First Synthesis of the β-d-Rhamnosylated Trisaccharide Repeating Unit of the O-Antigen from Xanthomonas campestris pv. campestris 8004

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.

Synthesis of the trisaccharide repeating unit of Stenotrophomonas maltophilia O6 antigen through step-wise and one-pot approaches

Synthetic conjugate vaccines are an important area of research for the prevention and occurrence of diseases caused by Gram-negative bacteria. For the development of such vaccines, access to the pure and homogeneous oligosaccharide fragments of the bacterial cell surface polysaccharides are necessary. Stenotrophomonas maltophilia is a typical opportunistic Gram-negative bacteria that causes severe pulmonary and other infections; often in hospitalized patients. With the emergence of multidrug resistant strains and increased virulence, new therapeutic strategies are needed to combat the threat. Herein, we report the syntheses of the trisaccharide repeating unit of S. maltophilia O6 antigen through stepwise and one-pot assemblies of the trisaccharide. The target trisaccharide was appended with a 2-aminoethyl linker that could provide the opportunity for conjugation to carrier proteins for the synthesis of vaccine candidates.

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.

Development of a Real-Time Quantitative PCR Assay for the Specific Detection of Bacillus velezensis and Its Application in the Study of Colonization Ability

Bacillus velezensis is a widely used biocontrol agent closely related to B. amyloliquefaciens, and the two species cannot be distinguished by universal primers that are currently available. The study aimed to establish a rapid, specific detection approach for B. velezensis. Many unique gene sequences of B. velezensis were selected through whole genome sequence alignment of B. velezensis strains and were used to design a series of forward and reverse primers, which were then screened by PCR and qPCR using different Bacillus samples as templates. The colonization ability of B. velezensis ZF2 in different soils and different soil environmental conditions was measured by qPCR and a 10-fold dilution plating assay. A specific primer pair targeting the sequence of the D3N19_RS13500 gene of B. velezensis ZF2 was screened and could successfully distinguish B. velezensis from B. amyloliquefaciens. A rapid specific real-time qPCR detection system for B. velezensis was established. B. velezensis ZF2 had a very strong colonization ability in desert soil, and the optimal soil pH was 7-8. Moreover, the colonization ability of strain ZF2 was significantly enhanced when organic matter from different nitrogen sources was added to the substrate. This study will provide assistance for rapid specificity detection and biocontrol application of B. velezensis strains.

Identification of Aristolactam Derivatives That Act as Inhibitors of Human Immunodeficiency Virus Type 1 Infection and Replication by Targeting Tat-Mediated Viral Transcription

Despite the success of antiretroviral therapy (ART), efforts to develop new classes of antiviral agents have been hampered by the emergence of drug resistance. Dibenzo-indole-bearing aristolactams are compounds that have been isolated from various plants species and which show several clinically relevant effects, including anti-inflammatory, antiplatelet, and anti-mycobacterial actions. However, the effect of these compounds on human immunodeficiency virus type 1 (HIV-1) infection has not yet been studied. In this study, we discovered an aristolactam derivative bearing dibenzo[cd,f]indol-4(5H)-one that had a potent anti-HIV-1 effect. A structure-activity relationship (SAR) study using nine synthetic derivatives of aristolactam identified the differing effects of residue substitutions on the inhibition of HIV-1 infection and cell viability. Among the compounds tested, 1,2,8,9-tetramethoxy-5-(2-(piperidin-1-yl)ethyl)-dibenzo[cd,f]indol-4(5H)-one (Compound 2) exhibited the most potent activity by inhibiting HIV-1 infection with a half-maximal inhibitory concentration (IC₅₀) of 1.03 μmol/L and a half-maximal cytotoxic concentration (CC₅₀) of 16.91 μmol/L (selectivity index, 16.45). The inhibitory effect of the compounds on HIV-1 infection was linked to inhibition of the viral replication cycle. Mode-of-action studies showed that the aristolactam derivatives did not affect reverse transcription or integration; instead, they specifically inhibited Tat-mediated viral transcription. Taken together, these findings show that several aristolactam derivatives impaired HIV-1 infection by inhibiting the activity of Tat-mediated viral transcription, and suggest that these derivatives could be antiviral drug candidates.

Invited review: Priming, induction and modulation of plant defence responses by bacterial lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) have multiple roles in plant—microbe interactions. LPS contributes to the low permeability of the outer membrane, which acts as a barrier to protect bacteria from plant-derived antimicrobial substances. Conversely, perception of LPS by plant cells can lead to the triggering of defence responses or to the priming of the plant to respond more rapidly and/or to a greater degree to subsequent pathogen challenge. LPS from symbiotic bacteria can have quite different effects on plants to those of pathogens. Some details are emerging of the structures within LPS that are responsible for induction of these different plant responses. The lipid A moiety is not solely responsible for all of the effects of LPS in plants; core oligosaccharide and O-antigen components can elicit specific responses. Here, we review the effects of LPS in induction of defence-related responses in plants, the structures within LPS responsible for eliciting these effects and discuss the possible nature of the (as yet unidentified) LPS receptors in plants.

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.

Efficient routes toward the synthesis of the D-rhamno-trisaccharide related to the A-band polysaccharide of Pseudomonas aeruginosa

The present work describes efficient avenues for the synthesis of the trisaccharide repeating unit [α-D-Rhap-(1→3)-α-D-Rhap-(1→3)-α-D-Rhap] associated with the A-band polysaccharide of Pseudomonas aeruginosa. One of the key steps involved 6-O-deoxygenation of either partially or fully acylated 4,6-O-benzylidene-1-thiomannopyranoside by radical-mediated redox rearrangement in high yields and regioselectivity. The D-rhamno-thioglycosides so obtained allowed efficient access to the trisaccharide target via stepwise glycosylation as well as a one-pot glycosylation protocol. In a different approach, a 4,6-O-benzylidene D-manno-trisaccharide derivative was synthesized, which upon global 6-O-deoxygenation followed by deprotection generated the target D-rhamno-trisaccharide. The application of the reported regioselective radical-mediated deoxygenation on 4,6-O-benzylidene D-manno thioglycoside (hitherto unexplored) has potential for ramification in the field of synthesis of oligosaccharides based on 6-deoxy hexoses.

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.

Stereocontrolled synthesis of D- and L-beta-rhamnopyranosides with 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycosides

The synthesis of both enantiomers of a 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycoside is described starting from D-mannnose and L-arabinose derivatives for the D- and L-series, respectively. This donor is effective in the preparation of the corresponding beta-glycosides using the 1-benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride protocol. Following desulfurization and concomitant debenzylation with Raney nickel, the so-formed 6-thio-beta-mannosides are converted in high yield to the beta-rhamnopyranosides.

Steroidal glycosides from the underground parts of Helleborus caucasicus

Four polyhydroxylated and polyunsaturated furostanol glycosides (1-4), named caucasicosides A (1), B (2), C (3) and D (4), were isolated from the MeOH extract of the underground parts of Helleborus caucasicus, along with four spirostanol derivatives, a furostanol glycoside, a furospirostanol glycoside, 20-hydroxyecdysone and the bufadienolides hellebrigenin and deglucohellebrin. The structures of 1-4 were elucidated as furosta-5,20(22),25(27)-triene-1beta,3beta,11alpha,26-tetrol 26-O-beta-D-glucopyranoside (1), 26-O-beta-D-glucopyranosylfurosta-5,20(22),25(27)-triene-1beta,3beta,11alpha,26-tetrol 3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside (2), 26-O-beta-d-glucopyranosyl-22alpha-methoxyfurosta-5,25(27)-diene-1beta,3beta,11alpha,26-tetrol 3-O-alpha-L-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside (3), 26-O-beta-D-glucopyranosylfurosta-5,20(22),25(27)-triene-1beta,3beta,26-triol 3-O-beta-D-xylopyranosyl-(1-->3)-alpha-L-rhamnopyranosyl-(1-->2)-4-O-sulfo-alpha-L-arabinopyranoside (4). Structure elucidation was accomplished through the extensive use of 1D- and 2D NMR experiments including 1H-1H (COSY, 1D-TOCSY) and 1H-13C (HSQC, HMBC) spectroscopy along with ESI-MS and HR-ESI-MS. The aglycones of 1-4 have never been reported before.

Synthesis of a beta-(1-->3)-D-rhamnotetraose by a one-pot, multiple radical fragmentation

A naturally occurring beta-(1-->3)-D-rhamnotetraose has been constructed under conditions of sequential beta-selective mannosylation controlled by the 4,6-O-[1-cyano-2-(2-iodophenyl)-ethylidene] protecting group. The route is concise, proceeding through a late-stage radical deoxygenation that successfully uncovers all four deoxy subunits at once.

The behaviour of deoxyhexose trihaloacetimidates in selected glycosylations

Armed deoxyhexose glycosyl donors are very reactive and sometimes too uncontrollably activated in glycosylation reactions; yields can be thereby reduced, especially when unreactive glycosyl acceptors are involved. In this paper, the behaviour of a range of deoxyhexose trihaloacetimidate (trichloro- and N-phenyl trifluoro-) donors is compared in some selected glycosylations towards biologically relevant targets. The selected N-phenyl trifluoroacetimidates often afforded best results in terms of both donor synthesis and glycosylation yield.

4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetals. improved second-generation acetals for the stereoselective formation of beta-D-mannopyranosides and regioselective reductive radical fragmentation to beta-D-rhamnopyranosides. scope and limitations

The [1-cyano-2-(2-iodophenyl)]ethylidene group is introduced as an acetal-protecting group for carbohydrate thioglycoside donors. The group is easily introduced under mild conditions, over short reaction times, and in the presence of a wide variety of other protecting groups by the reaction of the 4,6-diol with triethyl (2-iodophenyl)orthoacetate and camphorsulfonic acid, followed by trimethylsilyl cyanide and boron trifluoride etherate. The new protecting group conveys strong beta-selectivity with thiomannoside donors and undergoes a tin-mediated radical fragmentation to provide high yields of the synthetically challenging beta-rhamnopyranosides. The method is also applicable to the glucopyranosides when high alpha-selectivity is observed in the coupling reaction and alpha-quinovosides are formed selectively in the radical fragmentation step. In the galactopyranoside series, beta-glycosides are formed selectively on coupling to donors protected by the new system, but the radical fragmentation is unselective and gives mixtures of the 4- and 6-deoxy products. Variable-temperature NMR studies for the glycosylation step, which helped define an optimal protocol, are described.

On the nitrile effect in L-rhamnopyranosylation

It is shown that the use of 5% acetonitrile or propionitrile in dichloromethane functions to increase the beta-selectivity of a number of L-rhamnopyranosylation reactions conducted by the thioglycoside method with activation by the 1-benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride couple. The use of more significant quantities of acetonitrile or propionitrile results in the formation of complex reaction mixtures containing little coupled product, but from which Ritter-type products can be isolated.