11B and 1H-11B HMBC NMR as a Tool for Identification of a Boron-Containing Nucleoside Dimer.

Boron-containing compounds are commonly used in synthetic chemistry and are known to play important roles in biology. Despite the widespread relevance of boronated compounds, there have been limited methods to discover, characterize, and study them. Here, we describe the use of 11B NMR, including 1H-11B HMBC, for the isolation and characterization of the boron-containing natural product diadenosine borate. Utilizing synthetic standards, we optimized coupling parameters for 1H-11B HMBC experiments to allow for the analysis of small quantities (∼1 mg) of boron-containing compounds. This work can facilitate the broader application of 11B NMR to the study of boron in a range of applications, from synthetic chemistry to the role of boron in naturally occurring systems.

Boron NMR as a Method to Screen Natural Product Libraries for B-Containing Compounds.

Natural products are biologically relevant metabolites exploited for biomedicine and biotechnology. The frequent reisolation of known natural products questions whether existing discovery models are still capable of identifying novel compounds. As innovative NMR-based screening techniques can help overcome these challenges, we applied a phase cycling composite pulse sequence to 11B NMR experiments to enhance their sensitivity to screen libraries for novel boron-containing molecules. Aplasmomycin and autoinducer-2 were detected in crude and enhanced microbial fractions, via their boron signals, as proof of concept. Subsequently, a screen of 21 crude plant and 50 crude marine microbial extracts were chosen at random and analyzed with the optimized 11B experiment for feasibility as a high throughput discovery method. Eight of the plant samples and 13 of the microbial samples were identified as boron-containing, suggesting that there is a higher presence of boron metabolites available from natural sources than previously known due to a lack of appropriate discovery methods. As a result, we believe that this optimized 11B NMR experiment can serve as a robust method for quick and facile discovery of novel boron-containing metabolites from a variety of natural sources.

The Natural Products Atlas: An Open Access Knowledge Base for Microbial Natural Products Discovery.

Despite rapid evolution in the area of microbial natural products chemistry, there is currently no open access database containing all microbially produced natural product structures. Lack of availability of these data is preventing the implementation of new technologies in natural products science. Specifically, development of new computational strategies for compound characterization and identification are being hampered by the lack of a comprehensive database of known compounds against which to compare experimental data. The creation of an open access, community-maintained database of microbial natural product structures would enable the development of new technologies in natural products discovery and improve the interoperability of existing natural products data resources. However, these data are spread unevenly throughout the historical scientific literature, including both journal articles and international patents. These documents have no standard format, are often not digitized as machine readable text, and are not publicly available. Further, none of these documents have associated structure files (e.g., MOL, InChI, or SMILES), instead containing images of structures. This makes extraction and formatting of relevant natural products data a formidable challenge. Using a combination of manual curation and automated data mining approaches we have created a database of microbial natural products (The Natural Products Atlas, www.npatlas.org) that includes 24 594 compounds and contains referenced data for structure, compound names, source organisms, isolation references, total syntheses, and instances of structural reassignment. This database is accompanied by an interactive web portal that permits searching by structure, substructure, and physical properties. The Web site also provides mechanisms for visualizing natural products chemical space and dashboards for displaying author and discovery timeline data. These interactive tools offer a powerful knowledge base for natural products discovery with a central interface for structure and property-based searching and presents new viewpoints on structural diversity in natural products. The Natural Products Atlas has been developed under FAIR principles (Findable, Accessible, Interoperable, and Reusable) and is integrated with other emerging natural product databases, including the Minimum Information About a Biosynthetic Gene Cluster (MIBiG) repository, and the Global Natural Products Social Molecular Networking (GNPS) platform. It is designed as a community-supported resource to provide a central repository for known natural product structures from microorganisms and is the first comprehensive, open access resource of this type. It is expected that the Natural Products Atlas will enable the development of new natural products discovery modalities and accelerate the process of structural characterization for complex natural products libraries.

Chromomycin A2 potently inhibits glucose-stimulated insulin secretion from pancreatic ß cells.

Modulators of insulin secretion could be used to treat diabetes and as tools to investigate ß cell regulatory pathways in order to increase our understanding of pancreatic islet function. Toward this goal, we previously used an insulin-linked luciferase that is cosecreted with insulin in MIN6 ß cells to perform a high-throughput screen of natural products for chronic effects on glucose-stimulated insulin secretion. In this study, using multiple phenotypic analyses, we found that one of the top natural product hits, chromomycin A2 (CMA2), potently inhibited insulin secretion by at least three potential mechanisms: disruption of Wnt signaling, interference of ß cell gene expression, and partial suppression of Ca2+ influx. Chronic treatment with CMA2 largely ablated glucose-stimulated insulin secretion even after washout, but it did not inhibit glucose-stimulated generation of ATP or Ca2+ influx. However, by using the KATP channel opener diazoxide, we uncovered defects in depolarization-induced Ca2+ influx that may contribute to the suppressed secretory response. Glucose-responsive ERK1/2 and S6 phosphorylation were also disrupted by chronic CMA2 treatment. By querying the FUSION bioinformatic database, we revealed that the phenotypic effects of CMA2 cluster with a number of Wnt-GSK3 pathway-related genes. Furthermore, CMA2 consistently decreased GSK3ß phosphorylation and suppressed activation of a ß-catenin activity reporter. CMA2 and a related compound, mithramycin, are known to have DNA interaction properties, possibly abrogating transcription factor binding to critical ß cell gene promoters. We observed that CMA2 but not mithramycin suppressed expression of PDX1 and UCN3. However, neither expression of INSI/II nor insulin content was affected by chronic CMA2. The mechanisms of CMA2-induced insulin secretion defects may involve components both proximal and distal to Ca2+ influx. Therefore, CMA2 is an example of a chemical that can simultaneously disrupt ß cell function through both noncytotoxic and cytotoxic mechanisms. Future therapeutic applications of CMA2 and similar aureolic acid analogues should consider their potential effects on pancreatic islet function.

Design, synthesis and antimicrobial evaluation of novel benzoxazole derivatives.

The synthesis of (S)-2-(4-tert-butylphenoxy)-3-(benzoxazol-5-yl) propanoic acid derivatives (2a-k) were described and their in vitro antibacterial activities were determined against Gram-negative and -positive bacteria. These compounds were found to exert a broad spectrum of activity against the screened bacteria, but poor MIC values were found for Candida albicans fungi. Compound 2b bearing a hydrophobic aromatic tie was the most active derivative against all bacteria studied with MIC values ranging from 0.098 to 0.78 µg/mL. The activity of 2b against B. subtilis was 2-fold higher than Penicillin, and 8- to 510-fold higher than other control antibiotics.

Assembly of Terpenoid Cores by a Simple, Tunable Strategy.

Oxygenated, polycyclic terpenoid natural products have important biological activities. Although total synthesis of such terpenes is widely studied, synthetic strategies that allow for controlled placement of oxygen atoms and other functionality remains a challenge. Herein, we present a simple, scalable, and tunable synthetic strategy to assemble terpenoid-like polycycloalkanes from cycloalkanones, malononitrile, and allylic electrophiles, abundantly available reagent classes.

Structure-activity of antifungal compounds inspired by aminobisabolenes from the sponge Halichondria sp.

Structure-activity relationships of the antifungal aminobisabolene natural product, 1 isolated from Halichondria sp., and synthetic analogs were explored, in parallel with the antidermatophytic allylamine, Terbinafine®, against a panel of pathogenic fungi: Candida spp., Cryptococcus spp. and Trichophyton rubrum. Interpretation of the results suggest different modes of action in antifungal activity for the two classes of compounds.

A benzotriazole-mediated route to protected marine-derived hetero-2,5-diketopiperazines containing proline.

A procedure for the cyclization of dipeptidoyl benzotriazolides containing proline derivatives promoted by triethylamine under MW activation is introduced. The reaction is general for a variety of dipeptidoyl benzotriazolides and represents a very practical and convenient method for the preparation of Pro- or Hyp-derived 2,5-diketopiperazines (2,5-DKPs) and bis-DKPs with a disulfide linker. This method can be used for the construction of 2,5-DKP compound libraries and for the synthesis of natural products with diketopiperazine cores.