Scalable and efficient synthesis of the mycolactone core

Exploring Mycolactone-The Unique Causative Toxin of Buruli Ulcer: Biosynthetic, Synthetic Pathways, Biomarker for Diagnosis, and Therapeutic Potential

Mycolactone is a complex macrolide toxin produced by Mycobacterium ulcerans, the causative agent of Buruli ulcer. The aim of this paper is to review the chemistry, biosynthetic, and synthetic pathways of mycolactone A/B to help develop an understanding of the mode of action of these polyketides as well as their therapeutic potential. The synthetic work has largely been driven by the desire to afford researchers enough (≥100 mg) of the pure toxins for systematic biological studies toward understanding their very high biological activities. The review focuses on pioneering studies of Kishi which elaborate first-, second-, and third-generation approaches to the synthesis of mycolactones A/B. The three generations focused on the construction of the key intermediates required for the mycolactone synthesis. Synthesis of the first generation involves assignment of the relative and absolute stereochemistry of the mycolactones A and B. This was accomplished by employing a linear series of 17 chemical steps (1.3% overall yield) using the mycolactone core. The second generation significantly improved the first generation in three ways: (1) by optimizing the selection of protecting groups; (2) by removing needless protecting group adjustments; and (3) by enhancing the stereoselectivity and overall synthetic efficiency. Though the synthetic route to the mycolactone core was longer than the first generation, the overall yield was significantly higher (8.8%). The third-generation total synthesis was specifically aimed at an efficient, scalable, stereoselective, and shorter synthesis of mycolactone. The synthesis of the mycolactone core was achieved in 14 linear chemical steps with 19% overall yield. Furthermore, a modular synthetic approach where diverse analogues of mycolactone A/B were synthesized via a cascade of catalytic and/or asymmetric reactions as well as several Pd-catalyzed key steps coupled with hydroboration reactions were reviewed. In addition, the review discusses how mycolactone is employed in the diagnosis of Buruli ulcer with emphasis on detection methods of mass spectrometry, immunological assays, RNA aptamer techniques, and fluorescent-thin layer chromatography (f-TLC) methods as diagnostic tools. We examined studies of the structure-activity relationship (SAR) of various analogues of mycolactone. The paper highlights the multiple biological consequences associated with mycolactone such as skin ulceration, host immunomodulation, and analgesia. These effects are attributed to various proposed mechanisms of actions including Wiskott-Aldrich Syndrome protein (WASP)/neural Wiskott-Aldrich Syndrome protein (N-WASP) inhibition, Sec61 translocon inhibition, angiotensin II type 2 receptor (AT2R) inhibition, and inhibition of mTOR. The possible application of novel mycolactone analogues produced based on SAR investigations as therapeutic agents for the treatment of inflammatory disorders and inflammatory pain are discussed. Additionally, their therapeutic potential as anti-viral and anti-cancer agents have also been addressed.

Collective and Diverted Total Synthesis of the Strasseriolides: A Family of Macrolides Endowed with Potent Antiplasmodial and Antitrypanosomal Activity

The strasseriolide macrolides show promising in vitro and in vivo activities against P. falciparum and T. cruzi, the parasites causing malaria and Chagas disease, respectively. However, the as yet poor understanding of structure/activity relationships and the fact that one family member proved systemically toxic for unknown reasons render a more detailed assessment of these potential lead compounds difficult. To help overcome these issues, a collective total synthesis was devised. The key steps consisted of a ring closing alkyne metathesis (RCAM) reaction to forge a common macrocyclic intermediate followed by a hydroxy-directed ruthenium catalyzed trans-hydrostannation of the propargyl alcohol site thus formed. The resulting alkenyltin derivative served as the central node of the synthesis blueprint, which could be elaborated into the natural products themselves as well as into a set of non-natural analogues according to the concept of diverted total synthesis. The recorded biological data confirmed the potency of the compounds and showed the lack of any noticeable cytotoxicity. The "northern" allylic alcohol subunit was recognized as an integral part of the pharmacophore, yet it provides opportunities for chemical modification.

Modular total syntheses of mycolactone A/B and its [2H]-isotopologue

A modular total synthesis of mycolactone A/B, the exotoxin produced by Mycobacterium ulcerans, has been achieved through the orchestration of several Pd-catalyzed key steps. While this route leads to a mixture of the natural product and its C12 epimer (4 : 1 ratio), this was inconsequential from the biological activity standpoint. Compared to the previously reported routes, this synthetic blueprint allows the late-stage modification of the toxin, as exemplified by the preparation of [22,22,22-²H₃]-mycolactone A/B.

The chemistry and biology of mycolactones

Mycolactones are a group of macrolides excreted by the human pathogen Mycobacterium ulcerans, which exhibit cytotoxic, immunosuppressive and analgesic properties. As the virulence factor of M. ulcerans, mycolactones are central to the pathogenesis of the neglected disease Buruli ulcer, a chronic and debilitating medical condition characterized by necrotic skin ulcers. Due to their complex structure and fascinating biology, mycolactones have inspired various total synthesis endeavors and structure-activity relationship studies. Although this review intends to cover all synthesis efforts in the field, special emphasis is given to the comparison of conceptually different approaches and to the discussion of more recent contributions. Furthermore, a detailed discussion of molecular targets and structure-activity relationships is provided.

Design, synthesis, and cytotoxicity of stabilized mycolactone analogs

On exposure to visible light, mycolactone A/B, the causative toxin of Buruli ulcer, rearranges to a mixture of four photo-mycolactones apparently via a rare photochemically-induced [₄π_s+₂π_a] cycloaddition. In order to prevent the rearrangement, two C6'-C7' dihydromycolactone analogs 6'α-15 and 6'β-15 were designed and synthesized. 6'α-15 and 6'β-15 were shown to be stable under not only photochemical, but also acidic and basic conditions. Cytotoxicity was tested against arbitrarily chosen four cell lines (human Hek-293, human lung carcinoma A-549, human melanoma LOX-IMVI, and mouse L-929), thereby revealing that: (1) both analogs maintain potent cytotoxicity; (2) 6'β-15 exhibits significantly higher potency against human cell lines than 6'α-15; (3) in comparison with parent mycolactone A/B, 6'β-15 exhibits equal potency against human Hek-293, whereas significantly lower potency against human lung carcinoma A-549 and human melanoma LOX-IMVI.

Synthetic strategies towards mycolactone A/B, an exotoxin secreted by Mycobacterium ulcerans

Pitfalls and dead-ends pave the way to mycolactone A/B. This full account reports synthetic efforts towards this natural product that eventually culminated in a de novo total synthesis.

Asymmetric synthesis of crambescin A–C carboxylic acids and their inhibitory activity on voltage-gated sodium channels

Both enantiomers of crambescin A–C carboxylic acids were synthesized via Katsuki asymmetric epoxidation and bromocation-triggered cascade cyclization.

Short Convergent Synthesis of the Mycolactone Core Through Lithiation-Borylation Homologations

Using iterative lithiation-borylation homologations, the mycolactone toxin core has been synthesized in 13 steps and 17% overall yield. The rapid build-up of molecular complexity, high convergence and high stereoselectivity are noteworthy features of this synthesis.

Total synthesis of herbimycin A

Benzoquinone ansamycin antibiotic herbimycin A was synthesized in 19 linear steps and 4.2% yield. Highlighted is the design of a chiral γ-lactone as the C11-C15 synthon that enabled a facile catalytic asymmetric synthesis of the challenging C8-C20 fragment of the target molecule. The easy access to the stereogenic centers and high overall yield made the strategy applicable in the molecular editing of benzoquinone ansamycins.

History, biology and chemistry of Mycobacterium ulcerans infections (Buruli ulcer disease)

Mycobacterium ulcerans infections (Buruli ulcer disease) have a long history that can be traced back 150 years. The successive discoveries of the mycobacteria in 1948 and of mycolactone A/B in 1999, the toxin responsible for this dramatic necrotic skin disease, resulted in a paradigm shift concerning the disease itself and in a broader sense, delineated an entirely new role for bioactive polyketides as virulence factors. The fascinating history, biology and chemistry of M. ulcerans infections are discussed in this review.

Structure-activity relationship studies on the macrolide exotoxin mycolactone of Mycobacterium ulcerans

Background

Mycolactones are a family of polyketide-derived macrolide exotoxins produced by Mycobacterium ulcerans, the causative agent of the chronic necrotizing skin disease Buruli ulcer. The toxin is synthesized by polyketide synthases encoded by the virulence plasmid pMUM. The apoptotic, necrotic and immunosuppressive properties of mycolactones play a central role in the pathogenesis of M. ulcerans.

Methodology/Principal Findings

We have synthesized and tested a series of mycolactone derivatives to conduct structure-activity relationship studies. Flow cytometry, fluorescence microscopy and Alamar Blue-based metabolic assays were used to assess activities of mycolactones on the murine L929 fibroblast cell line. Modifications of the C-linked upper side chain (comprising C12–C20) caused less pronounced changes in cytotoxicity than modifications in the lower C5-O-linked polyunsaturated acyl side chain. A derivative with a truncated lower side chain was unique in having strong inhibitory effects on fibroblast metabolism and cell proliferation at non-cytotoxic concentrations. We also tested whether mycolactones have antimicrobial activity and found no activity against representatives of Gram-positive (Streptococcus pneumoniae) or Gram-negative bacteria (Neisseria meningitis and Escherichia coli), the fungus Saccharomyces cerevisae or the amoeba Dictyostelium discoideum.

Conclusion

Highly defined synthetic compounds allowed to unambiguously compare biological activities of mycolactones expressed by different M. ulcerans lineages and may help identifying target structures and triggering pathways.

Buruli ulcer is a chronic necrotizing skin disease caused by Mycobacterium ulcerans. The characteristic histopathological features of Buruli ulcer, severe destruction of subcutaneous tissue with minimal inflammation in the core of the lesion, are primarily attributed to the cytotoxic activity of mycolactone, the macrolide exotoxin of M. ulcerans. Different geographical lineages of M. ulcerans produce different structural variants of mycolactone. By using highly defined synthetic mycolactones, including both naturally occurring molecular species and additional non-natural variants, we have assessed the influence of the structure of the C-linked upper side chain and the lower C5-O-linked polyunsaturated acyl side chain on biological activity. Changes in the lower side chain affected the cytotoxic activity against mammalian cells more profoundly than changes in the upper side chain. Mycolactone A/B had no antimicrobial activity against Gram-positive and Gram-negative bacteria and was also inactive against Saccharomyces and Dictyostelium.

A diverted total synthesis of mycolactone analogues: an insight into Buruli ulcer toxins

Mycolactones are complex macrolides responsible for a severe necrotizing skin disease called Buruli ulcer. Deciphering their functional interactions is of fundamental importance for the understanding, and ultimately, the control of this devastating mycobacterial infection. We report herein a diverted total synthesis approach of mycolactones analogues and provide the first insights into their structure-activity relationship based on cytopathic assays on L929 fibroblasts. The lowest concentration inducing a cytopathic effect was determined for selected analogues, allowing a clear picture to emerge by comparison with the natural toxins.

A ring-closing metathesis (RCM)-based approach to mycolactones A/B

The total synthesis of the mycobacterial toxins mycolactones A/B (1 a/b) has been accomplished based on a strategy built around the construction of the mycolactone core through ring-closing metathesis. By employing the Grubbs second-generation catalyst, the 12-membered core macrocycle of mycolactones, with a functionalized C2 handle attached to C11, was obtained in 60-80 % yield. The C-linked upper side chain (comprising C12-C20) was completed by a highly efficient modified Suzuki coupling between C13 and C14, while the attachment of the C5-O-linked polyunsaturated acyl side chain was achieved by Yamaguchi esterification. Surprisingly, a diene containing a simple isopropyl group attached to C11 could not be induced to undergo ring-closing metathesis. By employing fluorescence microscopy and flow cytometry techniques, the synthetic mycolactones A/B (1 a/b) were demonstrated to display similar apoptosis-inducing and cytopathic effects as mycolactones A/B extracted from Mycobacterium ulcerans. In contrast, a simplified analogue with truncated upper and lower side chains was found to be inactive.

Highly stereoselective total synthesis of fully hydroxy-protected mycolactones A and B and their stereoisomerization upon deprotection

Unprecedentedly efficient and highly (≥98 %) stereoselective syntheses of mycolactones A and B side chains relied heavily on Pd-catalyzed alkenylation (Negishi version) and were completed in 11 longest linear steps from ethyl (S)-3-hydroxybutyrate in 12% and 11% overall yield, respectively, roughly corresponding to an average of 82% yield per step. The synthesis of mycolactone core was realized by using Pd-catalyzed alkenyl-allyl coupling and an epoxide-opening reaction with a trialkylalkenylaluminate as key steps. Fully hydroxy-protected mycolactones A and B of ≥98% isomeric purity were synthesized successfully for the first time. However, unexpected 4:3-5:4 inseparable mixtures of mycolactones A and B were obtained upon deprotection.

Chemistry of mycolactones, the causative toxins of Buruli ulcer

Buruli ulcer is a severe and devastating skin disease caused by Mycobacterium ulcerans infection, yet it is one of the most neglected diseases. The causative toxin, referred to as mycolactone A/B, was isolated and characterized as a polyketide-derived macrolide in 1999. The current status of the mycolactone chemistry is described, highlighting the stereochemistry assignment of mycolactone A/B; total synthesis; the structure determination of mycolactone congeners from the human pathogen M. ulcerans, the frog pathogen Mycobacterium liflandii, and the fish pathogen Mycobacterium marinum; the structural diversity in the mycolactone class of natural products; the highly sensitive detection/structure-analysis of mycolactones; and some biological activity.