Total synthesis of bacilosarcins A and B

Development of a drug discovery approach from microbes with a special focus on isolation sources and taxonomy

After the successful discoveries of numerous antibiotics from microorganisms, frequent reisolation of known compounds becomes an obstacle in further development of new drugs from natural products. Exploration of biological sources that can provide novel scaffolds is thus an urgent matter in drug lead screening. As an alternative source to the conventionally used soil microorganisms, we selected endophytic actinomycetes, marine actinomycetes, and actinomycetes in tropical areas for investigation and found an array of new bioactive compounds. Furthermore, based on the analysis of the distribution pattern of biosynthetic gene clusters in bacteria together with available genomic data, we speculated that biosynthetic gene clusters for secondary metabolites are specific to each genus. Based on this assumption, we investigated actinomycetal and marine bacterial genera from which no compounds have been reported, which led to the discovery of a variety of skeletally novel bioactive compounds. These findings suggest that consideration of environmental factor and taxonomic position is critically effective in the selection of potential strains producing structurally unique compounds.

Concise synthesis of amino acid component of amicoumacins via dihydrooxazine formation through intramolecular conjugate addition

Amicoumacins are a family of antibiotics with a variety of important bioactivities. A concise and efficient method was developed for synthesizing the amino acid component of amicoumacins via the corresponding dihydrooxazine intermediate. The dihydrooxazine ring was formed with complete stereoselectivity through an intramolecular conjugate addition of a δ-trichloroacetimidoyloxy-α,β-unsaturated ester, which was obtained from a known 4,6-O-p-methoxybenzylidene-protected d-glucose. The synthesis developed in this study can be used to synthesize the building blocks of amicoumacins and can likely be adapted for the synthesis of other types of molecules possessing dihydrooxazine rings or amino alcohol moieties.

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

A Novel Dual Organocatalyst for the Asymmetric Pinder Reaction and a Mechanistic Proposal Consistent with the Isoinversion Effect Thereof

In general, the Pinder reaction concerns the reaction between an enolisable anhydride and an aldehyde proceeding initially through a Knoevenagel reaction followed by the ring closing process generating lactones with at least two chiral centers. These scaffolds are frequently present in natural products and synthetic bioactive molecules, hence it has attracted intense interest in organic synthesis and medicinal chemistry, particularly with respect to controlling the diastereo- and enantioselectivity. To the best of our knowledge, there has been only one attempt prior to this work towards the development of a catalytic enantioselective Pinder reaction. In our approach, we designed, synthesized, and tested dual chiral organocatalysts by combining BIMAH amines, (2-(α-(alkyl)methanamine)-1H-benzimidazoles, and a Lewis acid motif, such as squaramides, ureas and thioureas. The optimum catalyst was the derivative of isopropyl BIMAH bearing a bis(3,5-trifluoromethyl) thiourea, which afforded the Pinder products from various aromatic aldehydes with diastereomeric ratio >98:2 and enatioselectivity up to 92 ee%. Interestingly, the enantioselectivity of this catalyzed process is increased at higher concentrations and exhibits an isoinversion effect, namely an inverted "U" shaped dependency with respect to the temperature. Mechanistically, these features, point to a transition state involving an entropy-favored heterodimer interaction between a catalyst/anhydride and a catalyst/aldehyde complex when all other processes leading to this are much faster in comparison above the isoinversion temperature.

Application of Pd-Catalyzed C-H Alkylation Reaction in Total Syntheses of Twelve Amicoumacin-Type Natural Products

Enantioselective total syntheses of 12 amicoumacin-type natural products are accomplished with a palladium(II)-catalyzed C-H alkylation as the key step to furnish the 3,4-dihydroisocoumarin scaffold. The target chemicals are assembled in a convergent protocol by merging 3,4-dihydroisocoumarin derived amine part with categories of acid segments that are efficiently prepared by chemoselective catalytic oxidation of chiral 1,2-dihydroxyethylfuran-2(5H)-ones. Afterward, the cytotoxicity of amicoumacins on five cancer cell lines and one normal cell line is investigated.

Asymmetric Total Synthesis of Hetiamacins A-F

Herein, we report a concise and stereoselective approach for the asymmetric total synthesis of hetiamacins A-F on the basis of the total synthesis of amicoumacin C, which could be synthesized from a known l-aspartic acid derivative. The synthesis of hetiamacin A was accomplished by an 11-step sequence that featured 1,3-oxazinane ring formation of amicoumacin B followed by amidation in one pot. Hetiamacins B-F were synthesized from amicoumacin A in only one step.

Genome Mining, Heterologous Expression, Antibacterial and Antioxidant Activities of Lipoamides and Amicoumacins from Compost-Associated Bacillus subtilis fmb60

Bacillus subtilis fmb60, which has broad-spectrum antimicrobial activities, was isolated from plant straw compost. A hybrid NRPS/PKS cluster was screened from the genome. Sixteen secondary metabolites produced by the gene cluster were isolated and identified using LC-HRMS and NMR. Three lipoamides D–F (1–3) and two amicoumacin derivatives, amicoumacins D, E (4, 5), were identified, and are reported here for the first time. Lipoamides D–F exhibited strong antibacterial activities against harmful foodborne bacteria, with the MIC ranging from 6.25 to 25 µg/mL. Amicoumacin E scavenged 38.8% of ABTS⁺ radicals at 1 mg/mL. Direct cloning and heterologous expression of the NRPS/PKS and ace gene cluster identified its importance for the biosynthesis of amicoumacins. This study demonstrated that there is a high potential for biocontrol utilization of B. subtilis fmb60, and genome mining for clusters of secondary metabolites of B. subtilis fmb60 has revealed a greater biosynthetic potential for the production of novel natural products than previously anticipated.

3,4-Dihydroisocoumarins, Interesting Natural Products: Isolation, Organic Syntheses and Biological Activities

BACKGROUND

3,4-dihydroisocoumarins are an important small group belonging to the class of naturally occurring lactones isolated from different bacterial strains, molds, lichens, and plants. The structures of these natural compounds show various types of substitution in their basic skeleton and this variability influences deeply their biological activities. These lactones are structural subunits of several natural products and serve as useful intermediates in the synthesis of different heterocyclic molecules, which exhibit a wide range of biological activities, such as anti-inflammatory, antiplasmodial, antifungal, antimicrobial, antiangiogenic and antitumoral activities, among others. Their syntheses have attracted attention of many researchers reporting many synthetic strategies to achieve 3,4-dihydroisocoumarins and other related structures.

OBJECTIVE

In this context, the isolation of these natural compounds from different sources, their syntheses and biological activities are reviewed, adding the most recent advances and related developments.

CONCLUSION

This review aims to encourage further work on the isolation and synthesis of this class of natural products. It would be beneficial for synthetic as well as the medicinal chemists to design selective, optimized dihydroisocoumarin derivatives as potential drug candidates, since dihydroisocoumarin scaffolds have significant utility in the development of therapeutically relevant and biologically active compounds.

Advances in the Asymmetric Total Synthesis of Natural Products Using Chiral Secondary Amine Catalyzed Reactions of α,β-Unsaturated Aldehydes

Chirality is one of the most important attributes for its presence in a vast majority of bioactive natural products and pharmaceuticals. Asymmetric organocatalysis methods have emerged as a powerful methodology for the construction of highly enantioenriched structural skeletons of the target molecules. Due to their extensive application of organocatalysis in the total synthesis of bioactive molecules and some of them have been used in the industrial synthesis of drugs have attracted increasing interests from chemists. Among the chiral organocatalysts, chiral secondary amines (MacMillan's catalyst and Jorgensen's catalyst) have been especially considered attractive strategies because of their impressive efficiency. Herein, we outline advances in the asymmetric total synthesis of natural products and relevant drugs by using the strategy of chiral secondary amine catalyzed reactions of α,β-unsaturated aldehydes in the last eighteen years.

Bioactive Secondary Metabolites from Bacillus subtilis: A Comprehensive Review

Bacillus subtilis is widely underappreciated for its inherent biosynthetic potential. This report comprehensively summarizes the known bioactive secondary metabolites from B. subtilis and highlights potential applications as plant pathogen control agents, drugs, and biosurfactants. B. subtilis is well known for the production of cyclic lipopeptides exhibiting strong surfactant and antimicrobial activities, such as surfactins, iturins, and fengycins. Several polyketide-derived macrolides as well as nonribosomal peptides, dihydroisocoumarins, and linear lipopeptides with antimicrobial properties have been reported, demonstrating the biosynthetic arsenal of this bacterium. Promising efforts toward the application of B. subtilis strains and their natural products in areas of agriculture and medicine are underway. However, industrial-scale availability of these compounds is currently limited by low fermentation yields and challenging accessibility via synthesis, necessitating the development of genetically engineered strains and optimized cultivation processes. We hope that this review will attract renewed interest in this often-overlooked bacterium and its impressive biosynthetic skill set.

Catalytic Asymmetric Cycloadditions between Aldehydes and Enolizable Anhydrides: cis-Selective Dihydroisocoumarin Formation

In the presence of a trityl-substituted cinchona alkaloid-based catalyst, homophthalic, aryl succinic, and glutaconic anhydride derivatives reacted with aromatic and aliphatic aldehydes to produce cis-lactones in up to 90:10 dr and 99% ee. A DFT study has shown how the catalyst is uniquely able to bring about the opposite sense of diastereocontrol to that usually observed.

A DFT mechanistic study of the organocatalytic asymmetric reaction of aldehydes and homophthalic anhydride

The first study of the cycloaddition of benzaldehyde with homophthalic anhydride under the influence of an alkaloid-based, squaramide substituted bifunctional organocatalyst using DFT methodology is reported.

Catalytic formal cycloadditions between anhydrides and ketones: excellent enantio and diastereocontrol, controllable decarboxylation and the formation of adjacent quaternary stereocentres

It has been shown for the first time that enolisable anhydrides can participate in highly efficient and diastereo/enantioselective additions to activated ketones.

Antiplasmodial activity of bacilosarcin A isolated from the octocoral-associated bacterium Bacillus sp. collected in Panama

Aim:

This study was designed for isolating and characterizing antiplasmodial compounds from marine octocoral-associated bacteria.

Materials and Methods:

The organic extract of the Bacillus sp. was subjected to purification using several chromatography techniques guided by bioassays to yield three isocoumarin derivatives (1–3). Chemical structures of the compounds were elucidated on the basis of HRMS spectra and NMR spectroscopy. The antiplasmodial activity of the isolated compounds was evaluated in vitro against the chloroquine-resistant Plasmodium falciparum strain W2.

Results:

Isolated compounds were identified as bacilosarcin A (1), AI77-F (2), and AI77-H (3). Bacilosarcin A (1) displayed a low micromolar activity (IC₅₀ = 2.2 μM) against P. falciparum while compounds 2 and 3 showed no activity.

Conclusions:

Bacilosarcin A was found to be responsible for the antiplasmodial activity observed in the crude extract obtained from the Bacillus sp.

Catalytic, enantio- and diastereoselective synthesis of γ-butyrolactones incorporating quaternary stereocentres

A new, highly enantio- and diastereoselective catalytic asymmetric formal cycloaddition of aryl succinic anhydrides and aldehydes which generates paraconic acid (γ-butyrolactone) derivatives is reported.

Nobilamides A-H, long-acting transient receptor potential vanilloid-1 (TRPV1) antagonists from mollusk-associated bacteria

New compounds nobilamides A-H and related known compounds A-3302-A and A-3302-B were isolated based upon their suppression of capsaicin-induced calcium uptake in a mouse dorsal root ganglion primary cell culture assay. Two of these compounds, nobilamide B and A-3302-A, were shown to be long-acting antagonists of mouse and human TRPV1 channels, abolishing activity for >1 h after removal of drug presumably via a covalent attachment. Other derivatives also inhibited the TRPV1 channel, albeit with low potency, affording a structure-activity profile to support the proposed mechanism of action. While the activities were modest, we propose a new mechanism of action and a new site of binding for these inhibitors that may spur development of related analogues for treatment of pain.