Component-based syntheses of trioxacarcin A, DC-45-A1 and structural analogues

Palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines

Herein, we report a visible-light-mediated palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines, affording unsaturated γ- and ε-amino acid derivatives with diverse structures. In this methodology, the diazo compound readily transforms into a hybrid α-ester alkylpalladium radical with the release of dinitrogen. The radical intermediate selectively adds to the double bond of a 1,3-diene or allene, followed by the allylpalladium radical-polar crossover path and selective allylic substitution with the amine substrate, thereby leading to a single unsaturated γ- or ε-amino acid derivative. This approach proceeds under mild and simple reaction conditions and shows high functional group tolerance, especially in the incorporation of various bioactive molecules. The studies on scale-up reactions and diverse derivatizations highlight the practical utility of this multicomponent reaction protocol.

Trioxacarcin A Interactions with G-Quadruplex DNA Reveal Its Potential New Targets as an Anticancer Agent

Trioxacarcin (TXN) A was reported to be an anticancer agent through alkylation of dsDNA. G-quadruplex DNA (G4-DNA) is frequently formed in the promoter regions of oncogenes and the ends of telomerase genes, considered as promising drug targets for anticancer therapy. There are no reports about TXN A interactions with G4-DNA. Here, we tested TXN A's interactions with several G4-DNA oligos with parallel, antiparallel, or hybrid folding, respectively. We demonstrated that TXN A preferred to alkylate one flexible guanine in the loops of parallel G4-DNA. The position of the alkylated guanine is in favor of interactions of G4-DNA with TXN A. The structure of TXN A covalently bound RET G4-DNA indicated that TXN A alkylation on RET G4-DNA stabilizes the G4-DNA conformation. These studies opened a new window of how TXN A interacted with G4-DNA, which might hint a new mode of its function as an anticancer agent.

Photoredox-Catalyzed Carbonyl Alkylative Amination with Diazo Compounds: A Three-Component Reaction for the Construction of γ-Amino Acid Derivatives

A photoredox-catalyzed reaction of secondary amines, aldehydes, diazo compounds, and Hantzsch ester is reported, affording biologically active γ-amino acid derivatives in high yields. This one-pot process tolerates a broad range of functional groups and various drug molecules and biologically active compounds. Remarkably, a gram-scale reaction and diverse transformations of γ-amino acid derivatives were successfully performed, and the utility of the products is demonstrated in the synthesis of therapeutic agent pregabalin.

Metabolomics Tools Assisting Classic Screening Methods in Discovering New Antibiotics from Mangrove Actinomycetia in Leizhou Peninsula

Mangrove actinomycetia are considered one of the promising sources for discovering novel biologically active compounds. Traditional bioactivity- and/or taxonomy-based methods are inefficient and usually result in the re-discovery of known metabolites. Thus, improving selection efficiency among strain candidates is of interest especially in the early stage of the antibiotic discovery program. In this study, an integrated strategy of combining phylogenetic data and bioactivity tests with a metabolomics-based dereplication approach was applied to fast track the selection process. A total of 521 actinomycetial strains affiliated to 40 genera in 23 families were isolated from 13 different mangrove soil samples by the culture-dependent method. A total of 179 strains affiliated to 40 different genera with a unique colony morphology were selected to evaluate antibacterial activity against 12 indicator bacteria. Of the 179 tested isolates, 47 showed activities against at least one of the tested pathogens. Analysis of 23 out of 47 active isolates using UPLC-HRMS-PCA revealed six outliers. Further analysis using the OPLS-DA model identified five compounds from two outliers contributing to the bioactivity against drug-sensitive A. baumannii. Molecular networking was used to determine the relationship of significant metabolites in six outliers and to find their potentially new congeners. Finally, two Streptomyces strains (M22, H37) producing potentially new compounds were rapidly prioritized on the basis of their distinct chemistry profiles, dereplication results, and antibacterial activities, as well as taxonomical information. Two new trioxacarcins with keto-reduced trioxacarcinose B, gutingimycin B (16) and trioxacarcin G (20), together with known gutingimycin (12), were isolated from the scale-up fermentation broth of Streptomyces sp. M22. Our study demonstrated that metabolomics tools could greatly assist classic antibiotic discovery methods in strain prioritization to improve efficiency in discovering novel antibiotics from those highly productive and rich diversity ecosystems.

Development of a Cross-Conjugated Vinylogous [4+2] Anionic Annulation and Application to the Total Synthesis of Natural Antibiotic (±)-ABX

The cross‐conjugated vinylogous [4+2] anionic annulation has been newly developed, the cascade process of which has a high preference for regiochemical control and chemoselectivity, giving rise to exclusively Michael‐type adducts in moderate to high yields (up to 94 %, 35 examples). By making use of this approach as a key operation, the first total synthesis of natural antibiotic ABX, in racemic form, has been successfully achieved in a concise 7‐step sequence with an overall yield of about 20 %.

Discovery of Druggability-Improved Analogues by Investigation of the LL-D49194α1 Biosynthetic Pathway

The biosynthetic gene cluster of antitumor antibiotic LL-D49194α1 (LLD) was identified and comparatively analyzed with that of trioxacarcins. The tailoring genes encoding glycosyltransferase, methyltransferase and cytochrome P450 were systematically deleted, which led to the discovery of eight compounds from the mutants. Preliminary pharmaceutical evaluation revealed two intermediates exhibiting higher cytotoxicity, stability and solubility. These results highlighted the modification pathway for LLD biosynthesis, and provided highly potent, structurally simplified "unnatural" natural products with improved druggability.

Total Synthesis in Search of Potent Antibody-Drug Conjugate Payloads. From the Fundamentals to the Translational

The emergence and evolution of antibody-drug conjugates (ADCs) as targeted cancer therapies in recent years is a living example of the "magic bullet" concept of Paul Ehrlich, introduced by him more than a century ago. Consisting of three components, the antibody serving as the delivery system, the payload drug that kills the cancer cell, and the chemical linker through which the payload is attached to the antibody, ADCs represent a currently hotly pursued paradigm of targeted cancer therapies. While the needed monoclonal antibody falls in the domains of biology and biochemistry, the potent payload and the linker belong to the realm of chemistry. Naturally occurring molecules and their derivatives endowed with high cytotoxic properties have proven to be useful payloads for the first approved ADCs (i.e., Mylotarg, Adcetris, Kadcyla, and Besponsa). The latest approaches and intensifying activities in this new paradigm of cancer therapy demands a variety of payloads with different mechanisms of action in order to address the medical needs for the various types of cancers, challenging synthetic organic chemists to enrich the library of potential payloads. Total synthesis of natural and designed molecules not only provides a powerful avenue to replicate rare naturally occurring compounds in the laboratory but also offers a unique opportunity to rationally design and synthesize analogues thereof for biological evaluation and optimization of ADC payloads. In this Account, we describe our efforts in this area highlighting a number of total synthesis endeavors through which we rendered scarce naturally occurring molecules readily available for biological evaluations and, most importantly, employed the developed synthetic strategies and methods to construct, otherwise unavailable or difficult to reach, designed analogues of these molecules. Specifically, we summarize the total syntheses of natural and designed molecules of the calicheamicin, uncialamycin, tubulysin, trioxacarcin, epothilone, shishijimicin, namenamicin, thailanstatin, and disorazole families of compounds and demonstrate how these studies led to the discovery of analogues of higher potencies, yet some of them possessing lower complexities than their parent compounds as potential ADC payloads. The highlighted examples showcase the continuing impact of total synthesis of natural products and their analogues on modern medicine, including the so-called biologics and should prove useful and inspirational in advancing both the fields of total synthesis and biomedical research and the drug discovery and development process.

Methods for 2-Deoxyglycoside Synthesis

Deoxy-sugars often play a critical role in modulating the potency of many bioactive natural products. Accordingly, there has been sustained interest in methods for their synthesis over the past several decades. The focus of much of this work has been on developing new glycosylation reactions that permit the mild and selective construction of deoxyglycosides. This Review covers classical approaches to deoxyglycoside synthesis, as well as more recently developed chemistry that aims to control the selectivity of the reaction through rational design of the promoter. Where relevant, the application of this chemistry to natural product synthesis will also be described.

Total Synthesis of Tetarimycin A, (±)-Naphthacemycin A9, and (±)-Fasamycin A: Structure-Activity Relationship Studies against Drug-Resistant Bacteria

Making use of a reductive olefin coupling reaction and Michael-Dieckmann condensation as two key operations, we have completed a concise total synthesis of tetarimycin A, (±)-naphthacemycin A₉, and (±)-fasamycin A in a highly convergent and practical protocol. Synthetic procedures thus developed have also been applied to provide related analogues for structure-activity relationship studies, thereby coming to the conclusion that the free hydroxyl group at C-10 is essential for exerting inhibitory activities against a panel of Gram-positive bacteria, including drug-resistant strains VRE and MRSA.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Natural product analogues: towards a blueprint for analogue-focused synthesis

A review of approaches to natural product analogues leads to the suggestion of new methods for the generation of biologically active natural product-like scaffolds.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

Marine natural products

This review covers the literature published in 2013 for marine natural products (MNPs), with 982 citations (644 for the period January to December 2013) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1163 for 2013), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Practical Synthesis of p- and o-Amino- and Methoxyphenolic Anthraquinones

New versatile and selective methods for the syntheses of substituted amino- and methoxyphenolic anthraquinones (I-IV) based on fusion of cyanophthalides (V) and semiquinone aminals (VI, VII) under basic conditions are described.

The SARP Family Regulator Txn9 and Two-Component Response Regulator Txn11 are Key Activators for Trioxacarcin Biosynthesis in Streptomyces bottropensis

Trioxacarcin A is a polyoxygenated, structurally complex antibiotic produced by Streptomyces spp., which possesses high anti-bacterial, anti-malaria, and anti-tumor activities. The trioxacarcin biosynthetic pathway involves type II polyketide synthases (PKSs) with L-isoleucine as a unique starter unit, as well as many complex post-PKS tailoring enzymes and resistance and regulatory proteins. In this work, two regulatory genes, txn9 coding for a Streptomyces antibiotic regulatory protein family regulator and txn11 for a two-component response regulator, were revealed to be absolutely required for trioxacarcin production by individually inactivating all the six annotated regulatory genes in the txn cluster. Complementation assay suggested that these two activators do not have a regulatory cascade relationship. Moreover, transcriptional analysis showed that they activate 15 of the 28 txn operons, indicating that a complicated regulatory network is involved in the trioxacarcin production. Information gained from this study may be useful for improving the production of the highly potent trioxacarcin A.

Biosynthesis of trioxacarcin revealing a different starter unit and complex tailoring steps for type II polyketide synthase

Different starter unit and complex tailoring steps for type II polyketide synthase in trioxacarcin biosynthesis.

Trioxacarcins (TXNs) are highly oxygenated, polycyclic aromatic natural products with remarkable biological activity and structural complexity. Evidence from ¹³C-labelled precursor feeding studies demonstrated that the scaffold was biosynthesized from one unit of l-isoleucine and nine units of malonyl-CoA, which suggested a different starter unit in the biosynthesis. Genetic analysis of the biosynthetic gene cluster revealed 56 genes encoding a type II polyketide synthase (PKS), combined with a large amount of tailoring enzymes. Inactivation of seven post-PKS modification enzymes resulted in the production of a series of new TXN analogues, intermediates, and shunt products, most of which show high anti-cancer activity. Structural elucidation of these new compounds not only helps us to propose the biosynthetic pathway, featuring a type II PKS using a novel starter unit, but also set the stage for further characterization of the enzymatic reactions and combinatorial biosynthesis.

O-Glycosylation methods in the total synthesis of complex natural glycosides

We highlight the total syntheses of 33 complex natural O-glycosides, with a particular focus on the O-glycosylation methods that enable the connection of the saccharides and aglycones.

Crystalline guanine adducts of natural and synthetic trioxacarcins suggest a common biological mechanism and reveal a basis for the instability of trioxacarcin A

X-ray crystallographic characterization of products derived from natural and fully synthetic trioxacarcins, molecules with potent antiproliferative effects, illuminates aspects of their reactivity and mechanism of action. Incubation of the fully synthetic trioxacarcin analog 3, which lacks one of the carbohydrate residues present in the natural product trioxacarcin A (1) as well as oxygenation at C2 and C4 yet retains potent antiproliferative effects, with the self-complimentary duplex oligonucleotide d(AACCGGTT) led to production of a crystalline covalent guanine adduct (6). Adduct 6 is closely analogous to gutingimycin (2), the previously reported guanine adduct derived from incubation of natural trioxacarcin A (1) with duplex DNA, suggesting that 3 and 1 likely share a common basis of cytotoxicity. In addition, we isolated a novel, dark-red crystalline guanine adduct (7) from incubation of trioxacarcin A itself with the self-complimentary duplex oligonucleotide d(CGTATACG). Crystallographic analysis suggests that 7 is an anthraquinone derivative, which we propose arises by a sequence of guanosine alkylation within duplex DNA, depurination, base-catalyzed elimination of the trioxacarcinose A carbohydrate residue, and oxidative rearrangement to form an anthraquinone. We believe that this heretofore unrecognized chemical instability of natural trioxacarcins may explain why trioxacarcin analogs lacking C4 oxygenation exhibit superior chemical stabilities yet, as evidenced by structure 3, retain a capacity to form lesions with duplex DNA.

Diversity-oriented synthesis of Lycopodium alkaloids inspired by the hidden functional group pairing pattern

Natural products continue to provide a rich source of inspiration for both chemists and biologists. The efficient synthesis of bioactive natural products or natural product-like molecules has offered tremendous opportunities for complex biological processes exploration and drug discovery. However, because natural products usually contain numerous stereogenic centres and polycyclic ring systems, significant synthetic challenges remain. Here we employ the build/couple/pair strategy that is frequently used in diversity-oriented synthesis to obtain skeletally diverse compounds with complexities comparable to natural products. Inspired by the functional group pairing patterns hidden in Lycopodium alkaloids, we efficiently and in parallel construct four natural products, (+)-Serratezomine A, (-)-Serratinine, (+)-8α-Hydroxyfawcettimine and (-)-Lycoposerramine-U, as well as six different unnatural scaffolds, following the advanced build/couple/pair algorithm. This newly developed strategy is expected to be applied to the efficient synthesis of other complex natural products possessing functional group pairing patterns as well as skeletally diverse natural product-like molecules.

The endeavor of total synthesis and its impact on chemistry, biology and medicine

The synthesis of urea in 1828 set in motion the discipline of organic synthesis in general and of total synthesis in particular, the art and science of synthesizing natural products, the molecules of living nature. Early endeavors in total synthesis had as their main objective the proof of structure of the target molecule. Later on, the primary goal became the demonstration of the power of synthesis to construct complex molecules through appropriately devised strategies, making the endeavor an achievement whose value was measured by its elegance and efficiency. While these objectives continue to be important, contemporary endeavors in total synthesis are increasingly focused on practical aspects, including method development, efficiency, and biological and medical relevance. In this article, the emergence and evolution of total synthesis to its present state is traced, selected total syntheses from the author's laboratories are highlighted, and projections for the future of the field are discussed.