Unusual Outcome of Glycosylation: Hydrogen‐Bond Mediated Control of Stereoselectivity by N ‐Trifluoroacetyl Group?

Glycosylation of an N-Acetylated Glucosamine Disaccharide Using an Orthogonally Protected 3-Iodo-Kdo Fluoride Donor

Kdo (3-Deoxy-d-manno-oct-2-ulosonic acid) is an essential sugar found in bacterial lipopolysaccharides with significant biomedical relevance. This study introduces an orthogonally protected 3-iodo-Kdo fluoride donor and demonstrates its coupling to a pre-synthesized β-(1→6)-linked N-acetylglucosamine disaccharide acceptor as an example. Nuclear magnetic resonance data indicates the presence of an intraresidue hydrogen bond in the distal glucosamine unit. Two complementary glycosylation approaches are explored with an emphasis on achieving high stereoselectivity and minimizing protecting-group manipulation. The orthogonal protection of 3-iodo Kdo fluoride donor offers insights into tailoring Kdo-based donors for specific biomedical applications. While yields vary depending on the approach, they are sufficient to demonstrate the donor's applicability. These findings enable the design of advanced glycomimetic constructs for therapeutic and vaccine research.

Stereocontrolled 1,2-trans-arabinofuranosylation in the absence of 2-O-acyl group in glycosyl donor

Stereocontrolled 1,2-trans-α-arabinofuranosylation using polysilylated mono- and disaccharide glycosyl donors was investigated. A complete α-stereoselectivity of 1,2-trans-arabinofuranosylation was found for Ara-β-(1 → 2)-Ara disaccharide glycosyl donors containing five triisopropylsilyl (TIPS) groups with arylthiol (1) (as shown in our previous publications) or N-phenyltrifluoroacetimidoyl (2) (this work) leaving groups. Conversely, in case of monosaccharide thioglycosides polysilylated with acyclic silyl groups (TIPS, TBDPS), stereoselectivity of glycosylation was lower (α:β = 7-8:1), although the desired α-isomer still dominated. Disaccharide glycosyl donor 2 was successfully used in the synthesis of linear α-(1 → 5)-, β-(1 → 2)-linked hexaarabinofuranoside useful for further preparation of conjugates thereof as antigens valuable for the diagnosis of mycobacterioses.

Unusual triflic acid-promoted oligomerization of arabinofuranosides during glycosylation

We discovered an unusual triflic acid-promoted oligomerization of arabinofuranosides during glycosylation of the primary hydroxy group of α-(1 → 5)-linked tetraarabinofuranoside bearing 4-(2-chloroethoxy)phenyl aglycone with α-(1 → 5), β-(1 → 2)-linked tetraarabinofuranoside containing N-phenyltrifluoroacetimidoyl leaving group, which led to octa-, dodeca- and hexadecaarabinofuranosides. The possible mechanism of triflic acid-promoted oligomerization was proposed. The choice of promoter was found to be a critical factor for the discovered oligomerization of arabinofuranosides. The obtained octa-, dodeca- and hexadecaarabinofuranosides may serve as useful blocks in the synthesis of oligosaccharide fragments of polysaccharides of Mycobacterium tuberculosis.

TIPS group-assisted isomerization of benzyl protected d-manno- and d-glucopyranose to d-fructofuranose derivatives

Base-promoted (MeONa in MeOH or imidazole in DMF) isomerization of a series of 3,4,6-tri-O-benzyl-d-gluco- and d-mannopyranose derivatives with triisopropylsilyl (TIPS) substituents was studied. The presence of a bulky TIPS group at O-1 or O-2 was shown to be favorable for the isomerization of benzyl protected d-gluco- and d-mannopyranose derivatives to d-fructofuranose derivatives, in which the bulky silyl group occupies less sterically hindered primary position. The highest yield (33%) of the fructofuranose derivative was achieved when 3,4,6-tri-O-benzyl-2-O-triisopropylsilyl-d-mannopyranose was treated with MeONa in MeON at 50 °C.

Synthesis and interaction with growth factors of sulfated oligosaccharides containing an anomeric fluorinated tail

Compounds that mimic the biological properties of glycosaminoglycans (GAGs) and can be more easily prepared than the native GAG oligosaccharides are highly demanded. Here, we present the synthesis of sulfated oligosaccharides displaying a perfluorinated aliphatic tag at the reducing end as GAG mimetics. The preparation of these molecules was greatly facilitated by the presence of the fluorinated tail since the reaction intermediates were isolated by simple fluorous solid-phase extraction. Fluorescence polarization competition assays indicated that the synthesized oligosaccharides interacted with two heparin-binding growth factors, midkine (MK) and FGF-2, showing higher binding affinities than the natural oligosaccharides, and can be therefore considered as useful GAG mimetics. Moreover, NMR experiments showed that the 3D structure of these compounds is similar to that of the native sequences, in terms of sugar ring and glycosidic linkage conformations. Finally, we also demonstrated that these derivatives are able to block the MK-stimulating effect on NIH3T3 cells growth.

Exploiting non-covalent interactions in selective carbohydrate synthesis

Non-covalent interactions (NCIs) are a vital component of biological bond-forming events, and have found important applications in multiple branches of chemistry. In recent years, the biomimetic exploitation of NCIs in challenging glycosidic bond formation and glycofunctionalizations has attracted significant interest across diverse communities of organic and carbohydrate chemists. This emerging theme is a major new direction in contemporary carbohydrate chemistry, and is rapidly gaining traction as a robust strategy to tackle long-standing issues such as anomeric and site selectivity. This Review thus seeks to provide a bird's-eye view of wide-ranging advances in harnessing NCIs within the broad field of synthetic carbohydrate chemistry. These include the exploitation of NCIs in non-covalent catalysed glycosylations, in non-covalent catalysed glycofunctionalizations, in aglycone delivery, in stabilization of intermediates and transition states, in the existence of intramolecular hydrogen bonding networks and in aggregation by hydrogen bonds. In addition, recent emerging opportunities in exploiting halogen bonding and other unconventional NCIs, such as CH-π, cation-π and cation-n interactions, in various aspects of carbohydrate chemistry are also examined.