Sulfoglycolipids and Related Analogues of Mycobacterium tuberculosis: Chemical Synthesis and Immunological Studies.

Mycobacterium tuberculosis (Mtb) causes tuberculosis as one major threat to human health, which has been deteriorated owing to the emerging multidrug resistance. Mtb contains a complex lipophilic cell wall structure that is important for bacterial persistence. Among the lipid components, sulfoglycolipids (SGLs), known to induce immune cell responses, are composed of a trehalose core attached with a conserved sulfate group and 1-4 fatty acyl chains in an asymmetric pattern. At least one of these acyl chains is polymethylated with 3-12 methyl branches. Although Mtb SGL can be isolated from bacterial culture, resulting SGL is still a homologous mixture, impeding accurate research studies. This up-to-date review covers the chemical synthesis and immunological studies of Mtb SGLs and structural analogues, with an emphasis on the development of new glycosylation methods and the asymmetric synthesis of polymethylated scaffolds. Both are critical to advance further research on biological functions of these complicated SGLs.

Hydroxamate-directed access to ß-Kdo glycosides.

The reaction repertoire for forming transient aziridinone or azaoxyallyl cations from α-halohydroxamate is conceptually extended to design Kdo-glycosyl donors by installing the hydroxamate moiety at an anomeric centre, which is shown to be highly effective for stereoselective access to ß-Kdo glycosides. The pivotal roles of hydroxamate over amide are revealed in control experiments.

Cation-Promoted Strain-Release-Driven Access to Functionalized Azetidines from Azabicyclo[1.1.0]butanes.

Access to 1,3-functionalized azetidines through a diversity-oriented approach is highly sought-after for finding new applications in drug-discovery. To this goal, strain-release-driven functionalization of azabicyclo[1.1.0]-butane (ABB) has generated significant interest. Through appropriate N-activation, C3-substituted ABBs are shown to render tandem N/C3-fucntionalization/rearrangement, furnishing azetidines; although, modalities of such N-activation vis-à-vis N-functionalization remain limited to selected electrophiles. This work showcases a versatile cation-driven activation strategy of ABBs. And capitalizes on the use of Csp3 precursors amenable to forming reactive (aza)oxyallyl cations in situ. Herein, N-activation leads to formation of a congested C-N bond, and effective C3 activation. The concept was extended to formal [3+2] annulations involving (aza)oxyallyl cations and ABBs, leading to bridged bicyclic azetidines. Besides the fundamental appeal of this new activation paradigm, operational simplicity and remarkable diversity should engender its prompt use in synthetic and medicinal chemistry.

Tunable Strategy for the Asymmetric Synthesis of Sulfoglycolipids from Mycobacterium tuberculosis To Elucidate the Structure and Immunomodulatory Property Relationships.

We developed a versatile asymmetric strategy to synthesize different classes of sulfoglycolipids (SGLs) from Mycobacterium tuberculosis. The strategy features the use of asymmetrically protected trehaloses, which were acquired from the glycosylation of TMS α-glucosyl acceptors with benzylidene-protected thioglucosyl donors. The positions of the protecting groups at the donors and acceptors can be fine-tuned to obtain different protecting-group patterns, which is crucial for regioselective acylation and sulfation. In addition, a chemoenzymatic strategy was established to prepare the polymethylated fatty acid building blocks. The strategy employs inexpensive lipase as a desymmetrization agent in the preparation of the starting substrate and readily available chiral oxazolidinone as a chirality-controlling agent in the construction of the polymethylated fatty acids. A subsequent investigation on the immunomodulatory properties of each class of SGLs showed how the structures of SGLs impact the host innate immunity response.

Metabolic Isolation, Stereochemical Determination, and Total Synthesis of Predominant Native Cholesteryl Phosphatidyl-α-glucoside from Carcinogenic Helicobacter pylori.

We report the isolation and stereochemical determination of the predominant native cholesteryl 6-O-phosphatidyl α-glucoside (CPG) from Helicobacter pylori via an integrated biological and chemical strategy. The strategy employed (i) the metabolic isolation of a CPG analogue and (ii) the enzymatic degradation of the analogue to obtain the native lactobacillic acid for the stereochemical determination. The absolute stereochemistry of the acid was found to be 11R and 12S. Using the new stereochemical data, we accomplished the total synthesis of predominant native CPG and other predominant αCG derivatives.

Enhanced enzymatic production of cholesteryl 6'-acylglucoside impairs lysosomal degradation for the intracellular survival of Helicobacter pylori.

BACKGROUND: During autophagy defense against invading microbes, certain lipid types are indispensable for generating specialized membrane-bound organelles. The lipid composition of autophagosomes remains obscure, as does the issue of how specific lipids and lipid-associated enzymes participate in autophagosome formation and maturation. Helicobacter pylori is auxotrophic for cholesterol and converts cholesterol to cholesteryl glucoside derivatives, including cholesteryl 6'-O-acyl-α-D-glucoside (CAG). We investigated how CAG and its biosynthetic acyltransferase assist H. pylori to escape host-cell autophagy. METHODS: We applied a metabolite-tagging method to obtain fluorophore-containing cholesteryl glucosides that were utilized to understand their intracellular locations. H. pylori 26695 and a cholesteryl glucosyltransferase (CGT)-deletion mutant (ΔCGT) were used as the standard strain and the negative control that contains no cholesterol-derived metabolites, respectively. Bacterial internalization and several autophagy-related assays were conducted to unravel the possible mechanism that H. pylori develops to hijack the host-cell autophagy response. Subcellular fractions of H. pylori-infected AGS cells were obtained and measured for the acyltransferase activity. RESULTS: The imaging studies of fluorophore-labeled cholesteryl glucosides pinpointed their intracellular localization in AGS cells. The result indicated that CAG enhances the internalization of H. pylori in AGS cells. Particularly, CAG, instead of CG and CPG, is able to augment the autophagy response induced by H. pylori. How CAG participates in the autophagy process is multifaceted. CAG was found to intervene in the degradation of autophagosomes and reduce lysosomal biogenesis, supporting the idea that intracellular H. pylori is harbored by autophago-lysosomes in favor of the bacterial survival. Furthermore, we performed the enzyme activity assay of subcellular fractions of H. pylori-infected AGS cells. The analysis showed that the acyltransferase is mainly distributed in autophago-lysosomal compartments. CONCLUSIONS: Our results support the idea that the acyltransferase is mainly distributed in the subcellular compartment consisting of autophagosomes, late endosomes, and lysosomes, in which the acidic environment is beneficial for the maximal acyltransferase activity. The resulting elevated level of CAG can facilitate bacterial internalization, interfere with the autophagy flux, and causes reduced lysosomal biogenesis.

Total synthesis of landomycins Q and R and related core structures for exploration of the cytotoxicity and antibacterial properties.

Herein, we report the total synthesis of landomycins Q and R as well as the aglycone core, namely anhydrolandomycinone and a related core analogue. The synthesis features an acetate-assisted arylation method for construction of the hindered B-ring in the core component and a one-pot aromatization-deiodination-denbenzylation procedure to streamline the global functional and protecting group manuipulation. Subsequent cytotoxicity and antibacterial studies revealed that the landomycin R is a potential antibacterial agent against methicillin-resistant Staphylococcus aureus.

Diverse Synthesis of Natural Trehalosamines and Synthetic 1,1'-Disaccharide Aminoglycosides.

A general strategy for the diverse synthesis of ten disaccharide aminoglycosides, including natural 2-trehalosamine (1), 3-trehalosamine (2), 4-trehalosamine (3), and neotrehalosyl 3,3'-diamine (8) and synthetic aminoglycosides 4-7, 9, and 10, has been developed. The aminoglycoside compounds feature different anomeric configurations and numbers of amino groups. The key step for the synthesis was the glycosylation coupling of a stereodirecting donor with a configuration-stable TMS glycoside acceptor. Either the donor or acceptor could be substituted with an azido group. The aminoglycosides prepared in the present study were characterized by 1D and 2D NMR spectroscopy.

Carbasugar Synthesis via Vinylogous Ketal: Total Syntheses of (+)-MK7607, (-)-MK7607, (-)-Gabosine A, (-)-Epoxydine B, (-)-Epoxydine C, epi-(+)-Gabosine E and epi-(+)-MK7607.

Carbasugars, the carbocyclic analogues of sugars, constitute an important class of natural products with more than 140 members known and have attracted much attention due to their diverse biological activities like anticancer, antibacterial, herbicidal, and various enzyme inhibitory activities. As many carbohydrates are involved in various cellular signaling pathways, there is great interest in synthesis and biological exploration of carbasugars. Herein, we have developed a methodology to install an α,ß-unsaturated aldehyde functionality on different inositols and derivatives by vinylogous elimination of the O-protecting group under mildly acidic condition. We have illustrated the versatility and utility of our methodology by the total syntheses of seven carbasugars viz. (-)-MK7607, (-)-gabosine A, (-)-epoxydine B, (-)-epoxydine C, (+)-MK7607, 1-epi-(+)-MK7607 and 1-epi-(+)-gabosine E.

Total syntheses and structural validation of lincitol A, lincitol B, uvacalol I, uvacalol J, and uvacalol K.

Natural carbasugars are an important class of biologically active compounds. Due to their conformational freedom and the subtle difference in spectral characteristics between isomers, often their NMR-based structural assignments are erroneous. It is thus important to validate their structural identity through chemical synthesis. We report the first total syntheses and structural validation of five natural carbasugars, namely, lincitol A, lincitol B, uvacalol I, uvacalol J, and uvacalol K in their racemic forms, from a myo-inositol-derived common intermediate. This intermediate was synthesized by the vinylogous ring opening of myo-inositol orthoester cage under mild acidic conditions in six steps from myo-inositol. From this intermediate, we achieved the syntheses of (±)-lincitol A in six steps, (±)-lincitol B in seven steps, (±)-uvacalol I in five steps, (±)-uvacalol J in five steps, and (±)-uvacalol K in seven steps. The structure and relative stereochemistry of these natural products were confirmed by comparing the (1)H and (13)C NMR spectra of synthesised natural products with the reported data. These syntheses involved several unprecedented protecting-group manipulations and unexpected reactivities.

Vinylogy in orthoester hydrolysis: total syntheses of cyclophellitol, valienamine, gabosine K, valienone, gabosine G, 1-epi-streptol, streptol, and uvamalol A.

C7-cyclitols represent an important category of natural products possessing a broad spectrum of biological activities. As each member of these compounds is structurally unique, the usual practice is to synthesize them individually from appropriate polyhydroxylated chiral pools. We have observed an unusual vinylogy in acid mediated hydrolysis of enol ethers of myo-inositol 1,3,5-orthoesters giving a synthetically versatile polyhydroxylated cyclohexenal intermediate. We have exploited this unprecedented reaction for developing a general strategy for the rapid and efficient syntheses of several structurally diverse natural products of C7-cyclitol family. We have made an appropriately protected advanced intermediate 25 in five steps from the cheap and commercially available myo-inositol, and this common intermediate has been used to synthesize eight natural products in racemic form. We could synthesize (±)-cyclophellitol in seven steps, (±)-valienamine in five steps, (±)-gabosine I in five steps, (±)-gabosine G in six steps, (±)-gabosine K in three steps, (±)-streptol in six steps, (±)-1-epi-streptol in two steps, and (±)-uvamalol A in five steps from this intermediate.