Challenges in the stereocontrolled syntheses of β-rhamnosides

Chemical approaches for the stereocontrolled synthesis of 1,2-cis-β-D-rhamnosides

In carbohydrate chemistry, the stereoselective synthesis of 1,2-cis-glycosides remains a formidable challenge. This complexity is comparable to the synthesis of 1,2-cis-β-D-mannosides, primarily due to the adverse anomeric and Δ-2 effects. Over the past decades, to attain β-stereoselectivity in D-rhamnosylation, researchers have devised numerous direct and indirect methodologies, including the hydrogen-bond-mediated aglycone delivery (HAD) method, the synthesis of β-D-mannoside paired with C6 deoxygenation, and the combined approach of 1,2-trans-glycosylation and C2 epimerization. This review elaborates on the advancements in β-D-rhamnosylation and its implications for the total synthesis of tiacumicin B and other physiologically relevant glycans.

Direct Synthesis of 2,6-Dideoxy-β-glycosides and β-Rhamnosides with a Stereodirecting 2-(Diphenylphosphinoyl)acetyl Group

Anomeric stereocontrol is usually one of the major issues in the synthesis of complex carbohydrates, particularly those involving β-configured 2,6-dideoxyglycoside and d/l-rhamnoside moieties. Herein, we report that 2-(diphenylphosphinoyl)acetyl is highly effective as a remote stereodirecting group in the direct synthesis of these challenging β-glycosides under mild conditions. A deoxy-trisaccharide as a mimic of the sugar chain of landomycin E was prepared stereospecifically in high yield. The synthetic potential was also highlighted in the synthesis of Citrobacter freundii O-antigens composed of a [→4)-α-d-Manp-(1→3)-β-d-Rhap(1→4)-β-d-Rhap-(1→] repeating unit, wherein the convergent assembly up to a nonasaccharide was realized with a strongly β-directing trisaccharide donor. Variable-temperature NMR studies indicate the presence of intermolecular H-bonding between the donor and the bulky acceptor as direct spectral evidence in support of the concept of hydrogen-bond-mediated aglycone delivery.

Recent advances in β-l-rhamnosylation

l-Rhamnose forms the key components of important antigenic oligo- and polysaccharides of a variety of pathogens. Obtaining 1,2-cis stereoselectivity in the glycosylation of l-rhamnoside is quite challenging due to the unavailability of neighboring group participation and disfavoring of the anomeric effect and stereoelectronic effect of the substituents on the C-2 axial position. Nevertheless, various methodologies have been developed exploiting diverse pathways for obtaining β-stereoselectivity in the glycosylation of l-rhamnose. This review describes the recent advances in β-l-rhamnosylation and its applications in the total synthesis of β-l-rhamnose-containing biologically important oligosaccharides.

Chemical Synthesis of β-l-Rhamnose Containing the Pentasaccharide Repeating Unit of the O-Specific Polysaccharide from a Halophilic Bacterium Halomonas ventosae RU5S2EL in the Form of Its 2-Aminoethyl Glycoside

Total synthesis of the pentasaccharide repeating unit of the OPS from Halomonas ventosae RU5S2EL is accomplished through a [3+2] block strategy. Picoloyl-induced hydrogen-bond-assisted aglycon delivery (HAD) is used for two consecutive 1,2-cis-l-rhamnosylations, and remote participation is used for α-selective glucosylation. The choice of 2-aminoethyl glycoside at the reducing end is opted for, leaving the scope for further glycoconjugate formation without hampering the reducing-end stereochemistry.

Synthesis of a Highly Branched Nonasaccharide Chlorella Virus N-Glycan Using a "Counterclockwise" Assembly Approach

Chloroviruses produce a capsid protein containing N-linked glycans differing in structure from those found in all other organisms. These species feature a core "hyper-branched" fucose residue in which every hydroxyl group is glycosylated. We describe the synthesis of a nonasaccharide from Paramecium bursaria chlorella virus 1, one of most complex chlorovirus N-glycans reported, using a "counterclockwise" strategy involving the sequential addition of trisaccharide, disaccharide, and monosaccharide motifs to a trisaccharide containing the core fucose residue.

An eco-friendly N-benzoylglycine/thiourea cooperative catalyzed stereoselective synthesis of β-L-rhamnopyranosides

A new practical utility for β-stereoselective L-rhamnopyranosylations are conducted using rhamnosyl trichloroacetimidate donors in the presence of N-benzoylglycine/thiourea cooperative catalysis. This method represents the first instance where amino acid derivative N-benzoylglycine is used as a catalyst for β-L-rhamnopyranosylations. This method represents the first instance where environmentally benign amino acid derivative, such as N-benzoylglycine which is reported as less toxic and can be used as efficient catalyst for smooth transformation under eco friendly conditions. On the other hand β-stereoselectivity of rhamnosyl trichloroacetimidate donors protected with O-picoloyl groups at remote positions (C-2 and C-3) has been investigated while the glycosylation reactions of 2-O-picoloyl group substituted l-rhamnosyl donor displays predominant β-stereoselectivity. Reaction proceeded smoothly with moderate to high yield under mild reaction conditions at room temperature with 10 mol% catalyst loadings and tolerant of a wide range of glycoside acceptors.

Total Synthesis of Trisaccharide Repeating Unit of O-Specific Polysaccharide of Pseudomonas fluorescens BIM B-582

The first total synthesis of the trisaccharide repeating unit of the O-specific polysaccharide of Pseudomonas fluorescens BIM B-582 is reported. This efficient synthesis involves consecutive 1,2- cis glycosylations including β-l-rhamnosylation and α selective coupling of rare 4-deoxy-d- xylo-hexose as the key steps. The synthetic trisaccharide is equipped with an aminopropyl linker at the reducing end to allow for conjugation to proteins and microarrays for further immunological studies.

Stereoselective synthesis of β-rhamnopyranosides via gold(i)-catalyzed glycosylation with 2-alkynyl-4-nitro-benzoate donors

An effective β-rhamnosylation protocol has been developed by using α-rhamnopyranosyl ortho-hexynyl-para-nitro-benzoates as donors and Ph₃PAuBArF4 as a catalyst.

Synthesis of the biological repeating unit of Streptococcus pneumoniae serotype 23F capsular polysaccharide

The 3-aminopropyl glycoside of the biological repeating unit ofStreptococcus pneumoniaeserotype 23F capsular polysaccharide was efficiently synthesized by a linear assembly strategy.

Total Synthesis of the Glycosylated Macrolide Antibiotic Fidaxomicin

The first enantioselective total synthesis of fidaxomicin, also known as tiacumicin B or lipiarmycin A3, is reported. This novel glycosylated macrolide antibiotic is used in the clinic for the treatment of Clostridium difficile infections. Key features of the synthesis involve a rapid and high-yielding access to the noviose, rhamnose, and orsellinic acid precursors; the first example of a β-selective noviosylation; an effective Suzuki coupling of highly functionalized substrates; and a ring-closing metathesis reaction of a noviosylated dienoate precursor. Careful selection of protecting groups allowed for a complete deprotection yielding totally synthetic fidaxomicin.

Rhamnosylation: diastereoselectivity of conformationally armed donors

The α/β-selectivity of super-armed rhamnosyl donors have been investigated in glycosylation reactions. The solvent was found to have a minor influence, whereas temperature was crucial for the diastereoselectivity. At very low temperature, a modest β-selectivity could be obtained, and increasing temperature gave excellent α-selectivity. The donors were highly reactive, and activation was observed at temperatures as low as -107 °C. Different promoter systems and leaving groups were investigated, and only activation with a heterogeneous catalyst increased the amount of the β-anomer significantly. By introducing an electron-withdrawing nonparticipating group, benzyl sulfonyl, on 2-O, an increase in β-product was observed.

Methodology development and physical organic chemistry: a powerful combination for the advancement of glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.

Revisit to the reaction of o-phenylene diamine with thiosemicarbazide to give benzimidazole-2-thione rather than benzotriazine-2-thione and its glycosylation

Reaction of o-phenylene diamine with thiosemicarbazide did not give benzotriazine-2-thione 2 as reported, although the product was found to be benzimidazole-2-thione 3. Glycosylation of 3 with acetobromo sugars 4a-4b gave the respective thioglycosides 7a-7d in addition to minor products of the nucleosides 8a and 8b, in the case of the gluco- and galacto-analogs, respectively. The regioselectivity of glycosylation reaction has been investigated.

Benzyl 2-(4-bromo-anilino)-4,4-dimethyl-6-oxocyclo-hex-1-enecarbodithio-ate: first triclinic polymorph

The six-membered cyclo-hexene ring in the title compound, C(22)H(22)BrNOS(2), adopts an envelope conformation, with the C atom bearing the two methyl groups representing the flap. This atom deviates by 0.686 (4) Å from the plane passing through the other five atoms of the ring (r.m.s. deviation = 0.025 Å). The mol-ecular conformation is stabilized by an intra-molecular N-H⋯S hydrogen bond.