A multiply convergent platform for the synthesis of trioxacarcins

Different DNA Binding and Damage Mode between Anticancer Antibiotics Trioxacarcin A and LL-D49194α1

Trioxacarcin A (TXN) is a highly potent cytotoxic antibiotic with remarkable structural complexity. The crystal structure of TXN bound to double-stranded DNA (dsDNA) suggested that the TXN interaction might depend on positions of two sugar subunits on the minor and major grooves of dsDNA. LL-D49194α1 (LLD) is a TXN analogue bearing the same polycyclic polyketide scaffold with a distinct glycosylation pattern. Although LLD was in a phase I clinical trial, how LLD binds to dsDNA remains unclear. Here, we solved the solution structures at high resolutions of palindromic 2″-fluorine-labeled guanine-containing duplex d(A1A2C3C4GFGFT7T8)2 and of its stable LLD and TXN covalently bound complexes. Combined with biochemical assays, we found that TXN-alkylated dsDNA would tend to keep DNA helix conformation, while LLD-alkylated dsDNA lost its stability more than TXN-alkylated dsDNA, leading to dsDNA denaturation. Thus, despite lower cytotoxicity in vitro, the differences of sugar substitutions in LLD caused greater DNA damage than TXN, thereby bringing about a completely new biological effect.

Total synthesis of (-)-securingine G

In this study, we present the first total synthesis of (-)-securingine G. Diverging from the proposed biosynthetic pathway, our approach involves the addition of nucleophilic pyridine anion species to the electrophilic menisdaurilide congener. Crucially, incorporating a weakly basic yet nucleophilic tri(2-pyridinyl)lanthanum complex proved essential to circumvent undesired base-mediated pathways during the key coupling reaction. Notably, we introduce n-Bu3La·5LiCl as a new exchange reagent, facilitating efficient halide/lanthanum exchange of (hetero)aryl halides.

Total Syntheses of (±)-Melicolones A and B

The first total syntheses of (±)-melicolones A and B, which have a unique and densely functionalized framework derived from a rearranged prenylated acetophenone, were accomplished in 12.3% combined overall yield. The concise and divergent synthesis of these two natural products, which were isolated in racemic form, was achieved in a longest linear sequence requiring only 9 steps (11 total steps) and 8 isolated intermediates using commercially available starting materials. This approach, which might enable access to all tetracyclic melicolones, features the highly regioselective (16:1) and diastereoselective (15:1) dipolar cycloaddition of a carbonyl ylide generated by the unusual cyclization of a rhodium carbene with the carbonyl oxygen atom of an aliphatic aldehyde. This cycloaddition proceeds with dominant steric control to give a highly functionalized oxabicycloheptane core. Stereoselective enolate alkylation led to a prenylated intermediate that underwent an intramolecular aldol reaction to give the penultimate tricyclic intermediate. Tandem epoxidation of the pendant prenyl group followed by a regioselective, acid-catalyzed cyclization delivered (±)-melicolones A and B.

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.

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.

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.

Studies on Antibiotics Active against Resistant Bacteria. Total Synthesis of MRSA-Active Tetarimycin A and Its Analogues

Making use of the Hauser-Kraus annulation as a key step, the first total synthesis of tetarimycin A has been accomplished in a highly convergent and operationally simple manner. Preliminary SAR not only validated that tetarimycin A exhibited potent activity against MRSA and VRE at a low MIC value but also identified that the hydroxyl group at C-10 was essential for antibacterial activities.

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.

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

The trioxacarcins are polyoxygenated, structurally complex natural products that potently inhibit the growth of cultured human cancer cells. Here we describe syntheses of trioxacarcin A, DC-45-A1 and structural analogues by late-stage stereoselective glycosylation reactions of fully functionalized, differentially protected aglycon substrates. Key issues addressed in this work include the identification of an appropriate means to activate and protect each of the two 2-deoxysugar components, trioxacarcinose A and trioxacarcinose B, as well as a viable sequencing of the glycosidic couplings. The convergent, component-based sequence we present allows for rapid construction of structurally diverse, synthetic analogues that would be inaccessible by any other means, in amounts required to support biological evaluation. Analogues that arise from the modification of four of five modular components are assembled in 11 steps or fewer. The majority of these are found to be active in antiproliferative assays using cultured human cancer cells.