Stereoselective and controllable C3-Amination or C1,C3-di-amination of 2-nitroglycals

Precise and selective modification of carbohydrates is a critical strategy in producing diverse carbohydrate derivatives for exploiting their functions. We disclosed a simple, efficient, and highly regioselective and stereoselective protocol to controllable amination of 2-nitroglycals under mild conditions in 5 min. A range of 3-amino-carbohydrates including 3-arylamino-2-nitro-glycals and 1,3-di-amino-carbohydrate derivatives were obtained in good to excellent yield with excellent stereoselectivity. The produced 3-amino-2-nitro-glycals can be used as a precursor for further transformation.

A Short Route to the Synthesis of Digoxose Trisaccharide Glycal Donor via Mislow-Evans Rearrangement

The Mislow-Evans rearrangement was used as a key reaction to construct digitoxose-derived glycals. The same rearrangement was iteratively performed on di- and trisaccharides to form the digoxose glycal donor component present in the cardenolides digitoxin, digoxin, and gitoxin. The scalability of the trisaccharide synthesis was shown by performing the reactions on a multigram scale. Glycosylation reactions were also performed between the synthesized digoxin glycal donor and aglycons digoxigenin and gitoxigenin to synthesize novel cardenolide derivatives.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Ferrier/Aza-Wacker/Epoxidation/Glycosylation (FAWEG) Sequence to Access 1,2-Trans 3-Amino-3-deoxyglycosides

3-Amino-3-deoxyglycosides constitute an essential class of nitrogen-containing sugars. Among them, many important 3-amino-3-deoxyglycosides possess a 1,2-trans relationship. In view of their numerous biological applications, the synthesis of 3-amino-3-deoxyglycosyl donors giving rise to a 1,2-trans glycosidic linkage is thus an important challenge. Even though glycals are highly polyvalent donors, the synthesis and reactivity of 3-amino-3-deoxyglycals have been little studied. In this work, we describe a new sequence, involving a Ferrier rearrangement and subsequent aza-Wacker cyclization that allows the rapid synthesis of orthogonally protected 3-amino-3-deoxyglycals. Finally a 3-amino-3-deoxygalactal derivative was submitted for the first time to an epoxidation/glycosylation with high yield and great diastereoselectivity, highlighting FAWEG (Ferrier/Aza-Wacker/Epoxidation/Glycosylation) as a new approach to access 1,2-trans 3-amino-3-deoxyglycosides.

Selective Reduction Leading to 3,5-cis-3-Aminosugars: Synthesis and Stereoselective Glycosylation

Synthesis of 3,5-cis-3-amino glycals with a cis-fused cyclic sulfamidate group has been achieved by selective reduction of sulfamidate ketimine groups. The efficient access to the structurally unique glycals allowed the subsequent divergent synthesis of various naturally occurring 3-amino-2,3,6-trideoxysugars. In addition, Lewis acid-promoted glycosylation of the glycals provided a simple solution for the stereoselective installation of O- and C-linked aglycons on the amino sugar scaffolds.

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.

(C6F5)3B·(HF)n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides

(C₆F₅)₃B·(HF)_n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides with structurally diverse nucleophiles has been achieved.

Stereoselective Synthesis of Iminosugar 2-Deoxy(thio)glycosides from Bicyclic Iminoglycal Carbamates Promoted by Cerium(IV) Ammonium Nitrate and Cooperative Brønsted Acid-Type Organocatalysis

The first examples of iminosugar-type 2-deoxy(thio)glycoside mimetics are reported. The key step is the activation of a bicyclic iminoglycal carbamate to generate a highly reactive acyliminium cation. Cerium(IV) ammonium nitrate efficiently promoted the formation of 2-deoxy S-glycosides in the presence of thiols, probably by in situ generation of catalytic HNO₃, with complete α-stereoselectivity. Cooperative phosphoric acid/Schreiner's thiourea organocatalysis proved better suited for generating 2-deoxy O-glycosides, significantly broadening the scope of the approach.

Hydrogen-Bonding-Assisted Exogenous Nucleophilic Reagent Effect for β-Selective Glycosylation of Rare 3-Amino Sugars

Challenges for stereoselective glycosylation of deoxy sugars are notorious in carbohydrate chemistry. We herein report a novel strategy for the construction of the less investigated β-glycosidic bonds of 3,5- trans-3-amino-2,3,6-trideoxy sugars (3,5- trans-3-ADSs), which constitute the core structure of several biologically important antibiotics. Current protocol leverages a C-3 axial sulfonamide group in 3,5- trans-3-ADSs as a hydrogen-bond (H-bond) donor and repurposes substoichiometric phosphine oxide as an exogenous nucleophilic reagent (exNu) to establish an intramolecular H-bond between the former and the derived α-oxyphosphonium ion. This pivotal interaction stabilizes the α-face-covered intermediate to inhibit the formation of the more reactive β-intermediate, thereby yielding reversed β-selectivity, which is unconventional for an exNu-mediated glycosylation system. A wide range of substrates was accommodated, and good to excellent β-selectivities were ensured by this H-bonding-assisted exNu effect. The robustness of the current strategy was further attested by the architectural modification of natural products and drugs containing 3,5- trans-3-ADSs, as well as the synthesis of a trisaccharide unit in avidinorubicin.

Highly reactive 2-deoxy-2-iodo-d-allo and d-gulo pyranosyl sulfoxide donors ensure β-stereoselective glycosylations with steroidal aglycones

The preparation of well-defined d-xylo and d-ribo glycosides represents a synthetic challenge due to the limited configurational availability of starting materials and the laborious synthesis of homogeneous 2-deoxy-β-glycosidic linkages, in particular that of the sugar-steroid motif, which represents the “stereoselective determining step” of the overall synthesis. Herein we describe the use of 2-deoxy-2-iodo-glycopyranosyl sulfoxides accessible from widely available d-xylose and d-ribose monosaccharides as privileged glycosyl donors that permit activation at very low temperature. This ensures a precise kinetic control for a complete 1,2-trans stereoselective glycosylation of particularly challenging steroidal aglycones.

Highly stereoselective synthesis of challenging steroidal 2-deoxy-β-glycosides with d-xylo and d-ribo configurations enabled by low temperature activation of 2-deoxy-2-iodoglycopyranosyl sulfoxides.

Gold(I)-Catalyzed Glycosylation with Glycosyl o-Alkynylbenzoates as Donors

Naturally occurring glycans and glycoconjugates have extremely diverse structures and biological functions. Syntheses of these molecules and their artificial mimics, which have attracted the interest of those developing new therapeutic agents, rely on glycosylation methodologies to construct the various glycosidic linkages. In this regard, a wide array of glycosylation methods have been developed, and they mainly involve the substitution of a leaving group on the anomeric carbon of a glycosyl donor with an acceptor (a nucleophile) under the action of a particular promoter (usually a stoichiometric electrophile). However, glycosylations involving inherently unstable or unreactive donors/acceptors are still problematic. In those systems, reactions involving nucleophilic, electrophilic, or acidic species present on the leaving group and the promoter could become competitive and detrimental to the glycosylation. To address this problem, we applied the recently developed chemistry of alkynophilic gold(I) catalysts to the development of new glycosylation reactions that would avoid the use of the conventional leaving groups and promoters. Gratifyingly, glycosyl o-alkynylbenzoates (namely, glycosyl o-hexynyl- and o-cyclopropylethynylbenzoates) turned out to be privileged donors under gold(I) catalysis with Ph₃PAuNTf₂ and Ph₃PAuOTf. The merits of this new glycosylation protocol include the following: (1) the donors are easily prepared and are generally shelf-stable; (2) the promotion is catalytic; (3) the substrate scope is extremely wide; (4) relatively few side reactions are observed; (5) the glycosylation conditions are orthogonal to those of conventional methods; and (6) the method is operationally simple. Indeed, this method has been successfully applied in the synthesis of a wide variety of complex glycans and glycoconjugates, including complex glycosides of epoxides, nucleobases, flavonoids, lignans, steroids, triterpenes, and peptides. The direct glycosylation of some sensitive aglycones, such as dammarane C20-ol and sugar oximes, and the glycosylation-initiated polymerization of tetrahydrofuran were achieved for the first time. The gold(I) catalytic cycle of the present glycosylation protocol has been fully elucidated. In particular, key intermediates, such as the 1-glycosyloxyisochromenylium-4-gold(I) and isochromen-4-ylgold(I) complexes, have been unambiguously characterized. Exploiting the former glycosyloxypyrylium intermediate, S_N2-type glycosylations were realized in specific cases, such as β-mannosylation/rhamnosylation. The protodeauration of the latter vinylgold(I) intermediate has been reported to be critically important for the gold(I) catalytic cycle. Thus, the addition of a strong acid as a cocatalyst can dramatically reduce the required loading of the gold(I) catalyst (down to 0.001 equiv). C-Glycosylation with silyl nucleophiles can proceed catalytically when moisture, which is sequestered by molecular sieves, can serve as the H⁺ donor for the required protodeauration step. Indeed, the unique mechanism explains the merits and broad applicability of the present glycosylation method and provides a foundation for future developments in glycosylation methodologies that mainly involve improving the diastereoselectivity and catalytic efficiency of glycosylations.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Diversified synthesis and α-selective glycosylation of 3-amino-2,3,6-trideoxy sugars

Quick access to various unnatural 3-amino-2,3,6-trideoxy sugars was achieved by sequential functionalization of a glycal intermediate. This strategy and the further glycosylation method allowed the efficient late-stage modification of bioactive natural products and drugs.