Synthesis of 3-Deoxy-d-manno-oct-2-ulosonic Acid (KDO) and Pseudaminic Acid C-Glycosides

Stereocontrolled Synthesis of α-3-Deoxy-d-manno-oct-2-ulosonic Acid (α-Kdo) Glycosides Using C3-p-Tolylthio-Substituted Kdo Donors: Access to Highly Branched Kdo Oligosaccharides

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an eight-carbon monosaccharide found widely in bacterial lipopolysaccharides (LPSs) and capsule polysaccharides (CPSs). We developed an indirect method for the stereoselective synthesis of α-Kdo glycosides with a C3-p-tolylthio-substituted Kdo phosphite donor. The presence of the p-tolylthio group enhanced the reactivity, suppressed the formation of elimination by-products (2,3-enes), and provided complete α-stereocontrol. A variety of Kdo α-glycosides were synthesized by our method in excellent yields (up to 98 %). After glycosylation, the p-tolylthio group can be efficiently removed by free-radical reduction. Subsequently, the orthogonality of the phosphite donor and thioglycoside donor was demonstrated by the one-pot synthesis of a trisaccharide in Helicobacter pylori and Neisseria meningitidis LPS. Moreover, an efficient total synthesis route to the challenging 4,5-branched Kdo trisaccharide in LPSs from several A. baumannii strains was highlighted. To demonstrate the high reactivity of our approach further, the highly crowded 4,5,7,8-branched Kdo pentasaccharide was synthesized as a model molecule for the first time. Additionally, the reaction mechanism was investigated by DFT calculations.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Nucleoside Analog 2',3'-Isopropylidene-5-Iodouridine as Novel Efficient Inhibitor of HIV-1

Nucleoside reverse transcriptase inhibitors are the first class of drugs to be approved by the FDA for the suppression of HIV-1 and are widely used for this purpose in combination with drugs of other classes. Despite the progress in HIV-1 treatment, there is still the need to develop novel efficient antivirals. Here the efficiency of HIV-1 inhibition by a set of original 5-substituted uridine nucleosides was studied. We used the replication deficient human immunodeficiency virus (HIV-1)-based lentiviral particles and identified that among the studied compounds, 2',3'-isopropylidene-5-iodouridine was shown to cause anti-HIV-1 activity. Importantly, no toxic action of this compound against the cells of T-cell origin was found. We determined that this compound is significantly more efficient at suppressing HIV-1 compared to Azidothymidine (AZT) when taken at the high non-toxic concentrations. We did not find any profit when using AZT in combination with 2',3'-isopropylidene-5-iodouridine. 2',3'-Isopropylidene-5-iodouridine acts synergistically to repress HIV-1 when combined with the CDK4/6 inhibitor Palbociclib in low non-toxic concentration. No synergistic antiviral action was detected when AZT was combined with Palbociclib. We suggest 2',3'-isopropylidene-5-iodouridine as a novel perspective non-toxic compound that may be used for HIV-l suppression.

Influence of Configuration at the 4- and 6-Positions on the Conformation and Anomeric Reactivity and Selectivity of 7-Deoxyheptopyranosyl Donors: Discovery of a Highly Equatorially Selective l-glycero-d-gluco-Heptopyranosyl Donor

The preparation of four per-O-benzyl-d- or l-glycero-d-galacto and d- or l-glycero-d-gluco heptopyranosyl sulfoxides and the influence of their side-chain conformations on reactivity and stereoselectivity in glycosylation reactions are described. The side-chain conformation in these donors is determined by the relative configuration of its point of attachment to the pyranoside ring and the two flanking centers in agreement with a recent model. In the d- and l-glycero-d-galacto glycosyl donors, the d-glycero-d-galacto isomer with the more electron-withdrawing trans,gauche conformation of its side chain was the more equatorially selective isomer. In the d- and l-glycero-d-gluco glycosyl donors, the l-glycero-d-gluco isomer with the least disarming gauche,gauche side-chain conformation was the most equatorially selective donor. Variable temperature NMR studies, while supporting the formation of intermediate glycosyl triflates at -80 °C in all cases, were inconclusive owing to a change in the decomposition mechanism with the change in configuration. It is suggested that the equatorial selectivity of the l-glycero-d-gluco isomer arises from H-bonding between the glycosyl acceptor and O6 of the donor, which is poised to deliver the acceptor antiperiplanar to the glycosyl triflate, resulting in a high degree of S_N2 character in the displacement reaction.

Stereoselective Synthesis of β-C-Glycosides of 3-Deoxy-d-manno-oct-2-ulosonic Acid (Kdo) via SmI2-Mediated Reformatsky Reactions

An efficient and simple approach for stereoselective synthesis of β-Kdo C-glycosides was described, which relies on easily available peracetylated anomeric acetate or anomeric 2-pyridyl sulfide to couple with carbonyl compounds via SmI₂-mediated Reformatsky reactions. The utility of this methodology is exemplified by the streamlined synthesis of a practical β-Kdo C-glycoside with an anomeric aminopropyl linker to conjugate with other biomolecules for further biological studies.

Influence of substitution at the 5α-Position on the side chain conformation of glucopyranosides

We describe the preparation of methyl 5α-methyl-α-d-glucopyranoside and of 5α-fluoro-β-d-glucopyranose per acetate and the NMR-based conformational analysis of their side chains. Both the 5α-methyl and 5α-fluoro substituents increase the population of the gauche,gauche side chain conformer to the extent that it becomes the predominant conformation. In the 5α-methyl series this is attributed to steric effects, whereas in the 5α-fluoro series the optimization of attractive gauche effects is the more likely reason.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.