One-pot synthesis of orthogonally protected sugars through sequential base-promoted/acid-catalyzed steps: A solvent-free approach with self-generation of a catalytic species

Formal Single Atom Editing of the Glycosylated Natural Product Fidaxomicin Improves Acid Stability and Retains Antibiotic Activity

Fidaxomicin (Fdx) constitutes a glycosylated natural product with excellent antibacterial activity against various Gram-positive bacteria but is approved only for Clostridioides difficile infections. Poor water solubility and acid lability preclude its use for other infections. Herein, we describe our strategy to overcome the acid lability by introducing acid-stable S-linked glycosides. We describe the direct, diastereoselective modification of unprotected Fdx without the need to avoid air or moisture. Using our newly established approach, Fdx was converted to the single atom exchanged analogue S-Fdx, in which the acid labile O-glycosidic bond to the noviose sugar was replaced by the acid stable S-glycosidic bond. Studies of the antibacterial activity of a structurally diverse set of thioglycoside derivatives revealed high potency of acyl derivatives of S-Fdx against Clostridioides difficile (MIC range: 0.12-4 μg/mL) and excellent potency against Clostridium perfringens (MIC range: 0.06-0.5 μg/mL).

Orthogonal Glycosylation with Phosphate Acceptors for Expeditious Synthesis of Bacterial Inner Core Oligosaccharides

We report a practical one-pot glycosylation strategy for synthesis of bacterial inner core oligosaccharides that composed of unavailable L-glycero-D-manno and D-glycero-D-manno-heptopyranose components. The glycosylation method features a new orthogonal glycosylation procedure; whereby a phosphate acceptor is coupled with a thioglycosyl donor producing a disaccharide phosphate, which can be engaged in another orthogonal glycosylation procedure to couple with a thioglycosyl acceptor. The phosphate acceptors used in above one-pot procedure are directly prepared from thioglycosyl acceptors via the in-situ phosphorylation. Such phosphate acceptor preparation protocol eliminates the traditional protection and deprotection procedures. Based on the new one-pot glycosylation strategy, two partial inner core structures of Yersinia pestis lipopolysaccharide and Haemophilus ducreyi lipooligosaccharide were acquired.

Multi-step Strategies Toward Regioselectively Sulfated M-Rich Alginates

Sulfated alginates (ASs), as well as several artificially sulfated polysaccharides, show interesting bioactivities. The key factors for structure-activity relationships studies are the degree of sulfation and the distribution of the sulfate groups along the polysaccharide backbone (sulfation pattern). The former parameter can often be controlled through stoichiometry, while the latter requires the development of suitable chemical or enzymatic, regioselective methods and is still missing for ASs. In this work, a study on the regioselective installation of several different protecting groups on a d-mannuronic acid enriched (M-rich) alginate is reported in order to develop a semi-synthetic access to regioselectively sulfated AS derivatives. A detailed structural characterization of the obtained ASs revealed that the regioselective sulfation could be achieved complementarily at the O-2 or O-3 positions of M units through multi-step sequences relying upon a silylating or benzoylating reagent for the regioselective protection of M-rich alginic acid, followed by sulfation and deprotection.

Self-esterified hyaluronan hydrogels: Advancements in the production with positive implications in tissue healing

Hyaluronan-(HA) short half-life in vivo limits its benefits in tissue repair. Self-esterified-HA is of great interest because it progressively releases HA, promoting tissue-regeneration longer than the unmodified-polymer. Here, the 1-ethyl-3-(3-diethylaminopropyl)carbodiimide(EDC)-hydroxybenzotriazole(HOBt) carboxyl-activating-system was evaluated for self-esterifying HA in the solid state. The aim was to propose an alternative to the time-consuming, conventional reaction of quaternary-ammonium-salts of HA with hydrophobic activating-systems in organic media, and to the EDC-mediated reaction, limited by by-product formation. Additionally, we aimed to obtain derivatives releasing defined molecular-weight(MW)-HA that would be valuable for tissue renewal. A 250 kDa-HA(powder/sponge) was reacted with increasing EDC/HOBt amounts. HA-modification was investigated through Size-Exclusion-Chromatography-Triple-Detector-Array-analyses, FT-IR/¹H NMR and the products(XHAs) extensively characterized. Compared to conventional protocols, the set procedure is more efficient, avoids side-reactions, allows for an easier processing to diverse clinically-usable 3D-forms, leads to products gradually releasing HA under physiological conditions with the possibility to tune the MW of the biopolymer-released. Finally, the XHAs exhibit sound stability to Bovine-Testicular-Hyaluronidase, hydration/mechanical properties suitable for wound-dressings, with improvements over available matrices, and prompt in vitro wound-regeneration, comparably to linear-HA. To the best of our knowledge, the procedure is the first valid alternative to conventional protocols for HA self-esterification with advances in the process itself and in product performance.

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.

Catalytic, Regioselective Sulfonylation of Carbohydrates with Dibutyltin Oxide under Solvent-Free Conditions

A simple approach was developed for the solvent-free regioselective functionalization of carbohydrate polyols with 4-toluesulfonyl (tosyl) group, allowing the easy and quick activation of a saccharide site with a tosylate leaving group. The method is based on the use of catalytic dibutyltin oxide and tetrabuylammonium bromide (TBAB), and a moderate excess of N,N-diisopropylethyl amine (DIPEA) and tosyl chloride (TsCl), leading to the selective functionalization at 75 °C of a secondary equatorial hydroxy function flanked by an axial one in a pyranoside. The procedure is endowed with several advantages, such as the use of cheap reagents, experimental simplicity, and the need for reduced reaction times in comparison with other known approaches.

Solvent-Free Approaches in Carbohydrate Synthetic Chemistry: Role of Catalysis in Reactivity and Selectivity

Owing to their abundance in biomass and availability at a low cost, carbohydrates are very useful precursors for products of interest in a broad range of scientific applications. For example, they can be either converted into basic chemicals or used as chiral precursors for the synthesis of potentially bioactive molecules, even including nonsaccharide targets; in addition, there is also a broad interest toward the potential of synthetic sugar-containing structures in the field of functional materials. Synthetic elaboration of carbohydrates, in both the selective modification of functional groups and the assembly of oligomeric structures, is not trivial and often entails experimentally demanding approaches practiced by specialized groups. Over the last years, a large number of solvent-free synthetic methods have appeared in the literature, often being endowed with several advantages such as greenness, experimental simplicity, and a larger scope than analogous reactions in solution. Most of these methods are catalytically promoted, and the catalyst often plays a key role in the selectivity associated with the process. This review aims to describe the significant recent contributions in the solvent-free synthetic chemistry of carbohydrates, devoting a special critical focus on both the mechanistic role of the catalysts employed and the differences evidenced so far with corresponding methods in solution.