantages in clinical progression (Luker, T.; et al. Bioorg. Med. Chem. Lett. 2011, 21, 5673, DOI: 10.1016/j.bmcl.2011.07.074 ; Ritchie, T. J.; Macdonald, S. J. F. Drug Discovery Today 2009, 14, 1011, DOI: 10.1016/j.drudis.2009.07.014 ). Research that expedites synthetic access to NP motifs can prevent homogeneity of chemical matter available for lead discovery. Easily accessed, focused libraries of NP scaffolds can fill empty but active gaps in screening sets and expand the molecular diversity of synthetic collections.

Direct C-C Bond Formation from Alkanes Using Ni-Photoredox Catalysis.

A method for direct cross coupling between unactivated C(sp3)-H bonds and chloroformates has been accomplished via nickel and photoredox catalysis. A diverse range of feedstock chemicals, such as (a)cyclic alkanes and toluenes, along with late-stage intermediates, undergo intermolecular C-C bond formation to afford esters under mild conditions using only 3 equiv of the C-H partner. Site selectivity is predictable according to bond strength and polarity trends that are consistent with the intermediacy of a chlorine radical as the hydrogen atom-abstracting species.

New radical methods for the potential synthesis of carbon-13 and carbon-14 labeled complex products.

The isotopic labeling of molecules for agrichemical and pharmaceutical uses is becoming more challenging as molecules become larger, involve more stereochemistry, and as intellectual property rights become more complex. As such, isotope chemists need to continually add new isotopic methods to their armamentarium to successfully label complex molecules with carbon-13 and carbon-14. Recently, there has been a surge in the use of radicals to form new carbon-carbon bonds and for the incorporation of functional groups which can be used to incorporate isotopically labeled carbons. This review will describe some potential new radical methods for incorporating isotopically labeled carbon into complex molecules or into substrates that can be attached to late stage intermediates to generate labeled products.

Total Synthesis of Complex Biosynthetic Late-Stage Intermediates and Bioconversion by a Tailoring Enzyme from Jerangolid Biosynthesis.

A highly convergent access to the late-stage biosynthetic intermediates projerangolid and jerangolid E is presented, and its utility is demonstrated by the synthesis of novel non-natural jerangolid derivatives. The key steps are fragment couplings by Julia-Kocienski olefination and olefin cross metathesis, as well as a stereoselective tetrahydropyran formation by intramolecular oxa-Michael addition. Bioconversion experiments with the tailoring O-methyltransferase JerF confirmed its proposed biosynthetic role and revealed relaxed substrate specificity of this enzyme as well as tolerance to organic cosolvents.

A novel fluorescence microscopic approach to quantitatively analyse protein-induced membrane remodelling

Membrane remodelling or the bending and rupture of the lipid bilayer occurs during diverse cellular processes such as cell division, synaptic transmission, vesicular transport, organelle biogenesis and sporulation. These activities are brought about by the localized change in membrane curvature, which in turn causes lipid-packing stress, of a planar lipid bilayer by proteins. For instance, vesicular transport processes are typically characterized by the cooperative recruitment of proteins that induce budding of a planar membrane and catalyse fission of the necks of membrane buds to release vesicles. The analysis of such membrane remodelling reactions has traditionally been restricted to electron microscopy-based approaches or force spectroscopic analysis of membrane tethers pulled from liposome-based model membrane systems. Our recent work has demonstrated the facile creation of tubular model membrane systems of supported membrane tubes (SMrTs), which mimic late-stage intermediates of typical vesicular transport reactions. This review addresses the nature of such an assay system and a fluorescence-intensity-based analysis of changes in tube dimensions that is indicative of the membrane remodelling capacity of proteins.

Electrochemical bromination of late stage intermediates and drug molecules

Abstract Aromatic C H bromination is one of the applications of late stage functionalization that provides precursors for generation of radio-labelled compounds to support drug metabolism and pharmacokinetic (DMPK) studies or provide a “handle” for further functionalization to expand SAR studies without having to resort to de novo synthesis. Electrochemical aromatic bromination was attempted on late stage intermediates and drug molecules such as cytidine, uridine, Tenofovir, MK-4618, Sch48973 and MK-8457. The reactions were conducted under mild galvanostatic electrolysis condition in aqueous NaBr solution. The brominated products were obtained and converted to the corresponding tritium labelled products. Electrochemical methodology provides an alternative way to quickly generate aryl bromides of late stage intermediates or drug molecules that are very useful in drug discovery.

Studies directed toward the exploitation of vicinal diols in the synthesis of (+)-nebivolol intermediates.

While the exploitation of the Sharpless asymmetric dihydroxylation as the source of chirality in the synthesis of acyclic molecules and saturated heterocycles has been tremendous, its synthetic utility toward chiral benzo-annulated heterocycles is relatively limited. Thus, in the search for wider applications of Sharpless asymmetric dihydroxylation-derived diols for the synthesis of benzo-annulated heterocycles, we report herein our studies in the asymmetric synthesis of (R)-1-((R)-6-fluorochroman-2-yl)ethane-1,2-diol, (R)-1-((S)-6-fluorochroman-2-yl)ethane-1,2-diol and (S)-6-fluoro-2-((R)-oxiran-2-yl)chroman, which have been used as late-stage intermediates for the asymmetric synthesis of the antihypertensive drug (S,R,R,R)-nebivolol. Noteworthy is that a large number of racemic and asymmetric syntheses of nebivolol and their intermediates have been described in the literature, however, the Sharpless asymmetric dihydroxylation has never been employed as the sole source of chirality for this purpose.