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The origin of HSQC artifacts in proton-detected INADEQUATE (PDI) NMR spectra is presented along with a remedy for their suppression.

Chloride is the most common counterion used to improve aqueous solubility and enhance stability of small molecule active pharmaceutical ingredients. While several analytical techniques, such as titration, HPLC with charged aerosol detection, and ion chromatography, are currently utilized to assay the level of chloride, they have notable limitations, and these instruments may not be readily available. Here, we present a generally applicable

Cannabicitran is an important cannabinoid natural product produced by Cannabis sativa and is often found at surprisingly high levels (up to ~10%) in "purified" commercial cannabidiol (CBD) extract preparations. Despite the prevalence of this molecule in CBD oil and other cannabinoid-related products, and the rapidly expanding interest in cannabinoids for treatment of a wide range of physiological conditions, only unassigned 1 H NMR data and partial unambiguous 13 C assignments have been published. Herein we report the complete 1 H and 13 C NMR assignments of cannabicitran and comparatively evaluate the performance of several DFT methods with varying levels of theory for the calculation of NMR chemical shifts.

Modification of the recently reported 19 F-detected 1,1-ADEQUATE experiment that incorporates dual-optimization to selectively invert a wide range of 1 JCC correlations in a 1,n-ADEQUATE experiment is reported. Parameters for the dual-optimization segment of the pulse sequence were modified to accommodate the increased size of 1 JCC homonuclear coupling constants of poly- and perfluorinated molecules relative to protonated molecules to allow broadband inversion of the 1 JCC correlations. The observation and utility of isotope shifts are reported for the first time for 1,1- and 1,n-ADEQUATE correlations.

The recently reported

Benzoic acid esters represent key building blocks for many drug discovery and development programs and have been advanced as potent PDE4 Inhibitors for inhaled administration for treatment of respiratory diseases. This class of compounds has also been employed in a myriad industrial processes and as common food preservatives. Recent work directed towards the synthesis of intermediates for a proprietary medicinal chemistry program, led us to observe that the

Prediction of anisotropic NMR data directly from solute-medium interaction is of significant theoretical and practical interest, particularly for structure elucidation, configurational analysis and conformational studies of complex organic molecules and natural products. Current prediction methods require an explicit structural model of the alignment medium: a requirement either impossible or impractical on a scale necessary for small organic molecules. Here we formulate a comprehensive mathematical framework for a parametrization protocol that deconvolutes an arbitrary surface of the medium into several simple local landscapes that are distributed over the medium's surface by specific orientational order parameters. The shapes and order parameters of these local landscapes are determined

Polyfluorinated and perfluorinated compounds in the environment are a growing health concern. 19 F-detected variants of commonly employed heteronuclear shift correlation experiments such as heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) are available; 19 F-detected experiments that employ carbon-carbon homonuclear coupling, in contrast, have never been reported. Herein, we report the measurement of the 1 JCC and n JCC coupling constants of a simple perfluorinated phthalonitrile and the first demonstration of a 19 F-detected 1,1-ADEQUATE experiment.

In this report, we revise the structure for a previously reported synthetic product proposed to be the 1R,2S-cannabidiol epoxide and reassign it as cannabielsoin using anisotropic NMR and synthetic chemistry methods. These results provide a direct link to the first known biological target and function of cannabielsoin.

A method for the measurement of residual chemical shift anisotropy in one experiment using a biphasic isotropic/anisotropic lyotropic liquid crystalline medium based on poly-γ-benzyl-l-glutamate as the alignment medium is presented. This approach is demonstrated on the model compound strychnine and neotricone, a depsidone natural product with a questionable structural assignment based on comparison with the closely related excelsione and in-depth density functional theory calculations.

Nonuniform sampling (NUS) strategies are developed for acquiring highly resolved 1,1-ADEQUATE spectra, in both conventional and homodecoupled (HD) variants with improved sensitivity. Specifically, the quantile-directed and Poisson gap methods were critically compared for distributing the samples nonuniformly, and the quantile schedules were further optimized for weighting. Both maximum entropy and iterative soft thresholding spectral estimation algorithms were evaluated. All NUS approaches were robust when the degree of data reduction is moderate, on the order of a 50% reduction of sampling points. Further sampling reduction by NUS is facilitated by using weighted schedules designed by the quantile method, which also suppresses sampling noise well. Seed independence and the ability to specify the sample weighting in quantile scheduling are important in optimizing NUS for 1,1-ADEQUATE data acquisition. Using NUS yields an improvement in sensitivity, while also making longer evolution times accessible that would be difficult or impractical to attain by uniform sampling. Theoretical predictions for the sensitivity enhancements in these experiments are in the range of 5-20%; NUS is shown to disambiguate weak signals, reveal some n JCC correlations obscured by noise, and improve signal strength relative to uniform sampling in the same experimental time. This work presents sample schedule development for applying NUS to challenging experiments. The schedules developed here are made available for general use and should facilitate the broader utilization of ADEQUATE experiments (including 1,1-, 1,n-, and HD- variants) for challenging structure elucidation problems.

Correct structural assignment of small molecules and natural products is critical for drug discovery and organic chemistry. Anisotropy‐based NMR spectroscopy is a powerful tool for the structural assignment of organic molecules, but it relies on the utilization of a medium that disrupts the isotropic motion of molecules in organic solvents. Here, we establish a quantitative correlation between the atomic structure of the alignment medium, the molecular structure of the small molecule, and molecule‐specific anisotropic NMR parameters. The quantitative correlation uses an accurate three‐dimensional molecular alignment model that predicts residual dipolar couplings of small molecules aligned by poly(γ‐benzyl‐l‐glutamate). The technique facilitates reliable determination of the correct stereoisomer and enables unequivocal, rapid determination of complex molecular structures from extremely sparse NMR data., A quantitative correlation between the atomic structure of the alignment medium, the molecular structure of a small molecule, and molecule‐specific anisotropic NMR parameters has been established that predicts residual dipolar couplings of small molecules aligned by poly(γ‐benzyl‐l‐glutamate). These molecular alignment simulations allow determination of the relative configurations of small molecules from extremely sparse NMR data.

The prevalence of kidney disease has increased rapidly in recent years and has emerged as one of the leading causes of mortality worldwide. Natural products have been suggested as valuable nephroprotective agents due to their multi-target and synergistic effects on modulating important proteins involved in kidney injury. There is a large number of plant species that have been used traditionally for kidney-related conditions in Mesoamerican medicine by different cultural groups that could provide a valuable source of nephroprotective therapeutic candidates and could lead to potential drug discovery.This review aims to provide an overview of the currently known efficacy of plant species used traditionally in Mesoamerica by Mayan groups to treat kidney-related conditions and to analyze the phytochemical, pharmacological, molecular, toxicological, and clinical evidence to contribute to public health efforts and for directing future research.Primary sources of plant use reports for traditional kidney-related disorders in Mesoamerica were searched systematically from library catalogs, theses, and scientific databases (PubMed, Google Scholar; and Science Direct), and were filtered according to usage frequency in Mayan groups and plant endemism. The database of traditional plants was further analyzed based on associations with published reports of the phytochemical, pharmacological, molecular, toxicological, and clinical evidence.The most reported kidney-related conditions used traditionally in Mayan medicine involve reducing renal damage (a cultural interpretation that considers an inflammatory or infectious condition), cleaning or purifying the blood and kidney, reducing kidney pain, and eliminating kidney stones. A total of 208 plants used for kidney-related problems by 10 Mayan groups were found, representing 143 native species, where only 42 have reported pharmacological activity against kidney damage, mainly approached by in vitro and in vivo models of chemical- or drug-induced nephrotoxicity, diabetes nephropathy, and renal injury produced by hypertension. Nephroprotective effects are mainly mediated by reducing oxidative stress, inflammatory response, fibrosis mechanisms, and apoptosis in the kidney. The most common nephroprotective compounds associated with traditional Mayan medicine were flavonoids, terpenoids, and phenolic acids. The most widely studied traditional plants in terms of pharmacological evidence, bioactive compounds, and mechanisms of action, are Annona muricata L., Carica papaya L., Ipomoea batatas (L.) Lam., Lantana camara L., Sechium edule (Jacq.) Sw., Tagetes erecta L., and Zea mays L. Most of the plant species with reported pharmacological activity against kidney damage were considered safe in toxicological studies.Available pharmacological reports suggest that several herbs used in traditional Mayan medicine for renal-associated diseases may have nephroprotective effects and consistent pharmacological evidence, nephroprotective compounds, and mechanisms of action in different models of kidney injury. However, more research is required to fully understand the potential of traditional Mayan medicine in drug discovery given the limited ethnobotanical studies and data available for most species with regards to identification on bioactive components, pharmacological mechanisms, and the scarce number of clinical studies.

Cannabicitran is an important cannabinoid natural product produced by Cannabis sativa and is often found at surprisingly high levels (up to ~10%) in "purified" commercial cannabidiol (CBD) extract preparations. Despite the prevalence of this molecule in CBD oil and other cannabinoid-related products, and the rapidly expanding interest in cannabinoids for treatment of a wide range of physiological conditions, only unassigned

Determining the configuration of proton-deficient molecules is challenging using conventional NMR methods including nuclear Overhauser effect (NOE) and the proton-dependent J-based configuration analysis (JBCA). The problem is exacerbated when only one stereoisomer is available. Alternative methods based on the utilization of 13C NMR chemical shifts, 13C-13C homonuclear couplings measured at natural abundance, and residual chemical shift anisotropy measurements in conjunction with density functional theory calculations are illustrated with a proton-deficient model compound.

Commonly used DFT methods for the calculation of 1JCH coupling constants have typically required the application of ad hoc correction factors, modification of functionals, or empirical scaling to improve the fit between predicted and experimental values. Here we demonstrate that highly accurate 1JCH coupling predictions can be obtained without such adjustments by careful selection of DFT methods for geometry optimization and J-coupling calculations (e.g. B3LYP/6-31G(d,p)(mixed)//mPW1PW/cc-pVTZ). The proposed method was cross-validated against a diverse set of 122 1JCH couplings and was successfully applied to the conformational and stereochemical analysis of strychnine and a previously unreported trachylobane diterpene natural product.

Host-associated bacteria are increasingly being recognized as underexplored sources of bioactive natural products with unprecedented chemical scaffolds. A recently identified example is the plant-root-associated marine bacterium Gynuella sunshinyii of the chemically underexplored order Oceanospirillales. Its genome contains at least 22 biosynthetic gene clusters, suggesting a rich and mostly uncharacterized specialized metabolism. Here, in silico chemical prediction of a non-canonical polyketide synthase cluster has led to the discovery of janustatins, structurally unprecedented polyketide alkaloids with potent cytotoxicity that are produced in minute quantities. A combination of MS and two-dimensional NMR experiments, density functional theory calculations of C-13 chemical shifts and semiquantitative interpretation of transverse rotating-frame Overhauser effect spectroscopy data were conducted to determine the relative configuration, which enabled the total synthesis of both enantiomers and assignment of the absolute configuration. Janustatins feature a previously unknown pyridodihydropyranone heterocycle and an unusual biological activity consisting of delayed, synchronized cell death at subnanomolar concentrations. ISSN:1755-4349 ISSN:1755-4330

Polyfluorinated and perfluorinated compounds in the environment are a growing health concern.

Thiopeptides are a broad class of macrocyclic, heavily modified peptide natural products that are unified by the presence of a substituted, nitrogen-containing heterocycle core. Early work indicated that this core might be fashioned from two dehydroalanines by an enzyme-catalyzed aza-[4 + 2] cycloaddition to give a cyclic-hemiaminal intermediate. This common intermediate could then follow a reductive path toward a dehydropiperidine, as in the thiopeptide thiostrepton, or an aromatization path to yield the pyridine groups observed in many other thiopeptides. Although several of the enzymes proposed to perform this cycloaddition have been reconstituted, only pyridine products have been isolated and any hemiaminal intermediates have yet to be observed. Here, we identify the conditions and substrates that decouple the cycloaddition from subsequent steps and allow interception and characterization of this long hypothesized intermediate. Transition state modeling indicates that the key amide-iminol tautomerization is the major hurdle in an otherwise energetically favorable cycloaddition. An anionic model suggests that deprotonation and polarization of this amide bond by TbtD removes this barrier and provides a sufficient driving force for facile (stepwise) cycloaddition. This work provides evidence for a mechanistic link between disparate cyclases in thiopeptide biosynthesis.

Characterization of the complex molecular scaffold of the marine polyketide natural product phormidolide A represents a challenge that has persisted for nearly two decades. In light of discordant results arising from recent synthetic and biosynthetic reports, a rigorous study of the configuration of phormidolide A was necessary. This report outlines a synergistic effort employing computational and anisotropic NMR investigation, that provided orthogonal confirmation of the reassigned side chain, as well as supporting a further correction of the C7 stereocenter.

Novel treatment routes are emerging for an array of diseases and afflictions. Complex dosage forms, based on active pharmaceutical ingredients (APIs) with previously undesirable physicochemical characteristics, are becoming mainstream and actively pursued in various pipeline initiatives. To fundamentally understand how constituents in these dosage forms interact on a molecular level, analytical methods need to be developed that encompass selectivity and sensitivity requirements previously reserved for a myriad of in vitro techniques. The knowledge of precise chemical interactions between drugs and excipients in a dosage form can streamline formulation development and process screening capabilities through the identification of properties that influence rates and mechanisms of drug release in a cost-effective manner, relative to long-term in vivo studies. Through this work, a noncompendial in vitro release (IVR) method was developed that distinguished the presence of individual components in a complex crystalline nanosuspension environment. Doravirine was formulated as a series of long-acting injectable nanosuspensions with assorted excipients, using low- and high-energy wet media milling methods. IVR behavior of all formulation components were monitored using a robust continuous flow-through (CFT) dissolution setup (USP-4 apparatus) with on-line 1H NMR end-analysis (flow-NMR). Results from this investigation led to a better understanding of formulation parameter influences on nanosuspension stability, surface chemistry, and dissolution behavior. Flow-NMR can be applied to a broad range of dosage forms in which specific molecular interactions from the solution microenvironment require further insight to enhance product development capabilities.

Structural features of proton-deficient heteroaromatic natural products, such as the breitfussins, can severely complicate their characterization by NMR spectroscopy. For the breitfussins in particular, the constitution of the five-membered oxazole central ring cannot be unequivocally established via conventional NMR methods when the 4′-position is halogenated. The level of difficulty is exacerbated by 4′-iodination, as the accuracy with which theoretical NMR parameters are determined relies extensively on computational treatment of the relativistic effects of the iodine atom. It is demonstrated in the present study, that the structure of a 4′-iodo breitfussin analog can be unequivocally established by anisotropic NMR methods, by adopting a reduced singular value decomposition (SVD) protocol that leverages the planar structures exhibited by its conformers., Structural features of proton-deficient heteroaromatic natural products, such as the breitfussins, can severely complicate their characterization by NMR spectroscopy.

NMR-guided isolation (based on 1D (1)H and (13)C NMR resonances consistent with a chlorovinylidene moiety) resulted in the characterization of five new highly functionalized polyketides, trichophycins B-F (1-5) and one non-chlorinated metabolite tricholactone (6) from a collection of Trichodesmium bloom material from the Gulf of Mexico. The planar structures of 1-6 were determined using 1D and 2D NMR spectroscopy, mass spectrometry and complementary spectroscopic procedures. Absolute configuration analysis of 1 and 2 were carried out by (1)H NMR analysis of diastereomeric Mosher esters in addition to ECD spectroscopy, J-based configuration analysis and DFT calculations. The absolute configurations of 3-6 were proposed based on comparative analysis of (13)C NMR chemical shifts, relative configurations, and optical rotation values to compounds 1 and 2. Compounds 1-5 represent new additions to the trichophycin family and are hallmarked by a chlorovinylidene moiety. These new trichophycins and tricholactone (1-6) feature intriguing variations with respect to putative biosynthetic starting units, halogenation, and terminations and trichophycin E (4) features a rare alkynyl bromide functionality. The phenyl-containing trichophycins showed low cytotoxicity to neuro-2A cells, while the alkyne-containing trichophycins showed no toxicity.

Jizanpeptins A-E (1-5) are micropeptin depsipeptides isolated from a Red Sea specimen of a Symploca sp. cyanobacterium. The planar structures of the jizanpeptins were established using NMR spectroscopy and mass spectrometry, and contain 3-amino-6-hydroxy-2-piperidone (Ahp) as one of eight residues in a typical micropeptin motif, as well as a side chain terminal glyceric acid sulfate moiety. The absolute configurations of the jizanpeptins were assigned using a combination of Marfey’s methodology and chiral-phase HPLC analysis of hydrolysis products compared to commercial and synthesized standards. Jizanpeptins A-E showed specific inhibition of the serine protease trypsin (IC(50) = 72 nM to 1 μM) compared to chymotrypsin (IC(50) = 1.4 μM to >10 μM) in vitro and were not overtly cytotoxic to HeLa cervical or NCI-H460 lung cancer cell lines at micromolar concentrations.

1,n-ADEQUATE is a powerful NMR technique for elucidating the structure of proton-deficient small molecules that can help establish the carbon skeleton of a given molecule by providing long-range three-bond 13 C─13 C correlations. Care must be taken when using the experiment to identify the simultaneous presence of one-bond 13 C─13 C correlations that are not filtered out, unlike the HMBC experiment that has a low-pass J-filter to filter 1 JCH responses out. Dual-optimized, inverted 1 JCC 1,n-ADEQUATE is an improved variant of the experiment that affords broadband inversion of direct responses, obviating the need to take additional steps to identify these correlations. Even though ADEQUATE experiments can now be acquired in a reasonable amount of experimental time if a cryogenic probe is available, low sensitivity is still the main impediment limiting the application of this elegant experiment. Here, we wish to report a further refinement that incorporates real-time bilinear rotation decoupling-based homodecoupling methodology into the dual-optimized, inverted 1 JCC 1,n-ADEQUATE pulse sequence. Improved sensitivity and resolution are achieved by collapsing homonuclear proton-proton couplings from the observed multiplets for most spin systems. The application of the method is illustrated with several model compounds.

A novel approach to produce chiral diaryl sulfoxides from aryl benzyl sulfoxides and aryl bromides via an enantioselective arylation of aryl sulfenate anions is reported. A (JosiPhos)Pd-based catalyst successfully promotes the asymmetric arylation reaction with good functional group compatibility. A wide range of enantioenriched diaryl, aryl heteroaryl, and even diheteroaryl sulfoxides were generated. Many of the sulfoxides prepared herein would be difficult to prepare via classic enantioselective oxidation of sulfides, including Ph(Ph-d5)SO (90% ee, 95% yield). A DFT-based computational study suggested that chiral induction originates from two primary factors: (i) both a kinetic and a thermodynamic preference for oxidative addition that places the bromide trans to the JosiPhos-diarylphosphine moiety and (ii) Curtin-Hammett-type control over the interconversion between O- and S-bound isomers of palladium sulfenate species following rapid interconversion between re- and si-bound transmetalation products, re/si-Pd-OSPh (re/si-PdO-trans). Fil: Jia, Tiezheng. University of Pennsylvania; Estados Unidos Fil: Zhang, Mengnan. University of Pennsylvania; Estados Unidos Fil: McCollom, Samuel P.. University of Pennsylvania; Estados Unidos Fil: Bellomo Peraza, Ana Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Centro de Investigaciones en Bionanociencias ; Argentina Fil: Montel, Sonia. University of Pennsylvania; Estados Unidos Fil: Mao, Jianyou. Nanjing Tech University; República de China Fil: Dreher, Spencer D.. Merck & Company; Estados Unidos Fil: Welch, Christopher J.. Merck & Company; Estados Unidos Fil: Regalado, Erik L.. Merck & Company; Estados Unidos Fil: Williamson, R. Thomas. Merck & Company; Estados Unidos Fil: Manor, Brian C.. Merck & Company; Estados Unidos Fil: Tomson, Neil C.. University of Pennsylvania; Estados Unidos Fil: Walsh, Patrick J.. University of Pennsylvania; Estados Unidos

Significant catalyst loading reduction and increased reaction robustness have been achieved for a Pd-catalyzed asymmetric intramolecular C–N coupling through comprehensive mechanistic studies. Detailed kinetic, spectroscopic, and crystallographic analyses revealed that the mono-oxidation of the bis-phosphine ligand is critical for a successful transformation. 31P NMR studies provided an understanding of the inefficient activation of the Pd(OAc)2/(R,R)-QuinoxP* pre-catalyst to form the active bis-phosphine mono-oxide–Pd(0) catalyst with competitive formation of a less active (R,R)-QuinoxP*·PdBr2 complex. Based on these detailed mechanistic studies, a new series of bis-phosphine mono-oxides (BPMO)-ligated Pd(II) pre-catalysts have been rationally developed that allow for reliable and complete catalyst activation which should have general utility in academic and industrial settings.

Retro-Brook rearrangements refer to the intramolecular migration of a silyl group from oxygen to carbon. In this study, we report a novel propargylic retro-Brook rearrangement observed in terminal alkynes bearing a silyl ether moiety. Retro-Brook rearrangements involving [1,2]-, [1,4]-, and [1,5]-migrations are described, affording propargylsilanes in reasonable yield. The reaction mechanism was investigated experimentally by deuterium quenching and rationalized by density functional theory calculations. The terminal alkyne and the subsequent propargyl/allenyl dianion were shown to be crucial for the reaction favoring the retro-Brook rearrangement product over the Brook rearrangement. The second deprotonation at the propargylic position was determined to be the rate-limiting step. In addition, a gas-phase Brook-type rearrangement of the propargylsilanes was observed under GC-MS conditions. This observation was also further confirmed by DFT calculations.

Commonly used DFT methods for the calculation of 1JCH coupling constants have typically required the application of ad hoc correction factors, modification of functionals, or empirical scaling to improve the fit between predicted and experimental values. Here we demonstrate that highly accurate 1JCH coupling predictions can be obtained without such adjustments by careful selection of DFT methods for geometry optimization and J-coupling calculations (e.g. B3LYP/6-31G(d,p)(mixed)//mPW1PW/cc-pVTZ). The proposed method was cross-validated against a diverse set of 122 1JCH couplings and was successfully applied to the conformational and stereochemical analysis of strychnine and a previously unreported trachylobane diterpene natural product.

Anisotropic NMR of small molecules has thus far utilized different solvent systems and alignment media to analyze molecules of different polarities. The value of utilizing PBLG liquid crystalline phase in a co-solvent system, expanding its range as a versatile alignment medium, is shown with three molecules of different polarities: strychnine, parthenolide, and sucrose.

ADEQUATE experiments were first described in the mid-1990s and are a family of proton-detected NMR experiments used to establish carbon–carbon correlations at natural abundance. Applications of these experiments were initially limited by the statistical probability of two 13C nuclides in the same molecule (~ 1:10,000) and the low intrinsic sensitivity of the NMR probe technology of the day. In the intervening two decades, there have been significant advances in NMR probe technology and it is now possible, for example, to perform multiplicity-edited pure shift HSQC NMR experiments on a microgram of material or less. The enhanced NMR sensitivity offered by cryogenic NMR probes has dropped sample requirements for ADEQUATE experiments from the ~ 10 mg range to less than a milligram when working with a 1.7 mm Micro Cryoprobe™ at 600 MHz on a modern spectrometer. In parallel with advances in NMR probe technology, this chapter will detail a number of modifications of the ADEQUATE experiment that have been reported to enhance both the utility and sensitivity of these experiments. Combining newly reported variants of the ADEQUATE experiments with anisotropic NMR and Computer-Assisted Structure Elucidation methods affords an orthogonal means of verifying both the constitution and configuration of complex molecular structures that can stem the flow of incorrectly reported structures into the chemical literature.

An extract of Eudistoma sp. provided eudistidine C (1), a heterocyclic alkaloid with a novel molecular framework. Eudistidine C (1) is a racemic natural product composed of a tetracyclic core structure further elaborated with a p-methoxyphenyl group and a phenol-substituted aminoimidazole moiety. This compound presented significant structure elucidation challenges due to the large number of heteroatoms and fully substituted carbons. These issues were mitigated by application of a new NMR pulse sequence (LR-HSQMBC) optimized to detect four- and five-bond heteronuclear correlations and the use of computer-assisted structure elucidation software. Synthesis of eudistidine C (1) was accomplished in high yield by treating eudistidine A (2) with 4(2-amino-1H-imidazol-5-yl)phenol (4) in DMSO. Synthesis of eudistidine C (1) confirmed the proposed structure and provided material for further biological characterization. Treatment of 2 with various nitrogen heterocycles and electron-rich arenes provided a series of analogues (5-10) of eudistidine C. Chiral-phase HPLC resolution of epimeric eudistidine C provided (+)-(R)-eudistidine C (1a) and (-)-(S)-eudistidine C (1b). The absolute configuration of these enantiomers was assigned by ECD analysis. (-)-(S)-Eudistidine C (1b) modestly inhibited interaction between the protein binding domains of HIF-1α and p300. Compounds 1, 2, and 6-10 exhibited significant antimalarial activity against Plasmodium falciparum.

A novel technique that allows efficient measurement of the 35,37Cl isotope pattern for any 13C resonance has been developed. The band-selective CLIP-HSQMBC experiment is reliable and universally applicable for the indirect measurement of the 35,37Cl isotope shift of 13C resonances. The experiment provides advantages over conventional 1D 13C NMR and the recently developed bs-HSQC experiments. The utility and performance of the bs-CLIP-HSQMBC experiment is demonstrated for polyhalogenated synthons in a synthetic route and for a polyhalogenated marine natural product.

Microbes sense and respond to their environment with small molecules, and discovering these molecules and identifying their functions informs chemistry, biology, and medicine. As part of a study of molecular exchanges between termite-associated actinobacteria and pathogenic fungi, we uncovered a remarkable fungal metabolite, homodimericin A, which is strongly upregulated by the bacterial metabolite bafilomycin C1. Homodimericin A is a hexacyclic polyketide with a carbon backbone containing eight contiguous stereogenic carbons in a C20 hexacyclic core. Only half of its carbon atoms have an attached hydrogen, which presented a significant challenge for NMR-based structural analysis. In spite of its microbial production and rich stereochemistry, homodimericin A occurs naturally as a racemic mixture. A plausible nonenzymatic reaction cascade leading from two identical achiral monomers to homodimericin A is presented, and homodimericin A's formation by this path, a six-electron oxidation, could be a response to oxidative stress triggered by bafilomycin C1.

As new chemical methodologies driven by single-electron chemistry emerge, process and analytical chemists must develop approaches to rapidly solve problems in this nontraditional arena. Electron paramagnetic resonance spectroscopy has been long known as a preferred technique for the study of paramagnetic species. However, it is only recently finding application in contemporary pharmaceutical development, both to study reactions and to track the presence of undesired impurities. Several case studies are presented here to illustrate its utility in modern pharmaceutical development efforts.

Heteronuclear long-range NMR experiments are well established as essential NMR techniques for the structure elucidation of unknown natural products and small molecules. It is generally accepted that the absence of a given (n)JXH correlation in an HMBC or HSQMBC spectrum would automatically place the proton at least four bonds away from the carbon in question. This assumption can, however, be misleading in the case of a mismatch between the actual coupling constant and the delay used to optimize the experiment, which can lead to structural misassignments. Another scenario arises when an investigator, for whatever reason, needs to have access to very long-range correlations to confirm or refute a structure. In such cases, a conventional HMBC experiment will most likely fail to provide the requisite correlation, regardless of the delay optimization. Two recent methods for visualizing extremely weak or very long-range connectivities are the LR-HSQMBC and the HSQMBC-TOCSY experiments. Although they are intended to provide similar structural information, they utilize different transfer mechanisms, which differentiates the experiments, making each better suited for specific classes of compounds. In this report we have sought to examine the considerations implicit in choosing the best experiment to access weak or very long-range correlations for different types of molecules.

Despite the tremendous usage of HMBC to establish long-range (1) H-(13) C and (1) H-(15) N heteronuclear correlations, an inherent drawback of the experiment is the indeterminate nature of the (n) JXH correlations afforded by the experiment. A priori there is no reliable way of determining whether a given (n) JCH correlation is, for example, via two-, three-, or sometimes even four-bonds. This limitation of the HMBC experiment spurred the development of the ADEQUATE family of NMR experiments that rely on, in the case of 1,1-ADEQUATE, an out-and-back transfer of magnetization via the (1) JCC homonuclear coupling constant, which is significantly larger than (n) JCC (where n = 2-4) couplings in most cases. Hence, the 1,1-ADEQUATE experiment has generally been assumed to unequivocally provide the equivalent of (2) JCH correlations. The recent development of the 1,1- and 1,n-HD-ADEQUATE experiments that can provide homodecoupling for certain (1) JCC and (n) JCC correlations has increased the sensitivity of the ADEQUATE experiments significantly and can allow acquisition of these data in a fraction of the time required for the original iterations of this pulse sequence. With these gains in sensitivity, however, there occasionally come unanticipated consequences. We have observed that the collapse of proton multiplets, in addition to providing better s/n for the desired (1) JCC correlations can facilitate the observation of typically weaker (2) JCC correlations across intervening carbonyl resonances in 1,1-HD-ADEQUATE spectra. Several examples are shown, with the results supported by the measurement of the (2) JCC coupling constants in question using J-modulated-HD-ADEQUATE and DFT calculations. Copyright © 2016 John Wiley & Sons, Ltd.

The use of anisotropic NMR data, such as residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs), has emerged as a powerful technique for structural characterization of organic small molecules. RDCs typically report the relative orientations of different 1H-13C bonds; RCSAs report the relative orientations of different carbon chemical shielding tensors and hence are more useful for proton-deficient molecules. This information is complementary to that obtained from conventional NMR data such as J couplings, isotropic chemical shifts, and nuclear Overhauser effects (NOEs)/rotational frame nuclear Overhauser effects (ROEs). Obtaining anisotropic NMR data requires the creation of an anisotropic sample environment through an alignment medium. Here, we focus on the use of compressed or stretched polymeric gels as two different but fundamentally equivalent methods for introducing sample anisotropy. Protocols are provided for the synthesis of the chloroform-compatible poly(methyl methacrylate) and dimethyl sulfoxide (DMSO)-compatible poly(2-hydroxyethyl methacrylate) gels and sample setup with a preparation time of 2-3 d. The bond-specific RDC data and the atom-specific RCSA data are extracted as changes in 1H-13C couplings and 13C chemical shifts, respectively, between two measurements under different alignment conditions, with a total experimental time of 0.5-4 d. NMR data acquisition and important considerations are described in detail. We also provide step-by-step procedures for the density functional theory (DFT) calculations involved and data analysis using the commercial software MSpin. We use three example compounds, namely cryptospirolepine (505 Da), retrorsine (351 Da), and estrone (270 Da), to demonstrate some important aspects of the workflow, such as input data preparation, handling of structural flexibility, and RCSA data correction when necessary.