Construction of Interglycosidic N–O Linkage via Direct Glycosylation of Sugar Oximes

Catalytic Light-Driven Strategy for Transforming Oximes to Carbonyls

Oxime and carbonyl functional groups serve as powerful chemical hubs for constructing complex synthetic targets and valuable molecular scaffolds. In furthering this value, we report a photopromoted catalytic deoximation protocol for converting oximes and their derivatives to carbonyl functional groups. This strategic approach benefits from the use of renewable light energy input and ambient air conditions, in addition to demonstrating good substrate scope, functional group tolerance, and product yields. In offering, insights into these reactivity mechanistic studies are communicated, and the value of this protocol is further shown through one-pot operations.

An ultra-low thiourea catalyzed strain-release glycosylation and a multicatalytic diversification strategy

The utility of thiourea catalysis in selective glycosylation strategies has gained significant momentum lately due to its versatility in hydrogen bonding or anionic recognition activation modes. The use of these non-covalent interactions constitute a powerful means to construct glycosidic linkages as it mimics physiologically occurring glycosyltransferases. However, glycosyl donor activation through the currently employed catalysts is moderate such that, in general, catalyst loadings are rather high in these transformations. In addition, thiourea catalysis has not been well explored for the synthesis of furanosides. Herein, we demonstrate an ultra-low loadings stereoselective and stereospecific thiourea catalyzed strain-release furanosylation and pyranosylation strategy. Our ultra-low organocatalyzed furanosylation enables a multicatalytic strategy, which opens up a unique avenue towards rapid diversification of synthetic glycosides. In-situ NMR monitoring unravel insights into unknown reaction intermediates and initial rate kinetic studies reveal a plausible synergistic hydrogen bonding/Brønsted acid activation mode.

Non-covalent glycosyl donor activation often requires high organocatalyst loadings. Here, the authors demonstrate that strain-release glycosylations can take place at very low thiourea catalyst loadings. In addition, the authors developed a one-pot multicatalytic strategy that can diversify glycosides rapidly.

Silver-catalyzed stereoselective formation of glycosides using glycosyl ynenoates as donors

A silver-catalyzed glycosylation reaction employing readily accessible and stable glycosyl ynenoates is developed. This reaction is mostly high yielding and exhibits varying levels of stereoinversion at the anomeric position. Compared to established and versatile Yu's gold catalysis, this chemistry features the use of substantially cheaper AgNTf₂.

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.

Aminooxylated Carbohydrates: Synthesis and Applications

Among other classes of biomolecules, carbohydrates and glycoconjugates are widely involved in numerous biological functions. In addition to addressing the related synthetic challenges, glycochemists have invested intense efforts in providing access to structures that can be used to study, activate, or inhibit these biological processes. Over the past few decades, aminooxylated carbohydrates have been found to be key building blocks for achieving these goals. This review provides the first in-depth overview covering several aspects related to the syntheses and applications of aminooxylated carbohydrates. After a brief introduction to oxime bonds and their relative stabilities compared to related C═N functions, synthetic aspects of oxime ligation and methodologies for introducing the aminooxy functionality onto both glycofuranosyls and glycopyranosyls are described. The subsequent section focuses on biological applications involving aminooxylated carbohydrates as components for the construcion of diverse architectures. Mimetics of natural structures represent useful tools for better understanding the features that drive carbohydrate-receptor interaction, their biological output and they also represent interesting structures with improved stability and tunable properties. In the next section, multivalent structures such as glycoclusters and glycodendrimers obtained through oxime ligation are described in terms of synthetic design and their biological applications such as immunomodulators. The second-to-last section discusses miscellaneous applications of oxime-based glycoconjugates, such as enantioselective catalysis and glycosylated oligonucleotides, and conclusions and perspectives are provided in the last section.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

N–O linkage in carbohydrates and glycoconjugates

The synthesis and chemical and physicochemical properties as well as biological and medical applications of various hydroxylamine-functionalized carbohydrate derivatives are summarized.

ortho-(Methyltosylaminoethynyl)benzyl glycosides as new glycosyl donors for latent-active glycosylation

A new glycosylation protocol with ortho-(methyltosylaminoethynyl)benzyl glycosides as donors is disclosed.

Probing the influence of protecting groups on the anomeric equilibrium in sialic acid glycosides with the persistent radical effect

A method for the investigation of the influence of protecting groups on the anomeric equilibrium in the sialic acid glycosides has been developed on the basis of the equilibration of O-sialyl hydroxylamines by reversible homolytic scission of the glycosidic bond following the dictates of the Fischer-Ingold persistent radical effect. It is found that a trans-fused 4O,5N-oxazolidinone group stabilizes the equatorial glycoside, i.e., reduces the anomeric effect, when compared to the 4O,5N-diacetyl protected systems. This effect is discussed in terms of the powerful electron-withdrawing nature of the oxazolidinone system, which in turn is a function of its strong dipole moment in the mean plane of the pyranose ring system. The new equilibration method displays a small solvent effect and is most pronounced in less polar media consistent with the anomeric effect in general. The unusual (for anomeric radicals) poor kinetic selectivity of anomeric sialyl radicals is discussed in terms of the planar π-type structure of these radicals and of competing 1,3-diaxial interactions in the diastereomeric transition states for trapping on the α- and β-faces of the radical.

Total synthesis of starfish saponin goniopectenoside B

Star quality: Goniopectenoside B, a minor asterosaponin from starfish Goniopecten demonstrans with antifouling activity, has been synthesized in a convergent 21 steps and in 4.3 % overall yield starting from adrenosterone. This represents the first synthesis of a complex asterosaponin, which are ubiquitous and characteristic in starfish as defense chemicals (see figure).