The synthesis of single enantiomers of mycobacterial ketomycolic acids containing cis-cyclopropanes

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Synthesis of trehalose glycolipids

Chemical synthesis of trehalose glycolipids such as DAT, TDM, SL-1, SL-3, and Ac₂SGL from MTb, emmyguyacins from fungi, succinoyl trehalose from rhodococcus, and maradolipids from worms, as well as mycobacterial oligosaccharides is reviewed.

Synthesis of a Di-Mycoloyl Tri-Arabinofuranosyl Glycerol Fragment of the Mycobacterial Cell Wall, Based on Synthetic Mycolic Acids

Fragments of mycobacterial cell walls such as arabinoglycerol mycolate and dimycoloyl diarabinoglycerol, comprising complex mixtures of mycolic acids, have immunostimulatory and antigenic properties. A related di-mycoloyl tri-arabinofuranosyl glycerol fragment has been isolated from cell wall hydrolysates. An effective stereoselective synthesis of tri-arabinofuranosyl glycerol, followed by coupling with stereochemically defined mycolic acids of different structural classes, to provide unique di-mycoloyl tri-arabinofuranosyl glycerols is now described.

The synthesis of mycobacterial dimycoloyl diarabinoglycerol based on defined synthetic mycolic acids

Complex mixtures of natural dimycoloyl diarabinoglycerols isolated from mycobacteria have been shown to be both potent immune signalling agents and potentially valuable antigens in the serodiagnosis of mycobacterial infections. We now report the highly stereocontrolled synthesis of diacyl l-glycerol-(1'→1)-β-d-arabinofuranosyl-α-d-arabinofuranosides based on simple fatty acids and single defined synthetic mycolic acids. NMR analysis confirmed that the synthetic core was identical to that in natural mixtures.

The antigenicity and cholesteroid nature of mycolic acids determined by recombinant chicken antibodies

Mycolic acids (MA) are major, species-specific lipid components of Mycobacteria and related genera. In Mycobacterium tuberculosis, it is made up of alpha-, methoxy- and keto-MA, each with specific biological functions and conformational characteristics. Antibodies in tuberculosis (TB) patient sera respond differently towards the three MA classes and were reported to cross-react with cholesterol. To understand the antigenicity and cholesterol cross-reactivity of MA, we generated three different chicken -derived phage-displayed single-chain variable fragments (scFv) that reacted similarly towards the natural mixture of MA, but the first recognized all three classes of chemically synthetic MAs, the second only the two oxygenated types of MAs and the third only methoxy MA. The cholesterol cross-reactivity was investigated after grafting each of the three scFv types onto two configurations of constant chain domains–CH1-4 and CH2-4. Weak but significant cross-reactivity with cholesterol was found only with CH2-4 versions, notably those two that were also able to recognize the trans-keto MA. The cholesteroid nature of mycobacterial mycolic acids therefore seems to be determined by the trans-keto MA subclass. The significantly weaker binding to cholesterol in comparison to MA confirms the potential TB diagnostic application of these antibodies.

Identification and Biological Activity of Synthetic Macrophage Inducible C-Type Lectin Ligands

The macrophage inducible C-type lectin (Mincle) is a pattern recognition receptor able to recognize both damage-associated and pathogen-associated molecular patterns, and in this respect, there has been much interest in determining the scope of ligands that bind Mincle and how structural modifications to these ligands influence ensuing immune responses. In this review, we will present Mincle ligands of known chemical structure, with a focus on ligands that have been synthetically prepared, such as trehalose glycolipids, glycerol-based ligands, and 6-acylated glucose and mannose derivatives. The ability of the different classes of ligands to influence the innate, and consequently, the adaptive, immune response will be described, and where appropriate, structure-activity relationships within each class of Mincle ligands will be presented.

Glycerol mycolates from synthetic mycolic acids

R- and S-Glycerol mycolates derived from single synthetic α-, keto- and methoxy-mycolic acids are described.

Mycobacterium tuberculosis-associated synthetic mycolates differentially exert immune stimulatory adjuvant activity

Mycolic acids (MAs) are highly hydrophobic long-chain α-alkyl β-hydroxy fatty acids present in the cell wall of Mycobacterium tuberculosis (Mtb) as a complex mixture of molecules with a common general structure but with variable functional groups in the meromycolate chain. In this study, we addressed the relationship between the MA molecular structure and their contribution to the development of T-cell immune responses. Hereto, we used the model antigen ovalbumin and single synthetic MAs, differing in oxygenation class and cis versus trans proximal cyclopropane configuration, as immune stimulatory agents. Subcutaneous delivery of liposome-formulated MAs with a proximal cis cyclopropane elicited antigen-specific Th1 and cytotoxic T-cell immune responses, whereas intratracheal immunization elicited pulmonary Th17 responses. These immune stimulatory activities depended not only on the cis versus trans proximal cyclopropane configuration but also on the MA oxygenation class. Our study thus shows that both the presence and nature of the functional groups in the meromycolate chain affect the immune stimulatory adjuvant activity of Mtb mycolates and suggests that Mtb bacilli may impact on the host protective immune response by modulating the cis versus trans stereochemistry of its mycolates as well as by altering the oxygenation class of the meromycolate functional group.

Synthesis of (-)-Cannabimovone and Structural Reassignment of Anhydrocannabimovone through Gold(I)-Catalyzed Cycloisomerization

The first total synthesis of cannabimovone from Cannabis sativa and anhydrocannabimovone was achieved by means of a highly stereoselective gold(I)-catalyzed cycloisomerization. The results led to reassignment of the structure of anhydrocannabimovone.

Towards understanding the functional diversity of cell wall mycolic acids of Mycobacterium tuberculosis

Mycolic acids constitute the waxy layer of the outer cell wall of Mycobacterium spp. and a few other genera. They are diverse in structure, providing a unique chromatographic foot-print for almost each of the more than 70 Mycobacterium species. Although mainly esterified to cell wall arabinogalactan, trehalose or glucose, some free mycolic acid is secreted during in vitro growth of Mycobacterium tuberculosis. In M. tuberculosis, α-, keto- and methoxy-mycolic acids are the main classes, each differing in their ability to attract neutrophils, induce foamy macrophages or adopt an antigenic structure for antibody recognition. Of interest is their particular relationship to cholesterol, discovered by their ability to attract cholesterol, to bind Amphotericin B or to be recognised by monoclonal antibodies that cross-react with cholesterol. The structural elements that determine this diverse functionality include the carboxylic acid in the mycolic motif, as well as the nature and stereochemistry of the two functional groups in the merochain. The functional diversity of mycolic acid classes implies that much information may be contained in the selective expression and secretion of mycolic acids to establish tuberculosis after infection of the host. Their cholesteroid nature may relate to how they utilize host cholesterol for their persistent survival.

Trehalose glycolipids--synthesis and biological activities

A variety of trehalose glycolipids have been isolated from natural sources, and several of these glycolipids exhibit important biological properties. These molecules also represent challenging synthetic targets due to their highly amphiphilic character, their large number of functional groups and additional chiral centres. This review highlights some of the recent advances made in the synthesis of trehalose glycolipids, and their associated biological activities.

Molecular structure of the Mycobacterium tuberculosis virulence factor, mycolic acid, determines the elicited inflammatory pattern

Mycolic acids (MAs) occur in the cell wall of Mycobacterium tuberculosis as variable mixtures of different classes and chain lengths. Here, we address the relationship between the structure and its inflammatory function of this virulence factor using single synthetic MA isomers, differing in oxygenation class and cis- versus α-methyl-trans proximal cyclopropane orientation. Analysis of bronchoalveolar inflammation, lung histopathology and alveolar macrophage transcription revealed a strong dependence on these meromycolic chemistries of mouse pulmonary inflammation in response to intratracheal treatments with MAs. Whereas α-MA was inert, oxygenated methoxy- and keto-MA with cis-cyclopropane stereochemistry elicited solid to mild inflammatory responses respectively. In trans-cyclopropane orientation, methoxy-MA partially lost its inflammatory activity and keto-MA exerted anti-inflammatory alternative activation of alveolar macrophages and counteracted cis-methoxy-MA induced airway inflammation. The differential innate immune activities of MAs demonstrated here, dependent on oxygenation class and cis versus α-methyl-trans cyclopropane chemistry, identify a novel means for M. tuberculosis to steer host immune responses during infection.

Structure-function relationships of the antigenicity of mycolic acids in tuberculosis patients

Cell wall mycolic acids (MA) from Mycobacterium tuberculosis (M.tb) are CD1b presented antigens that can be used to detect antibodies as surrogate markers of active TB, even in HIV coinfected patients. The use of the complex mixtures of natural MA is complicated by an apparent antibody cross-reactivity with cholesterol. Here firstly we report three recombinant monoclonal scFv antibody fragments in the chicken germ-line antibody repertoire, which demonstrate the possibilities for cross-reactivity: the first recognized both cholesterol and mycolic acids, the second mycolic acids but not cholesterol, and the third cholesterol but not mycolic acids. Secondly, MA structure is experimentally interrogated to try to understand the cross-reactivity. Unique synthetic mycolic acids representative of the three main functional classes show varying antigenicity against human TB patient sera, depending on the functional groups present and on their stereochemistry. Oxygenated (methoxy- and keto-) mycolic acid was found to be more antigenic than alpha-mycolic acids. Synthetic methoxy-mycolic acids were the most antigenic, one containing a trans-cyclopropane apparently being somewhat more antigenic than the natural mixture. Trans-cyclopropane-containing keto- and hydroxy-mycolic acids were also found to be the most antigenic among each of these classes. However, none of the individual synthetic mycolic acids significantly and reproducibly distinguished the pooled serum of TB positive patients from that of TB negative patients better than the natural mixture of MA. This argues against the potential to improve the specificity of serodiagnosis of TB with a defined single synthetic mycolic acid antigen from this set, although sensitivity may be facilitated by using a synthetic methoxy-mycolic acid.