Double diastereoselection explains limitations in synthesizing mannose-containing beta-(1,3)-glucans

Selection of Neighboring Group Participation Intermediates of Fully Acylated Donors around the Glycosylation Sites in Oligosaccharide Acceptors

Stereo- and regioselective formation of glycosidic linkages is a challenging topic in oligosaccharide syntheses. The stereoselective construction of 1,2-trans-glycosides generally involves neighboring group participation, which is less successful when synthesizing β-1,3-linked oligosaccharides. The combined steric effect of a 2-O-substituent and an aglycon moiety in acceptors increases the efficiency of glycosylation via neighboring group participation. This steric effect was reduced by using vicinal polyol acceptors and was demonstrated in the synthesis of 1,3-linked branched oligosaccharides.

Acceptor reactivity in glycosylation reactions

The effect of the reactivity of the glycosyl acceptor on the outcome of glycosylation reactions is reviewed.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

A Concise Synthesis of Three Branches Derived from Polysaccharide RN1 and Anti-Pancreatic Cancer Activity Study

RN1, a polysaccharide from flowers of Panax pseudo-ginsieng Wall. Var. notoginseng (Burkill) Hoo & Tseng, is a potential multi-targeting drug candidate for pancreatic cancer treatment. However, the active targeting domain of RN1 is still unknown. Herein, three RN1 derived branches were synthesized via [3+2] or [2+2] strategies, efficiently. Two pentasaccharides, 18 and 27, showed similar inhibition effect on pancreatic cancer BxPC-3 cells to that of RN1 at same concentration. Interestingly, tetrasaccharide 21 potently inhibited gemcitabineresistant cell line Panc-1 at high concentration. These suggest that the branches of RN1 might be the active targeting domain and tetrasaccharide 21 might be a potential leading compound for pancreatic cancer with gemcitabine resistance.

Synthesis of 3-aminopropyl glycoside of branched β-(1 → 3)-d-glucooctaoside

The synthesis was described of branched glucooctaoside bearing the β-(1 → 3)-glucotrioside side chain at O-6 of the second (from the reducing end) monosaccharide unit of the linear β-(1 → 3)-glucopentaoside core.

Synthesis of 3-aminopropyl glycosides of linear β-(1 → 3)-D-glucooligosaccharides

3-Aminopropyl glycosides of a series of linear β-(1 → 3)-linked D-glucooligosaccharides containing from 3 to 13 monosaccharide units were efficiently prepared. The synthetic scheme featured highly regioselective glycosylation of 4,6-O-benzylidene-protected 2,3-diol glycosyl acceptors with a disaccharide thioglycoside donor bearing chloroacetyl groups at O-2' and -3' as a temporary protection of the diol system. Iteration of the deprotection and glycosylation steps afforded the series of the title oligoglucosides differing in length by two monosaccharide units. A novel procedure for selective removal of acetyl groups in the presence of benzoyl ones consisting in a brief treatment with a large excess of hydrazine hydrate has been proposed.

β-(1→3)-Glucan-mannitol conjugates: scope and amazing results

It is well known that β-(1→3)-Glucans present high applicative potential in human health as immunostimulating agents. Numerous studies have highlighted this, but mainly used native polysaccharides extracted from various natural sources. These compounds are therefore inevitably polydisperse but also present structures that are not homogeneous, in an analytical point of view. This is the reason why we have achieved the chemical synthesis of small glucan-mannitol derivatives especially found in brown seaweeds. The targets differ from each other by the nature of the conjunction between the laminaribiose and the mannose or mannitol, i.e., (1→6) or (1→3). We established that (I) these molecules were efficiently obtained from glucose, laminaribiose and/or mannose derivatives; (II) the synthetic plan has to be adapted to the first connection between a glucosyl entity and the mannosyl residue; and (III) resulting pure compounds may be used as the standard for analytical purposes.

Synthesis and reactivity of 4'-deoxypentenosyl disaccharides

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4'-deoxypentenosyl (4'-DP) disaccharides, and we investigate their post-glycosylational C5' additions using the DMDO oxidation/ring-opening sequence. The α-linked 4'-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4'-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4'-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4'-DP unit is established at a post-glycosylative stage.

Protecting group-free glycoligation by the desulfurative rearrangement of allylic disulfides as a means of assembly of oligosaccharide mimetics

2-(2-Pyridyldithio-3-butenyl) glycosides react with carbohydrate-based thiols in a two-step process involving sulfenyl transfer followed by desulfurative 2,3-allylic rearrangement, promoted by either triphenylphosphine or silver nitrate, to give novel saccharide mimetics. In an alternative embodiment of the same chemistry anomeric thiols are coupled with carbohydrates derivatized in the form of 2-(2-pyridyldithio-3-butenyl) ethers. This new method of glycoligation does not require protection of hydroxyl groups and is compatible with the presence of acetamides, azides, trichloroethoxycarbamates, and thioglycosides. Variations on the general theme enable the preparation of mimetics of reducing and nonreducing oligosaccharides as well as of nonglycosidically linked systems.