Rethinking Carbohydrate Synthesis: Stereoretentive Reactions of Anomeric Stannanes

In this Concept article, recent advances are highlighted in the synthesis and applications of anomeric nucleophiles, a class of carbohydrates in which the C1 carbon bears a carbon–metal bond. First, the advantages of exploiting the carboanionic reactivity of carbohydrates and the methods for the synthesis of mono- and oligosaccharide stannanes are discussed. Second, recent developments in the glycosyl cross-coupling method resulting in the transfer of anomeric configuration from C1 stannanes to C-aryl glycosides are reviewed. These highly stereoretentive processes are ideally suited for the preparation of carbohydrate-based therapeutics and were demonstrated in the synthesis of antidiabetic drugs. Next, the application of the glycosyl cross-coupling method to the preparation of Se-glycosides and to glycodiversification of small molecules and peptides are highlighted. These reactions proceed with exclusive anomeric control for a broad range of substrates and tolerate carbohydrates with free hydroxyl groups. Taken together, anomeric nucleophiles have emerged as powerful tools for the synthesis of oligosaccharides and glycoconjugates and their future applications will open new possibilities to incorporate saccharides into small molecules and biologics.     

β‐Selective C‐Glycosylation and its Application in the Synthesis of Scleropentaside A

C‐Glycosides are carbohydrates that bear a C?C bond to an aglycon at the anomeric center. Due to their high stability towards chemical and enzymatic hydrolysis, these compounds are widely used as carbohydrate mimics in drug development. Herein, we report a general and exclusively β‐selective method for the synthesis of a naturally abundant acyl‐C‐glycosidic structural motif first found in the scleropentaside natural product family. A Corey–Seebach umpolung reaction as the key step in the synthesis of scleropentaside A and analogues enables the β‐selective construction of the anomeric C?C bond starting from unprotected carbohydrates in only four steps. The one‐pot approach is highly atom‐efficient and avoids the use of toxic heavy metals.     

Use of the Anomeric Carbon-13 Proton One Bond Scalar Coupling Constant as a Tool for Detecting 1,2-Orthoester Formation in Oligosaccharide Synthesis

Abstract

The anomeric ¹J_C,J value has for some time been successfully used for structural elucidation of oligosaccharides. The first recorded spectra on unenriched carbohydrates were published by Bock et al. in 1973 and pointed out that the anomeric ¹J_C,J value could be used for assignment of the anomeric configuration since pyranoses with an axial H-1 have a ¹J_C,J value which is approximately 10 Hz lower than the corresponding value in compounds with an equatorial H-1.^1–3 Their method recording non decoupled ¹³C NMR spectra had the disadvantage of being insensitive and thereby time-consuming. Introduction of INEPT⁴ followed by technical developments, has overcome this disadvantage. Nowadays, with reverse detection techniques, technical skill instead of sensitivity is a limiting factor. In this communication I would like to emphasize that the anomeric ¹J_C,J value can be used for detecting 1,2-orthoester formation as well as for establishing a- or P-configuration.     

CARBOHYDRATES CONTAINING SULFUR

Abstract

Thioglycosides with sulfur as the glycosidic bridge are naturally occurring. They have been examined chemically and also biochemically wherein they aid in establishment of enzyme activity. Sugars and sugar nucleoside derivatives with sulfur—and sometimes selenium and nitrogen—replacing the ring oxygen show interesting chemistry and are of great potential usefulness in medicine as indicated by initial biochemical investigations. Their value in biochemistry as analogs of natural structures is proving of useful interest. Carbohydrates with sulfur at positions other than at the anomeric carbon or with ring involvement have interesting chemical properties. This review presents a general survey of carbohydrates containing sulfur, of their special methods of synthesis, their special chemical properties and reactions and a very brief review of their biochemistry and potential clinical value.     

Dynamic Kinetic Stereoselective Glycosylation via RhII and Chiral Phosphoric Acid-Cocatalyzed Carbenoid Insertion to the Anomeric OH Bond for the Synthesis of Glycoconjugates

Chemical synthesis of glycoconjugates is essential for studying the biological functions of carbohydrates. We herein report an efficient approach for the stereoselective synthesis of challenging α-linked glycoconjugates via a Rh^II/chiral phosphoric acid (CPA)-cocatalyzed dynamic kinetic anomeric O-alkylation of sugar-derived lactols via carbenoid insertion to the anomeric OH bond. Notably, we observed excellent anomeric selectivity, excellent diastereoselectivity, broad substrate scope, and high efficiency for this glycosylation reaction by exploring various parameters of the cocatalytic system. DFT calculations suggested that the anomeric selectivity was mainly determined by steric interactions between the C2-carbon of the carbohydrate and the phenyl group of the metal carbenoid, while π/π interactions with the C2−OBn substituent on the carbohydrate substrate play a significant role for diastereoselectivity at the newly generated stereogenic center.     

Synthesis of Carbohydrate‐Functionalised Sequence‐Defined Oligo(amidoamine)s by Photochemical Thiol?Ene Coupling in a Continuous Flow Reactor

Poly/oligo(amidoamine)s (PAAs) have recently been recognised for their potential as well‐defined scaffolds for multiple carbohydrate presentation and as multivalent ligands. Herein, we report two complimentary strategies for the preparation of such sequence‐defined carbohydrate‐functionalised PAAs that use photochemical thiol? ene coupling (TEC) as an alternative to the established azide–alkyne cycloaddition (“click”) reaction. In the first approach, PAAs that contained multiple olefins were synthesised on a solid support from a new building block and subsequent conjugation with unprotected thio‐carbohydrates. Alternatively, a pre‐functionalised building block was prepared by using TEC and assembled on a solid support to provide a carbohydrate‐functionalised PAA. Both methods rely on the use of a continuous flow photoreactor for the TEC reactions. This system is highly efficient, owing to its short path length, and requires no additional radical initiator. Performing the reactions at 254 nm in Teflon AF‐2400 tubing provides a highly efficient TEC procedure for carbohydrate conjugation, as demonstrated in the reactions of O‐allyl glycosides with thiols. This method allowed the complete functionalisation of all of the reactive sites on the PAA backbone in a single step, thereby obtaining a defined homogeneous sequence. Furthermore, reaction at 366 nm in FEP tubing in the flow reactor enabled the large‐scale synthesis of an fluorenylmethyloxycarbonyl (Fmoc)‐protected glycosylated building block, which was shown to be suitable for solid‐phase synthesis and will also allow heterogeneous sequence control of different carbohydrates along the oligomeric backbone. These developments enable the synthesis of sequence‐defined carbohydrate‐functionalised PAAs with potential biological applications.     

‘ClickCarb’: modular sugar based ligands via click chemistry

Reported is a modular approach for the synthesis of a number of structurally diverse ligands derived from carbohydrates. The use of the highly functional hydroxy amino azide 1 derived from glucosamine allows the synthesis of a number of useful ligands for organic and organometallic catalyses. The key step of the approach is the Huisgen cycloaddition of the anomeric sugar azide and diverse alkynes.     

Enzymes in Organic Synthesis: Application to the Problems of Carbohydrate Recognition (Part 2)

Recognition of carbohydrates by proteins and nucleic acids is highly specific, but the dissociation constants are relatively high (generally in the mM to high μM range) because of the lack of hydrophobic groups in the carbohydrates. The high specificity of this weak binding often comes from many hydrogen bonds and the coordination of metal ions as bridge between sugars and receptors. Though weak hydrophobic interactions between sugars and proteins have also been identified, the unique shape of a complex carbohydrate under the influence of anomeric and exo anomeric effects (the glycosidic torsion angles are therefore often not flexible but are typically somewhat restricted) and the topographic orientation of the hydroxyl and charged groups contribute most significantly to the recognition process. Studies on the structure–function relationship of a complex carbohydrate therefore require deliberate manipulation of its shape and functional groups, and synthesis of oligosaccharide analogs from modified monosaccharides is often useful to address the problem. The availability of various monosaccharides and their analogs for the synthesis of complex carbohydrates together with the information resulting from structural studies (such a NMR or X-ray studies on sugar–protein complexes) will certainly provide a basic understanding of complex carbohydrate recognition. An ultimate goal is to develop simple and easy-to-make non-carbohydrate molecules that resemble the active structure involved in carbohydrate–receptor interaction or the transition-state of an enzyme-catalyzed transformation (for example, glycosidase or glycosyltransferase reactions) and have the approprite bioavailability to be used to control the carbohydrate function in a specific manner. In part one of this review we described various enzymatic approaches to the synthesis of monosaccharides, analogs, and related structures. We describe in this part enzymatic and chemoenzymatic approaches to the synthesis of oligosaccarides and analogs, including those involved in E-selectin recognition, and strategies to inhibit glycosidases and glycosyltransferases.     

A highly enantioselective total synthesis of (+)-goniodiol

A high-yielding enantioselective total synthesis of the bioactive styryllactone (+)-goniodiol has been realised, starting from readily available (S)-glycidol. A key step is an oxygen-to-carbon rearrangement of a silyl enol ether linked via an anomeric centre, facilitating the rapid and diastereoselective construction of this functionalised system.     

Synthese de O-Glycosides d'Alcenyles Catalysée par les Complexes du Palladium(0)

Abstract

Allylic carbonates reacted with carbohydrates having a free anomeric hydroxyl group in the presence of a catalytic amount of palladium(0) giving the unsaturated O-glycosides under neutral conditions with excellent yields. In the mannofuranose and ribofuranose series, the reaction was stereospecific leading only to the α and the β anomers respectively, although a mixture of α and β anomers was obtained in the glucopyranose series.     

Stereoselective diversity-oriented syntheses of functionalized saccharides from bicyclic carbohydrate 1,2-lactones

Bicyclic carbohydrate 1,2-lactones have been synthesized in only two steps and high yields by saponification and subsequent cyclization from known malonate addition products to glycals. The gluco-configured lactone serves as an important precursor for diversity-oriented syntheses. Thus, stereoselective opening of the lactone ring was realized with various nucleophiles in the presence of Sc(OTf)₃. This enabled the introduction of different substituents at the anomeric position, to afford a broad variety of 1-functionalized carbohydrates. On the other hand, stereoselective α-substitution of the gluco-configured lactone with different electrophiles and subsequent ring opening gives a collection of 2-functionalized saccharides. More than 30 products have been isolated in analytically pure form, and their configurations were unequivocally established by various NMR methods. Thus, carbohydrate 1,2-lactones are attractive precursors for the stereoselective synthesis of diverse saccharides.     

Syntheses of Branched Non‐1‐ynitols by Chemoselective Addition of (Trimethylsilyl)‐propargylmagnesium Bromide at the Anomeric Center of Side Chain Halogeno Functionalized Carbohydrates

A convenient method for the syntheses of non‐1‐ynitols 8a–8d, by chemoselective addition of (trimethylsilyl)‐propargylmagnesium bromide at the anomeric center of a 1‐unprotected sugar, in the presence of 3‐C′‐halide and 3‐C′‐silyl functions in the side chain is described. In addition, an efficient method for the synthesis of 3‐C′‐hydroxymethyl sugar 3 via the addition of C₁ silyl Grignard reagents to ulose 1 and subsequent oxidation by the Fleming‐Tamao method in excellent yields is reported. Also, a suitable acid‐catalyzed isomerization of the 1,2‐O‐isopropylidene group to the 2,3‐O‐isopropylidene group (5a–5f), to get access to the anomeric center, in good to excellent yields has been depicted.     

A Convenient Synthesis of 3,6-Anhydro-2-deoxy-D-glucose [Isoglucal] and Some of Its Derivatives

Abstract

The present paper describes a facile synthesis of isoglucal (3,6-anhydro-2-deoxy-D-glucose, 3), starting from the easily available α,β-unsaturated aldehyde (2E)-4,6-di-O-acetyl-2,3-didehydro-aldehydo-D-erythro-hex-2-enose (1). Compound 3 exists in equilibrium with its corresponding bicyclic forms whose anomeric configurations and ring-sizes have been determined.     

Synthesis and Complexation Studies of a Tetra(N,N-dimethyl) Aminoethylamide p-tert-butyl Calix[4]arene and Its Tetramethylammonium Derivative

We describe the synthesis of two functionalised p-tert-butyl calix[4]arenes: tetra(N,N-dimethyl) aminoethylamide derivative 1 and related tetramethylammonium 2. Their complexation properties towards alkali and zinc metal cations are reported along with complexation of perchlorate anion by 2.     

Synthesis of New Dimeric Amphiphiles. Influence of Anomeric Configuration and Spacer Functionality on Interfacial Properties

ABSTRACT

The synthesis of new dimeric carbohydrate-based surfactants was performed connecting two butyl glucopyranosides with a spacer through ester and ether linkages. Critical micellar concentrations were determined to study the influence of anomeric configuration and spacer functionality on surfactant properties.     

Synthesis and Photochromic Properties of Configurationally Varied Azobenzene Glycosides

Spatial orientation of carbohydrates is a meaningful parameter in carbohydrate recognition processes. To vary orientation of sugars with temporal and spatial resolution, photosensitive glycoconjugates with favorable photochromic properties appear to be opportune. Here, a series of azobenzene glycosides were synthesized, employing glycoside synthesis and Mills reaction, to allow “switching” of carbohydrate orientation by reversible E/Z isomerization of the azobenzene N=N double bond. Their photochromic properties were tested and effects of azobenzene substitution as well as the effect of anomeric configuration and the orientation of the sugars 2-hydroxy group were evaluated.     

Capillary Zone Electrophoresis of Iridium(III) and Ruthenium(II) Polypyridyl Complexes in Non‐Aqueous Solvents

Abstract

A method of non‐aqueous capillary zone electrophoresis has been optimised for the characterisation of five functionalised mixed‐ligand iridium(III) polypyridyl complexes and four functionalised ruthenium(II) bis‐terpyridine complexes. Their mobilities, the relation to their molar mass and the capillary lifetime (measured in number of injections) have been determined.     

Synthesis of 1,2,3-triazolyl analog of Neisseria meningitidis A capsular polysaccharide

Abstract

Towards constructing a stable, non-hydrolyzable linkage for Neisseria meningitidis type A (MenA) polysaccharide, the phosphate bridge of its (1→6)-linked 2-acetamido-2-deoxy-α-D-mannopyranosyl phosphate repeating unit was replaced by a triazolyl analog using Cu(I) catalyzed click reaction. The synthesis involved anomeric azidation of N-acetyl-D-mannosamine derivative from its acetate precursor using stoichiometric amount of FeCl₃ and one-pot synthesis of precursors for di- and tri- triazolyl saccharides with an azidoethyl spacer at the reducing end for bioconjugation.     

Chemistry of β-trimethylsilylethanol. II. A new method for protection of an anomeric center in pyranosides

Protection of the anomeric center in various carbohydrates as a β-trimethylsilylethyl glycoside is reported. The free sugar can be regenerated using LiBF₄ in acetonitrile.     

Anomeric configuration-directed diastereoselective C-C bond formation in vinyl sulfone-modified carbohydrates: a general route to branched-chain sugars

[structure: see text] This is the first report on the diastereoselective addition of carbon nucleophiles to vinyl sulfone-modified hex-2-enopyranosides and pent-2-enofuranosides. Nucleophiles add to the C-2 position from a direction opposite to that of the disposition of the anomeric methoxy group. This novel concept of anomeric configuration-directed stereocontrolled carbon-carbon bond formation in vinyl sulfone-modified carbohydrates is general in nature and has been implemented in the synthesis of new hexopyranosyl and pentofuranosyl branched-chain sugars and densely functionalized carbohydrates.