Synthesis of 2-deoxy-2-C-alkylglucosides of myo-inositol as possible inhibitors of a N-deacetylase enzyme in the biosynthesis of mycothiol

C2-ketonylation of carbohydrates via excited-state palladium-catalyzed 1,2-spin-center shift

C2-ketonyl-2-deoxysugars, sugars with the C2-hydroxyl group replaced by a ketone side chain, are important carbohydrate mimetics in glycobiology and drug discovery studies; however, their preparation remains a vital challenge in organic synthesis. Here we report the first direct strategy to synthesize this class of glycomimetics from readily available 1-bromosugars and silyl enol ethers via an excited-state palladium-catalyzed 1,2-spin-center shift (SCS) process. This step-economic reaction features broad substrate scope, has a high functional group tolerance, and can be used in late-stage functionalization of natural product- and drug-glycoconjugates. Preliminary experimental and computational mechanistic studies suggested a non-chain radical mechanism involving photoexcited palladium species, a 1,2-SCS process, and a radical Mizoroki-Heck reaction.

A protecting group-free approach for synthesizing C-glycosides through glycosyl dithiocarbamates

The first protection/deprotection-free process for radical C-glycosylation has been achieved through one-step preparable glycosyl dithiocarbamates (GDTCs). The Giese-type reaction and radical allylation of unprotected GDTCs were successfully performed to obtain the corresponding α-C-glycosides stereoselectively under mild reaction conditions.

Bioisosteres of Carbohydrate Functional Groups in Glycomimetic Design

The aberrant presentation of carbohydrates has been linked to a number of diseases, such as cancer metastasis and immune dysregulation. These altered glycan structures represent a target for novel therapies by modulating their associated interactions with neighboring cells and molecules. Although these interactions are highly specific, native carbohydrates are characterized by very low affinities and inherently poor pharmacokinetic properties. Glycomimetic compounds, which mimic the structure and function of native glycans, have been successful in producing molecules with improved pharmacokinetic (PK) and pharmacodynamic (PD) features. Several strategies have been developed for glycomimetic design such as ligand pre-organization or reducing polar surface area. A related approach to developing glycomimetics relies on the bioisosteric replacement of carbohydrate functional groups. These changes can offer improvements to both binding affinity (e.g., reduced desolvation costs, enhanced metal chelation) and pharmacokinetic parameters (e.g., improved oral bioavailability). Several examples of bioisosteric modifications to carbohydrates have been reported; this review aims to consolidate them and presents different possibilities for enhancing core interactions in glycomimetics.

Synthetic utility of endocyclic cleavage reaction

Existence of endocyclic cleavage reaction is now clearly shown from experimental evidence of endocyclic cleavage reaction as well as computational chemistry. Not only stereoelectronic factor, several factors could be main factors for endocyclic cleavage reaction. Endocyclic cleavage reaction is useful for 1,2-cis aminoglycoside formation, which is difficult by conventional glycosylation. By using endocyclic cleavage reaction, several glycosides with 1,2-cis aminoglycoside were prepared.

Mycothiol synthesis by an anomerization reaction through endocyclic cleavage

Mycothiol is found in Gram-positive bacteria, where it helps in maintaining a reducing intracellular environment and it plays an important role in protecting the cell from toxic chemicals. The inhibition of the mycothiol biosynthesis is considered as a treatment for tuberculosis. Mycothiol contains an α-aminoglycoside, which is difficult to prepare stereoselectively by a conventional glycosylation reaction. In this study, mycothiol was synthesized by an anomerization reaction from an easily prepared β-aminoglycoside through endocyclic cleavage.

Mycothiol: a promising antitubercular target

Tuberculosis (TB) is the world's second commonest cause of death next to HIV/AIDS. The increasing emergence of multi drug resistance and the recalcitrant nature of persistent infections pose an additional challenge for the treatment of TB. Due to the development of resistance to conventional antibiotics there is a need for new therapeutic strategies to combat M. tuberculosis. One such target is Mycothiol (MSH), a major low molecular-mass thiol in mycobacteria, an important cellular anti-oxidant. MSH is present only in actinomycetes and hence is a good target. This review explores mycothiol as a potential target against tuberculosis and various research ongoing worldwide.

Intramolecular alpha-glucosaminidation: synthesis of mycothiol

A protected cyclitol aglycon was tethered to an (N-arylsulfonyl)glucosamine donor by a methylene linker; the exclusively alpha-selective intramolecular glycosylation reaction was then initiated by electrophilic activation of the thioglycoside donor portion. Further transformations of the glycosylation product to give the M. tuberculosis detoxifier mycothiol and its oxidized congener, the disulfide mycothione, are detailed.

Biosynthesis and functions of mycothiol, the unique protective thiol of Actinobacteria

Mycothiol (MSH; AcCys-GlcN-Ins) is the major thiol found in Actinobacteria and has many of the functions of glutathione, which is the dominant thiol in other bacteria and eukaryotes but is absent in Actinobacteria. MSH functions as a protected reserve of cysteine and in the detoxification of alkylating agents, reactive oxygen and nitrogen species, and antibiotics. MSH also acts as a thiol buffer which is important in maintaining the highly reducing environment within the cell and protecting against disulfide stress. The pathway of MSH biosynthesis involves production of GlcNAc-Ins-P by MSH glycosyltransferase (MshA), dephosphorylation by the MSH phosphatase MshA2 (not yet identified), deacetylation by MshB to produce GlcN-Ins, linkage to Cys by the MSH ligase MshC, and acetylation by MSH synthase (MshD), yielding MSH. Studies of MSH mutants have shown that the MSH glycosyltransferase MshA and the MSH ligase MshC are required for MSH production, whereas mutants in the MSH deacetylase MshB and the acetyltransferase (MSH synthase) MshD produce some MSH and/or a closely related thiol. Current evidence indicates that MSH biosynthesis is controlled by transcriptional regulation mediated by sigma(B) and sigma(R) in Streptomyces coelicolor. Identified enzymes of MSH metabolism include mycothione reductase (disulfide reductase; Mtr), the S-nitrosomycothiol reductase MscR, the MSH S-conjugate amidase Mca, and an MSH-dependent maleylpyruvate isomerase. Mca cleaves MSH S-conjugates to generate mercapturic acids (AcCySR), excreted from the cell, and GlcN-Ins, used for resynthesis of MSH. The phenotypes of MSH-deficient mutants indicate the occurrence of one or more MSH-dependent S-transferases, peroxidases, and mycoredoxins, which are important targets for future studies. Current evidence suggests that several MSH biosynthetic and metabolic enzymes are potential targets for drugs against tuberculosis. The functions of MSH in antibiotic-producing streptomycetes and in bioremediation are areas for future study.

Mycothiol: synthesis, biosynthesis and biological functions of the major low molecular weight thiol in actinomycetes

Actinomycetes produce mycothiol as their major low molecular weight thiol, which parallels the functions of glutathione found in prokaryotes and most Gram-negative bacteria. This review covers progress that has so far been made in terms of its distribution, biosynthesis and metabolic functions, as well as chemical syntheses of mycothiol and alternative substrates and inhibitors of mycothiol biosynthesis and mycothiol-dependent enzymes. 152 references are cited.

Biochemistry of the initial steps of mycothiol biosynthesis

Mycothiol is the major thiol produced by mycobacteria and is required for growth of Mycobacterium tuberculosis. The final three steps in the biosynthesis of mycothiol have been fully elucidated but the initial steps have been unclear. A glycosyltransferase, MshA, is required for production of the mycothiol precursor, 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol, but its substrates and immediate products were unknown. In this study, we show that the N-acetylglucosamine donor is UDP-N-acetylglucosamine and that the N-acetylglucosamine acceptor is 1L-myo-inositol 1-phosphate. The reaction generates UDP and 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol 3-phosphate. Using cell-free extracts of M. smegmatis mc(2)155, little activity was obtained with myo-inositol, 1D-myo-inositol 1-phosphate, or myo-inositol 2-phosphate as the N-acetylglucosamine acceptor. A phosphatase, designated MshA2, is required to dephosphorylate 1-O-(2-acetamido-2-deoxy-alpha-glucopyranosyl)-D-myo-inositol 3-phosphate to produce 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol. The latter is deacetylated, ligated with cysteine, and the cysteinyl amino group acetylated by acetyl-CoA to complete the mycothiol biosynthesis pathway. Uptake and concentration of myo-[14C]inositol is rapid in Mycobacterium smegmatis and leads to production of radiolabeled inositol 1-phosphate and mycothiol. This demonstrates the presence of a myo-inositol transporter and a kinase that generates 1L-myo-inositol 1-phosphate. The biochemical pathway of mycothiol biosynthesis is now fully elucidated.

Purification and characterization of Mycobacterium tuberculosis 1d-myo-inosityl-2-acetamido-2-deoxy-α-d-glucopyranoside deacetylase, MshB, a mycothiol biosynthetic enzyme

Mycothiol (MSH, AcCys-GlcN-Ins) is the major low molecular weight thiol in actinomycetes and is essential for growth of Mycobacterium tuberculosis. MshB, the GlcNAc-Ins deacetylase, is a key enzyme in MSH biosynthesis. MshB from M. tuberculosis was cloned, expressed, purified, and its properties characterized. Values of k(cat) and K(m) for MshB were determined for the biological substrate, GlcNAc-Ins, and several other good substrates. The substrate specificity of MshB was compared to that of M. tuberculosis mycothiol S-conjugate amidase (Mca), a homologous enzyme having weak GlcNAc-Ins deacetylase activity. Both enzymes are metalloamidases with overlapping amidase activity toward mycothiol S-conjugates (AcCySR-GlcN-Ins). The Ins residue and hydrophobic R groups enhance the activity with both MshB and Mca, but changes in the acyl group attached to GlcN have opposite effects on the two enzymes.