Highly Diastereoselective Thioglycosylation of Functionalized Peracetylated Glycosides Catalyzed by MoO2Cl2

Expedient Azide-Alkyne Huisgen Cycloaddition Catalyzed by a Combination of VOSO4 with Cu(0) in Aqueous Media

A series of vanadium(III), vanadyl(IV/V) species, inorganic metal oxides, and transition-metal oxides was examined as cocatalysts with Cu(0) powder for copper(I)-catalyzed azide-alkyne cycloaddition. Among them, vanadyl(IV) species bearing acetylacetonate, acetate, and sulfate, vanadyl(V) isopropoxide, and vanadate were suitable for the click reactions of per-acetyl and per-benzyl β-azido glycosides with three different terminal alkynes in CH3CN. Water-soluble vanadyl(IV) sulfate was further selected for efficient click reactions for unprotected β-glycosyl azides and even compatible with a thiol-containing substrate in aqueous media at ambient temperature.

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

Straightforward Synthesis of the Poly(ADP-ribose) Branched Core Structure

Poly(ADP-ribosyl)ation is a post-translational modification that produces poly(ADP-ribose) with a branched structure every 20-50 units; such branching structure has been previously suggested to be involved in regulating chromatin remodeling. To elucidate its detailed functions, we developed a straightforward method for the synthesis of the poly(ADP-ribose) branched core structure, α-d-ribofuranosyl-(1‴ → 2″)-α-d-ribofuranosyl-(1″ → 2')-adenosine 5',5'',5‴-trisphosphate 1, from 6-chloropurine ribofuranoside 4 in 10 steps and 6.1% overall yield. The structure poses synthetic challenges for constructing iterative α-1,2-cis-glycosidic bonds in the presence of a purine base and the installation of three phosphate groups at primary hydroxyl groups. Iterative glycosidic bonds were formed by α-1,2-cis-selective ribofuranosylation using 2-O-(2-naphthylmethyl)-protected thioglycoside donor 6 and a thiophilic bismuth promoter. After the construction of diribofuranosyl adenosine 5 had been constructed, it was chemo- and regioselectively phosphorylated at a later stage. Subsequent deprotection provided the synthetic target 1.

Copper(I)-Catalyzed Sandmeyer-Type S-Arylation of 1-Thiosugars with Aryldiazonium Salts under Mild Conditions

Preparation of S-aryl thioglycosides from 1-thiosugars via S-arylation was demonstrated under mild reaction conditions. A wide range of protected and unprotected 1-thiosugars derived from glucose, glucosamine, galactose, mannose, ribose, maltose, and lactose underwent cross-coupling reactions with functionalized aryldiazonium salts in the presence of copper(I) chloride and DBU. The desired products were obtained in 55-88% yields within 5 min. Various functional groups, including halogens, were tolerated under standard reaction conditions. Synthesis of the biologically relevant antidiabetic dapagliflozin S-analogue and arbutin S-analogues (tyrosinase inhibitors) was demonstrated.

Solvent free, catalytic and diastereoselective preparation of aryl and alkyl thioglycosides as key components for oligosaccharide synthesis

A new and environmentally friendly protocol for the conversion of sugar per-acetates into thioglycosides under solvent free and catalytic conditions is presented. The procedure involves heating in the presence of InCl₃ and various aryl thiols. For alkyl thioglycoside synthesis, cyclohexane thiol was found to give good results and yield a glycosyl donor with reactivity similar to a thioethyl congener. The established optimum reaction conditions were found to provide the desired thioglycoside products in an easy and highly diastereoselective manner even when conducted on a multigram scale.

Boron-Promoted Umpolung Reaction of Sulfonyl Chlorides for the Stereospecific Synthesis of Thioglycosides via Reductive Deoxygenation Coupling Reactions

S-Glycosides have broad biological activities and serve as stable mimics of natural O-glycoside counterparts and thus are of great therapeutic potential. Herein we disclose an efficient method for the stereospecific synthesis of 1-thioglycosides via a boron-promoted reductive deoxygenation coupling reaction from readily accessible sulfonyl chlorides and glycosyl bromides. Our protocol features mild conditions and excellent functional group tolerance and stereoselectivity. The translational potential of this metal-free approach is demonstrated by the late-stage glycodiversification of natural products and drug molecules.

Photooxidation of thiosaccharides mediated by sensitizers in aerobic and environmentally friendly conditions

A series of β-d-glucopyranosyl derivates have been synthesized and evaluated in photooxidation reactions promoted by visible light and mediated by organic dyes under aerobic conditions. Among the different photocatalysts employed, tetra-O-acetyl riboflavin afforded chemoselectively the respective sulfoxides, without over-oxidation to sulfones, in good to excellent yields and short reaction times. This new methodology for the preparation of synthetically useful glycosyl sulfoxides constitutes a catalytic, efficient, economical, and environmentally friendly oxidation process not reported so far for carbohydrates.

One-pot construction of carbohydrate scaffolds mediated by metal catalysts

Owing to the environmental concern worldwide and also due to cost, time and labour issues, use of one-pot reactions [domino/cascade/tandem/multi-component (MC) or sequential] has gained much attention among the scientific and industrial communities for the generation of compound libraries having different scaffolds. Inclusion of sugars in such compounds is expected to increase the pharmacological efficacy because of the possibility of better interactions with the receptors of such unnatural glycoconjugates. In many of the one-pot transformations, the presence of a metal salt/complex can improve the reaction/change the course of reaction with remarkable increase in chemo-/regio-/stereo-selectivity. On the other hand because of the importance of natural polymeric glycoconjugates in life processes, the development and efficient synthesis of related oligosaccharides, particularly utilising one-pot MC-glycosylation techniques are necessary. The present review is an endeavour to discuss one-pot transformations involving carbohydrates catalysed by a metal salt/complex.

Triflic acid-mediated synthesis of thioglycosides

An efficient synthesis of thioglycosides from per-acetates in the presence of triflic acid is described. The developed protocol features high reaction rates and product yields. Some reactive sugar series give high efficiency in the presence of sub-stoichiometric trifluoromethanesulfonic acid (TfOH) in contrast to other known protocols that require multiple equivalents of Lewis acids to reach high conversion rates.

Efficient one-pot per-O-acetylation–thioglycosidation of native sugars, 4,6-O-arylidenation and one-pot 4,6-O-benzylidenation–acetylation of S-/O-glycosides catalyzed by Mg(OTf)2

Sequential one-pot per-O-acetylation–S-/O-glycosidation under neat condition, regioselective 4,6-O-arylidenation and sequential one-pot benzylidenation–acetylation of Mg(OTf)₂as non-hygroscopic, recyclable catalyst are reported.

Chemistry of xylopyranosides

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

Synthesis and biological evaluation of a novel MUC1 glycopeptide conjugate vaccine candidate comprising a 4'-deoxy-4'-fluoro-Thomsen-Friedenreich epitope

The development of selective anticancer vaccines that provide enhanced protection against tumor recurrence and metastasis has been the subject of intense research in the scientific community. The tumor-associated glycoprotein MUC1 represents a well-established target for cancer immunotherapy and has been used for the construction of various synthetic vaccine candidates. However, many of these vaccine prototypes suffer from an inherent low immunogenicity and are susceptible to rapid in vivo degradation. To overcome these drawbacks, novel fluorinated MUC1 glycopeptide-BSA/TTox conjugate vaccines have been prepared. Immunization of mice with the 4’F-TF-MUC1-TTox conjugate resulted in strong immune responses overriding the natural tolerance against MUC1 and producing selective IgG antibodies that are cross-reactive with native MUC1 epitopes on MCF-7 human cancer cells.

β-Xylopyranosides: synthesis and applications

In recent years, β-xylopyranosides have attracted interest due to the development of biomass-derived molecules. This review focuses on general routes for the preparation of β-xylopyranosides by chemical and enzymatic pathways and their main uses.

In(III) triflate-mediated solvent-free synthesis and activation of thioglycosides by ball milling and structural analysis of long chain alkyl thioglycosides by TEM and quantum chemical methods

Conventional solution-phase synthesis of thioglycosides from glycosyl acetates and thiols in the presence of In(III) triflate as reported for benzyl thioglucoside failed when applied to the synthesis of phenolic and alkyl thioglycosides. But, it was achieved in high efficiency and diastereospecificity with ease by solvent-free grinding in a ball mill. The acetates in turn were also obtained by the homogenization of free sugars with stoichiometric amounts of acetic anhydride and catalytic In(OTf)3 in the mill as neat products. Per-O-benzylated thioglycosides on grinding with an acceptor sugar in the presence of In(OTf)3 yield the corresponding O-glycosides efficiently. The latter in the case of a difficult secondary alcohol was nearly exclusive (>98%) in 1,2-cis-selectivity. In contrast, the conventional methods for this purpose require use of a coreagent such as NIS along with the Lewis acid to help generate the electrophilic species that actually is responsible for the activation of the thioglycoside donor in situ. The distinctly different self-assembling features of the peracetylated octadecyl 1-thio-α- and β-D-galactopyranosides observed by TEM could be rationalized by molecular modeling.

Appel-reagent-mediated transformation of glycosyl hemiacetal derivatives into thioglycosides and glycosyl thiols

A series of glycosyl hemiacetal derivatives have been transformed into thioglycosides and glycosyl thiols in a one-pot two-step reaction sequence mediated by Appel reagent (carbon tetrabromide and triphenylphosphine). 1,2-trans-Thioglycosides and β-glycosyl thiol derivatives were stereoselectively formed by the reaction of the in situ generated glycosyl bromides with thiols and sodium carbonotrithioate. The reaction conditions are reasonably simple and yields were very good.

Iron(III) chloride as an efficient catalyst for stereoselective synthesis of glycosyl azides and a cocatalyst with Cu(0) for the subsequent click chemistry

A highly efficient and mild method for azido glycosylation of glycosyl β-peracetates to 1,2-trans glycosyl azides was developed by using inexpensive FeCl(3) as the catalyst. In addition, we demonstrated, for the first time, that FeCl(3) in combination with copper powder can promote 1,3-dipolar cycloaddition (click chemistry) of azido glycosides with terminal alkynes. Good to excellent yields were obtained with exclusive formation of a single isomer in both glycosyl azidation and subsequent cycloaddition processes.

Thioglycosylation of 1,2-cis-glycosyl acetates: a long-standing overlooked issue in preparative carbohydrate chemistry

1,2-cis-Glycosyl acetates with the α-configuration were revealed to be very unreactive towards Lewis acid catalyzed thioglycosylations. Optimal and cost-effective conditions for enabling this direct conversion is absent in the literature. Our studies have shown that elevating the reaction temperature with a catalytic amount of BF(3)·OEt(2) was more effective than changing Lewis acids with higher acidities to accommodate the low reactivity of α-glycosyl acetates. The effect of impurities in stored BF(3)·OEt(2) on the reaction is also discussed.

Thio-arylglycosides with various aglycon para-substituents: a probe for studying chemical glycosylation reactions

Three series of thioglycosyl donors differing only in their respective aglycon substituents within each series have been prepared as representatives of typical glycosyl donors. The relative anomeric reactivities of these donors were quantified under competitive glycosylation conditions with various reaction time, promoters, solvents and acceptors. Over three orders of magnitude reactivity difference were generated by simple transformation of the para-substituent on the aglycon with methanol as the acceptor, while chemoselectivities became lower with carbohydrate acceptors. Excellent linear correlations were attained between relative reactivity values of donors and sigma(p) values of the substituents in the Hammett plots. This indicates that the glycosylation mechanism remains the same over a wide range of reactivities and glycosylation conditions. The negative slopes of the Hammett plots suggested that electron donating substituents expedite the reactions and the magnitudes of slopes can be rationalized by neighboring group participation as well as electronic properties of the glycon protective groups. Within the same series of donors, less nucleophilic acceptors gave smaller slopes in their Hammett plots. This is consistent with the notion that acceptor nucleophilic attack onto the reactive intermediate is part of the rate limiting step of the glycosylation reaction.

Catalytic conjugate additions of nitrogen-, phosphorus-, and carbon-containing nucleophiles by amphoteric vanadyl triflate

A series of carbamates, amides, N-tosyl amides, (hetero)arenes, and hydrogen phosphines/phosphites has been examined as nucleophiles for (hetero)Michael-type additions to enones and enamides catalyzed by amphoteric vanadyl triflate under mild and neutral conditions. The newly developed C-N, C-P, and C-C bond-formation protocols were carried out smoothly in good to high yields without intervention of any 1,2-additions.