Potential Safety Hazards Associated with Using Acetonitrile and a Strong Aqueous Base

Acetonitrile, a common solvent in organic synthesis, can be hydrolyzed in the presence of a strong aqueous base, such as NaOH or KOH, which can propagate into a runaway reaction. For a process recently reviewed in our laboratory, a possible loss of cooling incident during the desired reaction was found to have the potential to self-heat to the onset temperature of this hydrolysis reaction. This base-catalyzed acetonitrile hydrolysis was determined to potentially escalate into a runaway reaction, where it could lead to secondary exothermic runaway reactions of the reaction mixture. The use of acetonitrile was preferred for the initial cyclization step. Thus, adequate removal of acetonitrile prior to the following step avoided the process safety hazard posed by the hydrolysis of the acetonitrile solvent. The findings presented in this work serve as an alert to chemists and engineers for the potential safety hazards when scaling up a process in which the solvent becomes a reactant

Digital NMR Profiles as Building Blocks: Assembling ¹H Fingerprints of Steviol Glycosides

This report describes a fragment-based approach to the examination of congeneric organic compounds by NMR spectroscopy. The method combines the classic interpretation of 1D- and 2D-NMR data sets with contemporary computer-assisted NMR analysis. Characteristic NMR profiles of key structural motifs were generated by ¹H iterative full spin analysis and then joined together as building blocks to recreate the ¹H NMR spectra of increasingly complex molecules. To illustrate the methodology described, a comprehensive analysis of steviol (<b>1</b>), seven steviol glycosides (<b>2</b>–<b>8</b>) and two structurally related isosteviol compounds (<b>9</b>, <b>10</b>) was carried out. The study also assessed the potential impact of this method on relevant aspects of natural product research including structural verification, chemical dereplication, and mixture analysis

Digital NMR Profiles as Building Blocks: Assembling ¹H Fingerprints of Steviol Glycosides

This report describes a fragment-based approach to the examination of congeneric organic compounds by NMR spectroscopy. The method combines the classic interpretation of 1D- and 2D-NMR data sets with contemporary computer-assisted NMR analysis. Characteristic NMR profiles of key structural motifs were generated by ¹H iterative full spin analysis and then joined together as building blocks to recreate the ¹H NMR spectra of increasingly complex molecules. To illustrate the methodology described, a comprehensive analysis of steviol (<b>1</b>), seven steviol glycosides (<b>2</b>–<b>8</b>) and two structurally related isosteviol compounds (<b>9</b>, <b>10</b>) was carried out. The study also assessed the potential impact of this method on relevant aspects of natural product research including structural verification, chemical dereplication, and mixture analysis

Ligand-Controlled Synthesis of Azoles via Ir-Catalyzed Reactions of Sulfoxonium Ylides with 2‑Amino Heterocycles

An iridium-catalyzed method was developed for the synthesis of imidazo-fused pyrrolopyrazines. The presence or absence of a nitrogenated ligand controlled the outcome of the reaction, leading to simple β-keto amine products in the absence of added ligand and the cyclized 7- and 8-substituted-imidazo­[1,2<i>-a</i>]­pyrrolo­[2,3<i>-e</i>]­pyrazine products in the presence of ligand. This catalyst control was conserved across a variety of ylide and amine coupling partners. The substrate was shown to act as a ligand for the iridium catalyst in the absence of other ligands via NMR spectroscopy. Kinetic studies indicated that formation of the Ir-carbene was reversible and the slow step of the reaction. These mechanistic investigations suggest that the β-keto amine products form via an intramolecular carbene N–H insertion, and the imidazopyrrolopyrazines form via an intermolecular carbene N–H insertion

Belizentrin, a Highly Bioactive Macrocycle from the Dinoflagellate <i>Prorocentrum belizeanum</i>

Belizentrin (<b>1</b>), a novel 25-membered polyketide-derived macrocycle, was isolated from cultures of the marine dinoflagellate <i>Prorocentrum belizeanum.</i> This metabolite is the first member of an unprecedented class of polyunsaturated and polyhydroxylated macrolactams. The structure of <b>1</b> was primarily determined by NMR and computational methods. Pharmacological assays with cerebellar cells showed that <b>1</b> produces important changes in neuronal network integrity at nanomolar concentrations

Copper-Catalyzed Aerobic Oxidative Amidation of Benzyl Alcohols

A Cu-catalyzed synthesis of amides from alcohols and secondary amines using the oxygen in air as the terminal oxidant has been developed. The methodology is operationally simple requiring no high pressure equipment or handling of pure oxygen. The commercially available, nonprecious metal catalyst, Cu­(phen)­Cl₂, in conjunction with di-<i>tert</i>-butyl hydrazine dicarboxylate and an inorganic base provides a variety of benzamides in moderate to excellent yields. The p<i>K</i>_a of amine conjugate acid and electronics of alcohol were shown to impact the selection of base for optimal reactivity. A mechanism consistent with the observed reactivity trends, KIE, and Hammett study is proposed

Importance of Purity Evaluation and the Potential of Quantitative ¹H NMR as a Purity Assay

In any biomedical and chemical context, a truthful description of chemical constitution requires coverage of both structure and purity. This qualification affects all drug molecules, regardless of development stage (early discovery to approved drug) and source (natural product or synthetic). Purity assessment is particularly critical in discovery programs and whenever chemistry is linked with biological and/or therapeutic outcome. Compared with chromatography and elemental analysis, quantitative NMR (qNMR) uses nearly universal detection and provides a versatile and orthogonal means of purity evaluation. Absolute qNMR with flexible calibration captures analytes that frequently escape detection (water, sorbents). Widely accepted structural NMR workflows require minimal or no adjustments to become practical ¹H qNMR (qHNMR) procedures with simultaneous qualitative and (absolute) quantitative capability. This study reviews underlying concepts, provides a framework for standard qHNMR purity assays, and shows how adequate accuracy and precision are achieved for the intended use of the material

Hytramycins V and I, Anti-<i>Mycobacterium tuberculosis</i> Hexapeptides from a <i>Streptomyces hygroscopicus</i> Strain

Thirty-five thousand actinomycete extracts were screened for anti-<i>Mycobacterium tuberculosis</i> (<i>M. tb</i>) activity, followed by C₁₈ cartridge fractionation of 37 prioritized extracts. Based on MICs against replicating and nonreplicating <i>M. tb</i>, and IC₅₀ values against Vero cells to generate selectivity indices, seven fractions from seven different strains were selected for further examination. When cultured in G.S.S. media and extracted with ethyl acetate, the <i>Streptomyces hygroscopicus</i> strain ECUM 14046 yielded an extract with promising anti-<i>M. tb</i> activity and a well-defined chromatographic profile. Fractionation by preparative HPLC and subsequent structure elucidation of two active fractions using 1D- and 2D-NMR and MS methods revealed the presence of two cyclohexapeptides, hytramycins V and I, each containing three unusual piperazic acid moieties. The use of ¹H iterative full spin analysis (HiFSA) on both hytramycins confirmed that quantum mechanics-simulated spectra match the experimental data, and all <i>J</i>_H,H and δ_H values are consistent with the proposed structures. The absolute configuration of each amino acid moiety was determined by Marfey’s method. The MICs against replicating and, more importantly, nonreplicating <i>M. tb</i> fall into the range of some existing second-line anti-TB drugs, such as streptomycin and capreomycin, respectively. The activities were maintained against <i>M. tb</i> strains that represent the major global clades, as well as H₃₇Rv-isogenic strains that are resistant to individual clinical anti-TB drugs

<i>K</i>‑Targeted Metabolomic Analysis Extends Chemical Subtraction to DESIGNER Extracts: Selective Depletion of Extracts of Hops (Humulus lupulus)

This study introduces a flexible and compound targeted approach to <u>D</u>eplete and <u>E</u>nrich <u>S</u>elect <u>I</u>ngredients to <u>G</u>enerate <u>N</u>ormalized <u>E</u>xtract <u>R</u>esources, generating DESIGNER extracts, by means of chemical subtraction or augmentation of metabolites. Targeting metabolites based on their liquid–liquid partition coefficients (<i>K</i> values), <i>K</i> targeting uses countercurrent separation methodology to remove single or multiple compounds from a chemically complex mixture, according to the following equation: DESIGNER extract = total extract ± target compound(s). Expanding the scope of the recently reported depletion of extracts by immunoaffinity or solid phase liquid chromatography, the present approach allows a more flexible, single- or multi-targeted removal of constituents from complex extracts such as botanicals. Chemical subtraction enables both chemical and biological characterization, including detection of synergism/antagonism by both the subtracted targets and the remaining metabolite mixture, as well as definition of the residual complexity of all fractions. The feasibility of the DESIGNER concept is shown by <i>K</i>-targeted subtraction of four bioactive prenylated phenols, isoxanthohumol (<b>1</b>), 8-prenylnaringenin (<b>2</b>), 6-prenylnaringenin (<b>3</b>), and xanthohumol (<b>4</b>), from a standardized hops (Humulus lupulus L.) extract using specific solvent systems. Conversely, adding <i>K</i>-targeted isolates allows enrichment of the original extract and hence provides an augmented DESIGNER material. Multiple countercurrent separation steps were used to purify each of the four compounds, and four DESIGNER extracts with varying depletions were prepared. The DESIGNER approach innovates the characterization of chemically complex extracts through integration of enabling technologies such as countercurrent separation, <i>K</i>-by-bioactivity, the residual complexity concepts, as well as quantitative analysis by ¹H NMR, LC-MS, and HiFSA-based NMR fingerprinting

Chlorinated Coumarins from the Polypore Mushroom <i>Fomitopsis officinalis</i> and Their Activity against <i>Mycobacterium tuberculosis</i>

An EtOH extract of the polypore mushroom <i>Fomitopsis officinalis</i> afforded two new naturally occurring chlorinated coumarins, which were identified as the previously synthesized compounds 6-chloro-4-phenyl-2<i>H</i>-chromen-2-one (<b>1</b>) and ethyl 6-chloro-2-oxo-4-phenyl-2<i>H</i>-chromen-3-carboxylate (<b>2</b>). The structures of the two isolates were deduced by <i>ab initio</i> spectroscopic methods and confirmed by chemical synthesis. In addition, an analogue of each was synthesized as 7-chloro-4-phenyl-2<i>H</i>-chromen-2-one (<b>3</b>) and ethyl 7-chloro-2-oxo-4-phenyl-2<i>H</i>-chromen-3-carboxylate (<b>4</b>). All four compounds were characterized physicochemically, and their antimicrobial activity profiles revealed a narrow spectrum of activity with lowest MICs against the <i>Mycobacterium tuberculosis</i> complex