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.

Stereodirecting Effect of Esters at the 4-Position of Galacto- and Glucopyranosyl Donors: Effect of 4-C-Methylation on Side-Chain Conformation and Donor Reactivity, and Influence of Concentration and Stoichiometry on Distal Group Participation

When generated in a mass spectrometer bridged bicyclic 1,3-dioxenium ions derived from 4-O-acylgalactopyranosyl, donors can be observed by infrared spectroscopy at cryogenic temperatures, but they are not seen in the solution phase in contrast to the fused bicyclic 1,3-dioxalenium ions of neighboring group participation. The inclusion of a 4-C-methyl group into a 4-O-benzoyl galactopyranosyl donor enables nuclear magnetic resonance observation of the bicyclic ion arising from participation by the distal ester, with the methyl group influence attributed to ester ground state conformation destabilization. We show that a 4-C-methyl group also influences the side-chain conformation, enforcing a gauche,trans conformation in gluco and galactopyranosides. Competition experiments reveal that the 4-C-methyl group has only a minor influence on the rate of reaction of 4-O-benzoyl or 4-O-benzyl-galacto and glucopyranosyl donors and, consequently, that participation by the distal ester does not result in kinetic acceleration (anchimeric assistance). We demonstrate that the stereoselectivity of the 4-O-benzoyl-4-C-methyl galactopyranosyl donor depends on reaction concentration and additive (diphenyl sulfoxide) stoichiometry and hence that participation by the distal ester is a borderline phenomenon in competition with standard glycosylation mechanisms. An analysis of a recent paper affirming participation by a remote pivalate ester is presented with alternative explanations for the observed phenomena.

C7 Epimerization of Benzylidene-Protected β-d-Idopyranosides Brings Structural Insights into Idose Conformational Flexibility

Idose is unique among other aldohexoses because of its high conformational flexibility in solution. We herein show that benzylidene acetal-protected 3-O-acyl-β-d-idopyranosides undergo Lewis acid-catalyzed C7 epimerization with concomitant ⁴C₁ to ¹C₄ ring inversion. The reaction conditions and structural parameters for this transformation to occur have been thoroughly investigated through an extensive glycosylation study combined with NMR analyses, X-ray diffraction, and quantum molecular modeling. In addition to reporting a direct, β-stereoselective idosylation approach, our work brings fundamental structural insights into the conformational flexibility of idose.

Regioselective Reductive Opening of Benzylidene Acetals with Dichlorophenylborane/Triethylsilane: Previously Unreported Side Reactions and How to Prevent Them

Arylidene acetals are widely used protecting groups, because of not only the high regioselectivity of their introduction but also the possibility of performing further regioselective reductive opening in the presence of a hydride donor and an acid catalyst. In this context, the Et₃SiH/PhBCl₂ system presents several advantages: silanes are efficient, environmentally benign, and user-friendly hydride donors, while PhBCl₂ opens the way to unique regioselectivity with regard to all other Brønsted or Lewis acids used with silanes. This system has been extensively used by several groups, and we have demonstrated its high regioselectivity in the reductive opening of 4,6- and 2,4-O-p-methoxybenzylidene moieties in protected disaccharides. Surprisingly, its use on 4,6-O-benzylidene-containing substrates 1 and 2 led to unreproducible yields due to the unexpected formation of several side products. Their characterizations allowed us to identify different pitfalls potentially affecting the outcome of reductive opening of arylidenes with the Et₃SiH/PhBCl₂ reagent system: alkene hydroboration, azide reduction, and/or Lewis acid-promoted cleavage of the arylidene. With this knowledge, we optimized reproducible and high-yielding reaction conditions that secure and extend the scope of the Et₃SiH/PhBCl₂ system as a reagent for the regioselective opening of arylidenes in complex and multifunctional molecules.

Systematic Study of Regioselective Reductive Ring-Opening Reactions of 4,6-O-Halobenzylidene Acetals of Glucopyranosides

Reductive openings of cyclic acetals are widely used in modern synthetic organic chemistry for the regioselective introduction of protecting groups. A systematic study was performed on the applicability and efficacy of various hydride donor and protic or Lewis acid reagent combinations in the reductive ring opening of glucosidic 4,6-halobenzylidene acetals bearing an ortho-, meta-, and para-chloro- or -bromo substituent on the benzene ring. Most of the reagent combinations tested cleaved the 4,6-O-halobenzylidene acetal rings at O4 or O6 efficiently and with the expected regioselectivity. The LiAlH₄-AlCl₃ and the BH₃·THF-TMSOTf combinations produced the 4-O-halobenzyl ether/6-OH products with complete regioselectivity and high yields. The use of Me₃N·BH₃-AlCl₃ reagent system in toluene was also effective in cleaving the acetal ring at O6 but was accompanied by Al-chelation-assisted debenzylation side reactions. The NaCNBH₃-HCl and the Et₃SiH-BF₃·Et₂O combinations were highly effective in yielding the 6-halobenzyl ether/4-OH derivatives. Et₃SiH, in combination with TfOH, produced the 6-O-ether/4-OH products in rapid reactions but also triggered silylation and reductive halobenzylation as secondary transformations. Reductive opening of the 1,3-dioxane ring of pyranosidic 4,6-O-halobenzylidene acetals by the proper reagent combination was found to be an efficient method for the regioselective introduction of versatile halobenzyl protecting groups onto the pyranose ring.

Synthesis of Chiral Polycyclic Tetrahydrocarbazoles by Enantioselective Aminocatalytic Double Activation of 2-Hydroxycinnamaldehydes with Dienals

An efficient aminocatalytic enantioselective double-activation strategy has been developed that combines several different aminocatalytic modes in a cascade process, such as iminium ion, vinylogous iminium ion, trienamine, and dienamine activations. By using this strategy, 2-hydroxycinnamaldehydes worked well with various dienals via [4 + 2] cycloaddition and the oxa-Michael reaction-initiated cascade, respectively, leading to chiral polycyclic tetrahydrocarbazole and chromane derivatives with excellent diastereo- and enantioselectivities.

Experimental observations on the reductive cleavage of endo and exo 3,4-O-benzylidene fucopyranoside derivatives

Reductive cleavage of methyl 3,4-O-benzylidene-α-L-fucopyranosides with BH₃·THF-TfOH and NaCNBH₃-TfOH systems resulted in enhanced reaction rates and selectivity compared to BH₃·THF-Bu₂BOTf. With this latter system, the nature of the O-2 substituent exerted a clear control on the reactivity but practically did not affect the regioselectivity. With TfOH the direction of cleavage was determined, as expected, by the configuration of the acetal carbon atom, but slightly influenced by its competitive epimerization. Protic conditions provided higher regioselectivity in the openings of the exo isomers, affording a useful approach to the practical synthesis of 3-O-benzyl ethers.

Regioselectivity of Cobalamin-Dependent Methyltransferase Can Be Tuned by Reaction Conditions and Substrate

Regioselective reactions represent a significant challenge for organic chemistry. Here the regioselective methylation of a single hydroxy group of 4-substituted catechols was investigated employing the cobalamin-dependent methyltransferase from Desulfitobacterium hafniense. Catechols substituted in position four were methylated either in meta- or para-position to the substituent depending whether the substituent was polar or apolar. While the biocatalytic cobalamin dependent methylation was meta-selective with 4-substituted catechols bearing hydrophilic groups, it was para-selective for hydrophobic substituents. Furthermore, the presence of water miscible co-solvents had a clear improving influence, whereby THF turned out to enable the formation of a single regioisomer in selected cases. Finally, it was found that also the pH led to an enhancement of regioselectivity for the cases investigated.

Stereochemistry of Simple Molecules inside Nanotubes and Fullerenes: Unusual Behavior of Usual Systems

Over the past three decades, carbon nanotubes and fullerenes have become remarkable objects for starting the implementation of new models and technologies in different branches of science. To a great extent, this is defined by the unique electronic and spatial properties of nanocavities due to the ramified π-electron systems. This provides an opportunity for the formation of endohedral complexes containing non-covalently bonded atoms or molecules inside fullerenes and nanotubes. The guest species are exposed to the force field of the nanocavity, which can be described as a combination of electronic and steric requirements. Its action significantly changes conformational properties of even relatively simple molecules, including ethane and its analogs, as well as compounds with C-O, C-S, B-B, B-O, B-N, N-N, Al-Al, Si-Si and Ge-Ge bonds. Besides that, the cavity of the host molecule dramatically alters the stereochemical characteristics of cyclic and heterocyclic systems, affects the energy of pyramidal nitrogen inversion in amines, changes the relative stability of cis and trans isomers and, in the case of chiral nanotubes, strongly influences the properties of R- and S- enantiomers. The present review aims at primary compilation of such unusual stereochemical effects and initial evaluation of the nature of the force field inside nanotubes and fullerenes.

Iron-Catalyzed Radical Cleavage/C-C Bond Formation of Acetal-Derived Alkylsilyl Peroxides

A novel radical-based approach for the iron-catalyzed selective cleavage of acetal-derived alkylsilyl peroxides, followed by the formation of a carbon-carbon bond is reported. The reaction proceeds under mild reaction conditions and exhibits a broad substrate scope with respect to the acetal moiety and the carbon electrophile. Mechanistic studies suggest that the present reaction proceeds through a free-radical process involving carbon radicals generated by the homolytic cleavage of a carbon-carbon bond within the acetal moiety. A synthetic application of this method to sugar-derived alkylsilyl peroxides is also described.

Synthesis of carbohydrate building blocks via regioselective uniform protection/deprotection strategies

Discussed herein is the synthesis of partially protected carbohydrates by manipulating only one type of a protecting group for a given substrate. The first focus of this review is the uniform protection of an unprotected starting material in a way that only one (or two) hydroxyl group remains unprotected. The second focus involves regioselective partial deprotection of uniformly protected compounds in a way that only one (or two) hydroxyl group becomes liberated.