Koenigs-Knorr Glycosylation Reaction Catalyzed by Trimethylsilyl Trifluoromethanesulfonate

The 4K reaction

The classical Koenigs-Knorr glycosidation of bromides or chlorides promoted with Ag2O or Ag2CO3 works only with reactive substrates (ideally both donor and acceptor). This reaction was found to be practically ineffective with unreactive donors such as per-O-benzoylated mannosyl bromide. Recently, it was discovered that the addition of catalytic (Lewis) acids to a silver salt-promoted reaction has a dramatic effect on the reaction rate and yield. A tentative mechanism for this cooperatively-catalyzed glycosylation reaction has been proposed, and the improved understanding of the reaction led to more efficient protocols and broader applications to a variety of glycosidic linkages. Since Ag2O-mediated activation was introduced by German chemists Koenigs and Knorr, and "cooperatively catalyzed" is Kooperativ Katalysiert in German, we refer to this new reaction as "the 4K reaction."

Expedient Synthesis of Superarmed Glycosyl Donors via Oxidative Thioglycosidation of Glycals

Superarmed glycosyl donors have higher reactivity compared to their perbenzylated armed counterparts. Generally, the 2-O- benzoyl-3,4,6-tri-O-benzyl protecting group pattern gives rise to increased reactivity due to an O-2/O-5 cooperative effect. Despite having a high reactivity profile and applicability in many expeditious strategies for glycan synthesis, regioselective introduction of the superarming protecting group pattern is tedious for most sugar series. Reported herein is a streamlined synthetic route to yield superarmed glycosyl donors of the d-gluco and d-galacto series equipped with an ethylthio, phenylthio, p-tolylthio, benzoxazol-2-ylthio, O-allyl, or O-pentenyl anomeric leaving group. This streamlined approach was made possible due to the refinement of the oxidative thioglycosylation reaction of the respective glucal and galactal precursors. The applicability of this approach to the direct formation of disaccharides is also showcased.

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Overcoming Challenges in Chemical Glycosylation to Achieve Innovative Vaccine Adjuvants Possessing Enhanced TLR4 Activity

Lipopolysaccharide (LPS) mimicry leading to toll-like receptor 4 (TLR4) active compounds has been so far based mainly on reproducing the lipid A portion of LPS. Our work led to a series of structurally simplified synthetic TLR4 agonists in preclinical development as vaccine adjuvants called FPs. FPs bind MD2/TLR4 similarly to lipid A, inserting the lipid chains in the MD2 lipophilic cavity. A strategy to improve FPs' target affinity is introducing a monosaccharide unit in C6, mimicking the first sugar of the LPS core. We therefore designed a panel of FP derivatives bearing different monosaccharides in C6. We report here the synthesis and optimization of FPs' C6 glycosylation, which presented unique challenges and limitations. The biological activity of glycosylated FP compounds was preliminarily assessed in vitro in HEK-Blue cells. The new molecules showed a higher potency in stimulating TLR4 activation when compared to the parent molecule while maintaining TLR4 selectivity.

Cooperatively Catalyzed Activation of Thioglycosides That Bypasses Intermediacy of Glycosyl Halides

Reported herein is the development of a novel method for activating thioglycosides without a glycosyl halide intermediate. This has been achieved through the use of a silver salt coupled with an acid additive and molecular iodine. The enhanced stereocontrol was achieved via the H-bond mediated aglycone delivery (HAD) method, and the extended trisaccharide synthesis was achieved via iteration of deprotection and glycosylation steps.

Expanding the scope of stereoselective α-galactosylation using glycosyl chlorides

Recently, we reported that silver(I) oxide mediated Koenigs-Knorr glycosylation reaction can be dramatically accelerated in the presence of catalytic acid additives. We have also investigated how well this reaction works in application to differentially protected galactosyl bromides. Reported herein is the stereoselective synthesis of α-galactosides with galactosyl chlorides as glycosyl donors. Chlorides are easily accessible, stable, and can be efficiently activated for glycosylation. In this application, the most favorable reactions conditions comprised cooperative Ag2SO4 and Bi(OTf)3 promoter system.

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

Solvothermal conversion of spent aromatic waste to ethyl glucosides

On-farm extraction of commercially important essential oil from aromatic crops generates huge spent aromatic waste. This massive waste is often disposed in the unregulated landfills or burned in the open air to vacate the fields. Hence, a new method for processing of aromatic spent waste has been developed to obtain platform chemicals, such as, xylose and ethyl glucosides. The thermochemical liquefaction of acid pre-treated palmarosa (cymbopogon martini) biomass furnished a mixture of ethyl glucopyranosides in good yield (∼17 wt% relative to biomass) and selectivity (∼77%) by heating with p-cymen-2-sulphonic acid (p-CSA) in the presence of ethanol as a solvent. The detection, quantification and isolation of ethyl glucosides may provide a new application of spent aromatic biomass for use as a feed stock in the production of value added chemicals.

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.

Indolylthio Glycosides As Effective Building Blocks for Chemical Glycosylation

The S-indolyl (SIn) anomeric moiety was investigated as a new leaving group that can be activated for chemical glycosylation under a variety of conditions including thiophilic and metal-assisted pathways. Understanding of the reaction pathways for the SIn moiety activation was achieved via the extended mechanistic study. Also reported is how the new SIn donors fit into selective activation strategies for oligosaccharide synthesis.

Stereocontrolled α-Galactosylation under Cooperative Catalysis

A recent discovery of a cooperative catalysis comprising a silver salt and an acid led to a dramatic improvement in the way glycosyl halides are glycosidated. Excellent yields have been achieved, but the stereoselectivity achieved with 2-O-benzylated donors was poor. Reported herein is our first attempt to refine the stereoselectivity of the cooperatively catalyzed galactosylation reaction. Careful optimization of the reaction conditions along with studying effects of the remote protecting groups led to excellent stereocontrol of α-galactosylation of a variety of glycosyl acceptors with differentially protected galactosyl donors.

A Highly Efficient Glycosidation of Glycosyl Chlorides by Using Cooperative Silver(I) Oxide-Triflic Acid Catalysis

Following our discovery that silver(I) oxide-promoted glycosylation with glycosyl bromides can be greatly accelerated in the presence of catalytic TMSOTf or TfOH, we report herein a new discovery that glycosyl chlorides are even more effective glycosyl donors under these reaction conditions. The developed reaction conditions work well with a variety of glycosyl chlorides. Both benzoylated and benzylated chlorides have been successfully glycosidated, and these reaction conditions proved to be effective in coupling substrates containing nitrogen and sulfur atoms. Another convenient feature of this glycosylation is that the progress of the reaction can be monitored visually; its completion can be judged by the disappearance of the characteristic dark color of Ag2 O.

Diastereoselective sp3 C-O Bond Formation via Visible Light-Induced, Copper-Catalyzed Cross-Couplings of Glycosyl Bromides with Aliphatic Alcohols

Copper-catalyzed cross-coupling reactions have become one of the most powerful methods for generating carbon-heteroatom bonds, an important framework of many organic molecules. However, copper-catalyzed C(sp³)-O cross-coupling of alkyl halides with alkyl alcohols remains elusive because of the sluggish nature of oxidative addition to copper. To address this challenge, we have developed a catalytic copper system, which overcomes the copper oxidative addition barrier with the aid of visible light and effectively facilitates the cross-couplings of glycosyl bromides with aliphatic alcohols to afford C(sp³)-O bonds with high levels of diastereoselectivity. Importantly, this catalytic system leads to a mild and efficient method for stereoselective construction of α-1,2-cis glycosides, which are of paramount importance, but challenging. In general, stereochemical outcomes in α-1,2-cis glycosidic C-O bond-forming processes are unpredictable and dependent on the steric and electronic nature of protecting groups bound to carbohydrate coupling partners. Currently, the most reliable approaches rely on the use of a chiral auxiliary or hydrogen-bond directing group at the C2- and C4-position of carbohydrate electrophiles to control α-1,2-cis selectivity. In our approach, earth-abundant copper not only acts as a photocatalyst and a bond-forming catalyst, but also enforces the stereocontrolled formation of anomeric C-O bonds. This cross-coupling protocol enables highly diastereoselective access to a wide variety of α-1,2-cis-glycosides and biologically relevant α-glycan oligosaccharides. Our work provides a foundation for developing new methods for the stereoselective construction of natural and unnatural anomeric carbon(sp³)-heteroatom bonds.

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.

Bromine-Promoted Glycosidation of Conformationally Superarmed Thioglycosides

Presented herein is a study of the conformation and reactivity of highly reactive thioglycoside donors. The structural studies have been conducted using NMR spectroscopy and computational methods. The reactivity of these donors has been investigated in bromine-promoted glycosylations of aliphatic and sugar alcohols. Swift reaction times, high yields, and respectable 1,2-cis stereoselectivity were observed in a majority of these glycosylations.