One-pot α-glycosylation pathway via the generation in situ of α-glycopyranosyl imidates in N,N-dimethylformamide

Solvent-free, under air selective synthesis of α-glycosides adopting glycosyl chlorides as donors

α-Glycosides are highly relevant synthetic targets due to their abundance in natural oligosaccharides involved in many biological processes. Nevertheless their preparation is hampered by several issues, due to both the strictly anhydrous conditions typically required in glycosylation procedures and the non-trivial achievement of high α-stereoselectivity, one of the major challenges in oligosaccharide synthesis. In this paper we report a novel and efficient approach for the highly stereoselective synthesis of α-glycosides. This is based on the unprecedented solvent-free combination of triethylphosphite, tetrabutylammonium bromide and N,N-diisopropylethylamine for the activation of glycosyl chlorides under air. Despite the relative stability of glycosyl chlorides with respect to more reactive halide donors, the solvent-free procedure allowed a wide set of α-glycosides, including biorelevant fragments, to be obtained in much shorter times compared with similar glycosylation approaches in solution. The presented method features a wide target scope and functional group compatibility, also serving with partially disarmed substrates, and it does not require a high stoichiometric excess of reagents nor the preparation of expensive precursors. The solvent-free glycosylation can be even directly performed from 1-hydroxy sugars without purification of the in situ generated chloride, providing an especially useful opportunity in the case of highly reactive and labile glycosyl donors.

Bismuth(iii) triflate as a novel and efficient activator for glycosyl halides

Presented herein is the discovery that bismuth(iii) trifluoromethanesulfonate (Bi(OTf)3) is an effective catalyst for the activation of glycosyl bromides and glycosyl chlorides. The key objective for the development of this methodology is to employ only one promoter in the lowest possible amount and to avoid using any additive/co-catalyst/acid scavenger except molecular sieves. Bi(OTf)3 works well in promoting the glycosidation of differentially protected glucosyl, galactosyl, and mannosyl halides with many classes of glycosyl acceptors. Most reactions complete within 1 h in the presence of only 35% of green and light-stable Bi(OTf)3 catalyst.

Reagent controlled stereoselective synthesis of teichoic acid α-(1,2)-glucans

The stereoselective construction of 1,2-cis-glycosidic linkages is key in the assembly of biologically relevant glycans, but remains a synthetic challenge. Reagent-controlled glycosylation methodologies, in which external nucleophiles are employed to modulate the reactivity of the glycosylation system, have become powerful means for the construction of 1,2-cis-glycosidic linkages. Here we establish that nucleophilic additives can support the construction of α-1,2-glucans, and apply our findings in the construction of a d-alanine kojibiose functionalized glycerol phosphate teichoic acid fragment. This latter molecule can be found in the cell wall of the opportunistic Gram-positive bacterium, Enterococcus faecalis and represents a structural element that can possibly be used in the development of therapeutic vaccines and diagnostic tools.

Defining the Scope of the Acid-Catalyzed Glycosidation of Glycosyl Bromides

Following the recent discovery that traditional silver(I) oxide-promoted glycosidations of glycosyl bromides (Koenigs-Knorr reaction) can be greatly accelerated in the presence of catalytic TMSOTf, reported herein is a dedicated study of all major aspects of this reaction. A thorough investigation of numerous silver salts and careful refinement of the reaction conditions led to an improved mechanistic understanding. This, in turn, led to a significant reduction in the amount of silver salt required for these glycosylations. The progress of this reaction can be monitored by naked eye, and the completion of the reaction can be judged by the disappearance of characteristic dark color of Ag2 O. Further evidence on higher reactivity of benzoylated α-bromides in comparison to that of their benzylated counterparts has been acquired.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Glycosylation intermediates studied using low temperature1H- and19F-DOSY NMR: new insight into the activation of trichloroacetimidates

Low temperature¹H- and¹⁹F-DOSY have been used for analyzing reactive intermediates in glycosylation reactions, where a glycosyl trichloroacetimidate donor has been activated using different catalysts.

Modulating glycosylation with exogenous nucleophiles: an overview

The major challenge in carbohydrate synthesis is stereochemical control of glycosidic bond formation. Different glycosylation methods have been developed that are based on the modulation effect of external nucleophiles. This review highlights the development, synthetic application, challenges and outlook of the modulated glycosylation methods.

Selective synthesis of 1,2-cis-α-glycosides without directing groups. Application to iterative oligosaccharide synthesis

A method for the highly selective synthesis of 1,2-cis-α-linked glycosides that does not require the use of the specialized protecting group patterns normally employed to control diastereoselectivity is described. Thioglycoside acceptors can be used, permitting iterative oligosaccharide synthesis. The approach eliminates the need for lengthy syntheses of monosaccharides possessing highly specialized and unconventional protecting group patterns.

Straightforward synthesis of novel 1-(2'-α-O-D-glucopyranosyl ethyl) 2-arylbenzimidazoles

A series of novel 1-(2'-α-O-D-glucopyranosyl ethyl) 2-arylbenzimidazoles has been prepared via one-pot glycosylation of ethyl-1-(2'-hydroxyethyl)-2-arylbenzimidazole-5-carboxylate derivatives. Synthesis of the 2-arylbenzimidazole aglycones from 4-fluoro-3-nitrobenzoic acid was accomplished in four high-yielding steps. The reduction and cyclocondensation steps for the aglycone synthesis proceeded efficiently under microwave irradiation to afford the appropriate benzimidazoles in excellent yields within 2-3 min. Glycosylation of the hydroxyethyl aglycones with the perbenzylated 1-hydroxy- glucopyranose, pretreated with the Appel-Lee reagent, followed by catalytic hydrogenolysis delivered the desired 1-(2'-α-O-D-glucopyranosyl ethyl) 2-aryl-benzimidazoles in a simple and straightforward manner.

An easy α-glycosylation methodology for the synthesis and stereochemistry of mycoplasma α-glycolipid antigens

Mycoplasma fermentans possesses unique α-glycolipid antigens (GGPL-I and GGPL-III) at the cytoplasm membrane, which carry a phosphocholine group at the sugar primary (6-OH) position. This paper describes a practical synthetic pathway to a GGPL-I homologue (C(16:0)) and its diastereomer, in which our one-pot α-glycosylation method was effectively applied. The synthetic GGPL-I isomers were characterized with (1)H NMR spectroscopy to determine the equilibrium among the three conformers (gg, gt, tg) at the acyclic glycerol moiety. The natural GGPL-I isomer was found to prefer gt (54%) and gg (39%) conformers around the lipid tail, while adopting all of the three conformers with equal probability around the sugar position. This property was very close to what we have observed with respect to the conformation of phosphatidylcholine (DPPC), suggesting that the Mycoplasma glycolipids GGPLs may constitute the cytoplasm fluid membrane together with ubiquitous phospholipids, without inducing stereochemical stress.

Dehydrative glycosylation with the Hendrickson reagent

The Hendrickson reagent is able to perform efficiently dehydrative glycosylation of 1-hydroxyglycosyl donors. The reaction occurs under mild conditions through an anomeric oxophosphonium intermediate detected by nuclear magnetic resonance. Further insight into the mechanism was gained by (18)O labeling of anomeric OH.

Appel–Lee synthesis of glycosyl inositols, substrates for inositol dehydrogenase from Bacillus subtilis

We recently reported that inositol dehydrogenase (EC 1.1.1.18) from Bacillus subtilis can catalyze the highly stereoselective oxidation of 1l-4-O-substituted myo-inositol derivatives, as well as disaccharides melibiose and isomaltose, but not gentiobiose or maltose, consistent with the requirement of an alpha-(1-->6) linkage. We believed that the enzyme might therefore catalyze efficient stereoselective oxidation of the appropriate alpha-linked glycosyl inositols. We have synthesized alpha-D-glucopyranosyl-(1-->4)-(DL)-myo-inositol and alpha-d-galactopyranosyl-(1-->4)-(DL)-myo-inositol using the Appel-Lee protocol to couple benzyl-protected glycosyl donors to protected inositols. This method failed in our hands using glycosyl donors derived from D-mannose and 2-azido-2-deoxy-D-glucose. When myo-inositol 1,3,5-monoorthoformate is used as the acceptor, the reaction is regiospecific for the 4/6-position. We report here the mildest conditions known for the removal of the orthoformate group. 2-Azido-2-deoxy-alpha-D-glucopyranosyl-(1-->4)-(DL)-myo-inositol was synthesized using the trichloroacetimidate derivative as the donor, and all three pseudo-disaccharides were substrates for inositol dehydrogenase. The glucopyranosyl and galactopyranosyl derivatives displayed apparent second-order rate constants comparable to that of myo-inositol.