Stereochemistry of the core structure of the mycolactones

Alternative boronic acids in the detection of Mycolactone A/B using the thin layer chromatography (f-TLC) method for diagnosis of Buruli ulcer

BACKGROUND

Mycobacterium ulcerans is the causative agent of Buruli ulcer. The pathology of M. ulcerans disease has been attributed to the secretion of a potent macrolide cytotoxin known as mycolactone which plays an important role in the virulence of the disease. Mycolactone is a biomarker for the diagnosis of BU that can be detected using the fluorescent-thin layer chromatography (f-TLC) technique. The technique relies on the chemical derivatization of mycolactone A/B with 2-naphthylboronic acid (BA) which acts as a fluorogenic chemosensor. However, background interferences due to co-extracted human tissue lipids, especially with clinical samples coupled with the subjectivity of the method call for an investigation to find an alternative to BA.

METHODS

Twenty-six commercially available arylboronic acids were initially screened as alternatives to BA using the f-TLC experiment. UV-vis measurements were also conducted to determine the absorption maximum spectra of mycolactone A/B and myco-boronic acid adducts followed by an investigation of the fluorescence-enhancing ability of the boronate ester formation between mycolactone A/B and our three most promising boronic acids (BA15, BA18, and BA21). LC-MS technique was employed to confirm the adduct formation between mycolactone and boronic acids. Furthermore, a comparative study was conducted between BA18 and BA using 6 Polymerase Chain Reaction (PCR) confirmed BU patient samples.

RESULTS

Three of the boronic acids (BA15, BA18, and BA21) produced fluorescent band intensities superior to BA. Complexation studies conducted on thin layer chromatography (TLC) using 0.1 M solution of the three boronic acids and various volumes of 10 ng/µL of synthetic mycolactone ranging from 1 µL - 9 µL corresponding to 10 ng - 90 ng gave similar results with myco-BA18 adduct emerging with the most visibly intense fluorescence bands. UV-vis absorption maxima (λ_max) for the free mycolactone A/B was observed at 362 nm, and the values for the adducts myco-BA15, myco-BA18, and myco-BA21 were at 272 nm, 270 nm, and 286 nm respectively. The comparable experimental λ_max of 362 nm for mycolactone A/B to the calculated Woodward-Fieser value of 367 nm for the fatty acid side chain of mycolactone A/B demonstrate that even though 2 cyclic boronates were formed, only the boronate of the southern side chain with the chromophore was excited by irradiation at 365 nm. Fluorescence experiments have demonstrated that coupling BA18 to mycolactone A/B along the 1,3-diols remarkably enhanced the fluorescence intensity at 537 nm. High-Resolution Mass Spectrometer (HR-MS) was used to confirm the formation of the myco-BA15 adduct. Finally, f-TLC analysis of patient samples with BA18 gave improved BA18-adduct intensities compared to the original BA-adduct.

CONCLUSION

Twenty-six commercially available boronic acids were investigated as alternatives to BA, used in the f-TLC analysis for the diagnosis of BU. Three (3) of them BA15, BA18, and BA21 gave superior fluorescence band intensity profiles. They gave profiles that were easier to interpret after the myco-boronic acid adduct formation and in experiments with clinical samples from patients with BA18 the best. BA18, therefore, has been identified as a potential alternative to BA and could provide a solution to the challenge of background interference of co-extracted human tissue lipids from clinical samples currently associated with the use of BA.

Detection of Mycolactone by Thin Layer Chromatography

By means of thin layer chromatography coupled to a fluorescence enhancer, a highly sensitive and operationally simple method to detect the mycolactones stemming from the human pathogen Mycobacterium ulcerans was developed and applied to various sample sources.

Rational Functionalization Towards Redox-Active TEMPO Stable Free-Radical-Hydrochar Composites

Although both stable free organic radicals and biomass-derived hydrochars have emerged as appealing, green, multifunctional materials, their association has not been explored. In this study, strength is found to lie in their union, which primarily leads to stable redox-active free-radical-hydrochar composites that can generate unexpected opportunities for the development of advanced metal-free sustainable materials. The composites are obtained by a straightforward green one-pot hydrothermal procedure. The loading of stable free radicals of nitroxide type and their localization is engineered by the nature of the carbohydrate and the reaction status; vigorous reaction parameters promote faster nucleation and growth kinetics of the hydrochar products, leading to a covalent immobilization of redox species on the surface of the carbonaceous microspherical aggregates. The nitroxide free-radical-hydrochar materials demonstrate enhancements in terms of both electrocatalytic activity and capacitive features.

New alkaloids and isocoumarins from the marine gorgonian-derived fungus Aspergillus sp. SCSIO 41501

Two new β-carboline alkaloids, aspergillspins A-B (1-2), three new quinolone alkaloids, aspergillspins C-E (3-5), and two new isocoumarins, aspergillspins F-G (6-7), together with four known alkaloids were isolated from the marine gorgonian-derived fungus Aspergillus sp. SCSIO 41501. Their structures were identified by spectroscopic analysis, and the absolute configurations of several chiral carbons in 2 and 3 were further established by quantum chemical calculations of the electronic circular dichroism (ECD) spectra. Their cytotoxic and antibacterial activities were also evaluated.

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.

Membrane perturbing properties of toxin mycolactone from Mycobacterium ulcerans

Mycolactone is the exotoxin produced by Mycobacterium ulcerans and is the virulence factor behind the neglected tropical disease Buruli ulcer. The toxin has a broad spectrum of biological effects within the host organism, stemming from its interaction with at least two molecular targets and the inhibition of protein uptake into the endoplasmic reticulum. Although it has been shown that the toxin can passively permeate into host cells, it is clearly lipophilic. Association with lipid carriers would have substantial implications for the toxin’s distribution within a host organism, delivery to cellular targets, diagnostic susceptibility, and mechanisms of pathogenicity. Yet the toxin’s interactions with, and distribution in, lipids are unknown. Herein we have used coarse-grained molecular dynamics simulations, guided by all-atom simulations, to study the interaction of mycolactone with pure and mixed lipid membranes. Using established techniques, we calculated the toxin’s preferential localization, membrane translocation, and impact on membrane physical and dynamical properties. The computed water-octanol partition coefficient indicates that mycolactone prefers to be in an organic phase rather than in an aqueous environment. Our results show that in a solvated membrane environment the exotoxin mainly localizes in the water-membrane interface, with a preference for the glycerol moiety of lipids, consistent with the reported studies that found it in lipid extracts of the cell. The calculated association constant to the model membrane is similar to the reported association constant for Wiskott-Aldrich syndrome protein. Mycolactone is shown to modify the physical properties of membranes, lowering the transition temperature, compressibility modulus, and critical line tension at which pores can be stabilized. It also shows a tendency to behave as a linactant, a molecule that localizes at the boundary between different fluid lipid domains in membranes and promotes inter-mixing of domains. This property has implications for the toxin’s cellular access, T-cell immunosuppression, and therapeutic potential.

Mycolactone is a macrolide exotoxin secreted by Mycobacterium ulcerans, which causes a skin disease called Buruli ulcer, a neglected emerging disease. It is the third most common mycobacterial disease after tuberculosis and leprosy. Studies have shown how mycolactone plays a pivotal role in Buruli ulcer pathogenesis, and identified it as an attractive therapeutic target. This multifunctional cytotoxin exerts multiple local and global responses, including ulcerative, analgesic, and anti-inflammatory effects. Prompted by its lipid-like structure, we used extensive multi-resolution simulations to probe mycolactone’s interactions with model membranes. Our results suggest that mycolactone is sequestered in membranes where it alters several dynamical, physical, and mechanical properties. It also behaves as a linactant, localizing at the interface between lipid domains and decreasing the inter-domain line tension. Our results shed light on how mycolactone permeates host cell membranes and is distributed between lipid and aqueous environments. These findings have significant implications for the toxin’s distribution in the host environment and mechanisms of pathogenicity. Understanding the toxin’s distribution and mechanism of trafficking will have ramifications for targeted diagnostics, therapeutic approaches, and our understanding of Buruli ulcer pathogenesis.

Identification and Proposed Relative and Absolute Configurations of Niphimycins C-E from the Marine-Derived Streptomyces sp. IMB7-145 by Genomic Analysis

Analysis of the whole genome sequence of Streptomyces sp. IMB7-145 revealed the presence of seven type I polyketide synthase biosynthetic gene clusters, one of which was highly homologous to the biosynthetic gene cluster of azalomycin F. Detailed bioinformatic analysis of the modular organization of the PKS gene suggested that this gene is responsible for niphimycin biosynthesis. Guided by genomic analysis, a large-scale cultivation ultimately led to the discovery and characterization of four new niphimycin congeners, namely, niphimycins C-E (1-3) and 17-O-methylniphimycin (4). The configurations of most stereocenters of niphimycins have not been determined to date. In the present study, the relative configurations were elucidated by spectroscopic analysis, including J-based analysis and the CNMR database method. Further, the full absolute configurations of niphimycins were completely proposed for the first time based on biosynthetic gene cluster analysis of the ketoreductase and enoylreductase domains for hydroxy- and methyl-bearing stereocenters. Compounds 1, 3, 4, and niphimycin Iα (5) showed antimicrobial activity against methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci (MIC: 8-64 μg/mL), as well as cytotoxicity against the human HeLa cancer cell line (IC₅₀: 3.0-9.0 μM). In addition, compounds 1 and 5 displayed significant activity against several Mycobacterium tuberculosis clinical isolates (MIC: 4-32 μg/mL).

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.

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.

Iso-petromyroxols: novel dihydroxylated tetrahydrofuran enantiomers from sea lamprey (Petromyzon marinus)

An enantiomeric pair of new fatty acid-derived hydroxylated tetrahydrofurans, here named iso-petromyroxols, were isolated from sea lamprey larvae-conditioned water. The relative configuration of iso-petromyroxol was elucidated with 1D and 2D NMR spectroscopic analyses. The ratio of enantiomers (er) in the natural sample was measured by chiral-HPLC-MS/MS to be ca. 3:1 of (–)- to (+)-antipodes.

Autumnalamide, a prenylated cyclic peptide from the cyanobacterium Phormidium autumnale, acts on SH-SY5Y cells at the mitochondrial level

Filamentous cyanobacteria of the genus Phormidium have been rarely studied for their chemical diversity. For the first time, the cultivable Phormidium autumnale was shown to produce a prenylated cyclic peptide named autumnalamide (1). The structure of this peptide was fully determined after a deep exploration of the spectroscopic data, including NMR and HRMS. Interestingly, a prenyl moiety was located on the guanidine end of the arginine amino acid. The absolute configurations of most amino acids were assessed using enantioselective GC/MS analysis, with (13)C NMR modeling being used for the determination of d-arginine and d-proline. The effects of 1 on sodium and calcium fluxes were studied in SH-SY5Y and hNav 1.6 HEK cells. When the Ca(2+) influx was stimulated by thapsigargin, strong inhibition was observed in the presence of 1. As a consequence, this compound may act by disrupting the normal calcium uptake of this organelle, inducing the opening of the mitochondrial permeability transition pore, which results in the indirect blockade of store-operated channels.

Synthetic variants of mycolactone bind and activate Wiskott-Aldrich syndrome proteins

Mycolactone is a complex macrolide toxin produced by Mycobacterium ulcerans, the causative agent of skin lesions called Buruli ulcers. Mycolactone-mediated activation of neural (N) Wiskott-Aldrich syndrome proteins (WASP) induces defects in cell adhesion underpinning cytotoxicity and disease pathogenesis. We describe the chemical synthesis of 23 novel mycolactone analogues that differ in structure and modular assembly of the lactone core with its northern and southern polyketide side chains. The lactone core linked to southern chain was the minimal structure binding N-WASP and hematopoietic homolog WASP, where the number and configuration of hydroxyl groups on the acyl side chain impacted the degree of binding. A fluorescent derivative of this compound showed time-dependent accumulation in target cells. Furthermore, a simplified version of mycolactone mimicked the natural toxin for activation of WASP in vitro and induced comparable alterations of epithelial cell adhesion. Therefore, it constitutes a structural and functional surrogate of mycolactone for WASP/N-WASP-dependent effects.

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.

Langkolide, a 32-membered macrolactone antibiotic produced by Streptomyces sp. Acta 3062

A new 32-membered macrolactone antibiotic, named langkolide, was isolated from the mycelium of Streptomyces sp. Acta 3062. The langkolide structure was determined by HR-MS and 1D and 2D NMR as a 32-membered macrolactone connected from an overhanging polyketide tail to a naphthoquinone unit mediated by two carbohydrate moieties. The producing strain was isolated from a rhizosphere soil of Clitorea sp. collected at Burau Bay, Langkawi, Malaysia, and was characterized by its morphological and chemotaxonomic features in addition to its 16S rRNA gene sequence. It was identified as a member of the Streptomyces galbus clade. Langkolide exhibited various bioactivities including antimicrobial and antiproliferative activities. Furthermore, langkolide inhibited human recombinant phosphodiesterase 4 with an IC(50) value of 0.48 μM.

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.

Synthesis and structure of mycolactone E isolated from frog mycobacterium

The structure of mycolactone E, isolated from the frog pathogen Mycobacterium liflandii, was established via organic synthesis. Within the mycolactone family of metabolites, a structural variation has been seen only at the unsaturated fatty acid moiety thus far, and mycolactone E follows this observation. Interestingly, the absolute configuration of its unsaturated fatty acid matches that of the mycolactones from human mycobacteria, rather than the structurally more closely related mycolactone F from fish mycobacteria.

Synthesis and structure assignment of the minor metabolite arising from the frog pathogen Mycobacterium liflandii

Total synthesis and structure assignment of the minor metabolite present in lipid extracts of the frog pathogen Mycobacterium liflandii are reported.

Scalable and efficient synthesis of the mycolactone core

A highly efficient, scalable, and stereoselective synthesis of the mycolactone core is reported. The synthesis consists of 14 longest linear steps, with 19% overall yield.

Highly sensitive, operationally simple, cost/time effective detection of the mycolactones from the human pathogen Mycobacterium ulcerans

A boronate-assisted fluorogenic chemosensor in a solid phase is developed, selectively to detect the mycolactones produced by the human pathogen Mycobacterium ulcerans.

Highly regio- and stereoselective synthesis of (Z)-trisubstituted alkenes via propyne bromoboration and tandem Pd-catalyzed cross-coupling

Contrary to all previous reports, bromoboration of propyne with BBr(3) proceeds in >or=98% syn-selectivity to produce (Z)-2-bromo-1-propenyldibromoborane (1). Although 1 is readily prone to stereoisomerization, it can be converted to the pinacolboronate (2) of >or=98% isomeric purity by treatment with pinacol, which may then be subjected to Negishi coupling to give trisubstituted (Z)-alkenylpinacolboronates (3) containing various R groups in 73-90% yields. Iodinolysis of 3 affords alkenyl iodides (4) in 80-90% yields. All alkenes isolated and identified are >or=98% Z.

Mycolactones: immunosuppressive and cytotoxic polyketides produced by aquatic mycobacteria

Mycolactones are a family of highly related macrocyclic polyketides that exhibit immunosuppressive and cytotoxic properties. First discovered in 1999, they are the primary virulence factors produced by the environmental human pathogen Mycobacterium ulcerans, the causative agent of Buruli ulcer, and by some closely-related aquatic mycobacteria that cause disease in fish and frogs. Mycolactones are characterized by a common 12-membered lactone core to which is appended an unsaturated fatty acyl side-chain of variable length and oxidation state. This Highlight summarizes recent progress in understanding the structural diversity of the mycolactones, their biological activity and mode of action in mammalian cells, and the genetics, evolution, and enzymology of their biosynthesis.

A synthetic entry to pladienolide B and FD-895

Presented within are syntheses of the pladienolide B and FD-895 side-chains, as well as models of the essential ring-closing metathesis and Stille coupling that will be used to complete their total syntheses. Several analogs of the pladienolide B side-chain were also prepared in order to evaluate the scope of the methodology and to create a library of structures that could be used for stereochemical and SAR analyses.

Total Synthesis of Mycolactones A and B

First and second generation total syntheses of mycolactones A and B are reported. The first generation total synthesis unambiguously confirmed our earlier assignment of the relative and absolute stereochemistry of mycolactones A and B. Knowledge of the chemical properties of the mycolactones accumulated through the first generation total synthesis allowed us to implement several major improvements to the original synthesis, including: (1) optimizing the choice of protecting groups, (2) eliminating the unnecessary adjustment of protecting groups, and (3) improving the overall stereoselectivity and synthetic efficiency. The second generation total synthesis consists of 21 longest linear steps, with 8.8% overall yield.

Attempts to Assemble a Universal NMR Database without Synthesis of NMR Database Compounds

[structure: see text] The feasibility of assembling 3JH,H profiles from NMR data collected on relevant, but not necessarily specific, NMR database compounds representing a given stereocluster was demonstrated. By this approach, the 3JH,H profile was created for the contiguous tetraol peracetate stereocluster. The reliability and applicability of the database thus assembled were demonstrated for known peracetates derived from two heptoses.

Structure determination of mycolactone C via total synthesis

[structure: see text] By total synthesis, mycolactone C has been established as an approximately 1:1 mixture of Z-Delta4'5'- and E-Delta4'5'-geometric isomers of C12'-deoxymycolactones A and B.