Oxidative Deamination of N-Acetyl Neuraminic Acid: Substituent Effects and Mechanism

Synthesis of sialyl halides with various acyl protective groups

Glycosyl halides are historically one of the first glycosyl donors used in glycosylation reactions, and interest in glycosylation reactions involving this class of glycosyl donors is currently increasing. New methods for their activation have been proposed and effective syntheses of oligosaccharides with their participation have been developed. At the same time, the possibilities of using these approaches to the synthesis of sialosides are restricted by the limited diversity of known sialyl halides (previously, mainly sialyl chlorides, less often sialyl bromides and sialyl fluorides, with acetyl (Ac) groups at the oxygen atoms and AcNH, Ac2N and N3 groups at C-5 were used). This work describes the synthesis of six new N-acetyl- and N-trifluoroacetyl-sialyl chlorides and bromides with O-chloroacetyl and O-trifluoroacetyl protective groups. Preparation of N,O-trifluoroacetyl protected derivatives was made possible due to development of the synthesis of sialic acid methyl ester pentaol with N-trifluoroacetyl group.

Concise Synthesis of Tunicamycin V and Discovery of a Cytostatic DPAGT1 Inhibitor

A short total synthesis of tunicamycin V (1), a non-selective phosphotransferase inhibitor, is achieved via a Büchner-Curtius-Schlotterbeck type reaction. Tunicamycin V can be synthesized in 15 chemical steps from D-galactal with 21 % overall yield. The established synthetic scheme is operationally very simple and flexible to introduce building blocks of interest. The inhibitory activity of one of the designed analogues 28 against human dolichyl-phosphate N-acetylglucosaminephosphotransferase 1 (DPAGT1) is 12.5 times greater than 1. While tunicamycins are cytotoxic molecules with a low selectivity, the novel analogue 28 displays selective cytostatic activity against breast cancer cell lines including a triple-negative breast cancer.

Sialic Acid Derivatives Inhibit SiaT Transporters and Delay Bacterial Growth

Antibiotic resistance is a major worldwide concern, and new drugs with mechanistically novel modes of action are urgently needed. Here, we report the structure-based drug design, synthesis, and evaluation in vitro and in cellular systems of sialic acid derivatives able to inhibit the bacterial sialic acid symporter SiaT. We designed and synthesized 21 sialic acid derivatives and screened their affinity for SiaT by a thermal shift assay and elucidated the inhibitory mechanism through binding thermodynamics, computational methods, and inhibitory kinetic studies. The most potent compounds, which have a 180-fold higher affinity compared to the natural substrate, were tested in bacterial growth assays and indicate bacterial growth delay in methicillin-resistant Staphylococcus aureus. This study represents the first example and a promising lead in developing sialic acid uptake inhibitors as novel antibacterial agents.

Direct sialic acid 4-OAc substitution by nitrogen, sulfur and carbon nucleophiles with retention of stereochemistry

A direct one-step nucleophilic substitution of the 4-OAc of acetyl protected Neu5Ac is presented. Previously published methods for direct substitution of the 4-OAc are limited to cyclic secondary amines. Here we present conditions that allow for a much wider range of nitrogen nucleophiles as well as thiols and cyanide, to be used. The present investigation significantly broadens the scope of 4-aminations and allows for the introduction of a wide variety of different nucleophiles.

Synthesis of 4-O-Alkylated N-Acetylneuraminic Acid Derivatives

The synthesis of 4-O-alkyl analogues of N-acetylneuraminic acid (Neu5Ac) and the scope of the reaction are described. Activated alkyl halides and sulfonates and primary alkyl iodides give products in useful yields. The utility of the methodology is exemplified using a thiophenyl Neu5Ac building block to synthesize a 4-O-alkyl DANA analogue. These results expand the toolbox of Neu5Ac chemistry with value in drug discovery and for the design of novel tools to study the biology of Neu5Ac lectins.

Use of hydroxylamines, hydroxamic acids, oximes and amines as nucleophiles in the Zbiral oxidative deamination of N-acetyl neuraminic acid. Isolation and characterization of novel mono- and disubstitution products

The oxidative deamination of N-nitroso N-acetylneuraminic acid (NeuAc) derivatives is a useful reaction for the formation of 5-desamino-5-hydroxy NeuAc derivatives and their stereoisomers. We demonstrated previously that replacement of the classical nucleophile in these reactions, acetic acid, by phenols resulted in a novel double displacement process with substitution of the acetoxy group at the 4-position taking place in addition to that of the 5-acetamido group, for which we postulated a mechanism centered on the formation of a highly reactive vinyl diazonium ion. We now extend these studies to encompass the use of hydroxylamine-based systems and weakly basic amines as nucleophile. We find that the nature of the product depends significantly on the pKa of the nucleophile, with the more acidic species typically affording only substitution at the 5-position, while the less acidic species give mixtures of elimination products and disubstitution products. The use of aniline as nucleophile is of particular note as it affords a novel aziridine spanning positions 4- and 5- of the neuraminic acid skeleton.

Use of Phenols as Nucleophiles in the Zbiral Oxidative Deamination of N-Acetyl Neuraminic Acid: Isolation and Characterization of Tricyclic 3-Keto-2-deoxy-nonulosonic Acid (KDN) Derivatives via an Intermediate Vinyl Diazonium Ion

It is well established that the N-nitrosoamide derived from peracetylated derivatives of N-acetyl neuraminic acid on treatment with a mixture of sodium isopropoxide and trifluoroethanol, followed by the addition of acetic acid, gives an oxidative deamination product, in which the AcN(NO)-C5 bond is replaced with a AcO-C5 bond with the retention of configuration, affording a practical synthesis of 2-keto-3-deoxy-d-glycero-d-galactononulosonic acid (KDN) derivatives. Application of other strong acids, including hydrogen fluoride, thioacetic acid, trifluoromethanesulfonic acid, and hydrogen azide, functions similarly to afford KDN derivatives functionalized at the 5-position. We describe our attempts to extend the range of useful nucleophiles employed in this oxidative deamination process to include phenols and thiophenols, resulting in the discovery of a new branch of the general reaction and the formation of a series of products resulting from substitution of the 5-acetamido group and of the 4-acetoxy group from neuraminic acid. A mechanistic rationale for the formation of these products is advanced according to which, in the absence of acids of pKa ≤ 8, the intermediate diazonium ion resulting from the elimination of acetic acid and nitrogen from the nitrosoacetamide undergoes elimination of acetic acid from the 4-position to afford a highly electrophilic alkenediazonium ion. Reversible conjugate addition of the nucleophile to the 4-position then initiates the reaction cascade leading to the ultimate products.

Synthesis of saccharocin from apramycin and evaluation of its ribosomal selectivity

We describe a straightforward synthesis of the apramycin biosynthetic precursor saccharocin from apramycin by regioselective partial azidation followed by stereoretentive oxidative deamination. Saccharocin was found to exhibit excellent selectivity for inhibition of the bacterial ribosome over the eukaryotic ribosomes indicating that its presence as a minor impurity in apramycin itself should not be problematic in the development of the latter as a clinical candidate.

Synthesis and Stereocontrolled Equatorially Selective Glycosylation Reactions of a Pseudaminic Acid Donor: Importance of the Side-Chain Conformation and Regioselective Reduction of Azide Protecting Groups

Pseudaminic acid is an amino deoxy sialic acid whose glycosides are essential components of many pathogenic Gram-negative bacterial cell walls including those from Pseudomonas aeruginosa, Vibrio cholerae, Campylobacter jejuni, Campylobacter coli, Vibrio vulnificus, and Pseudoalteromonas distincta. The study of pseudaminic acid glycosides is however hampered by poor availability from nature and the paucity of good synthetic methods and limited to no understanding of the factors controlling stereoselectivity. Conformational analysis of the side chains of various stereoisomeric sialic acids suggested that the side chain of pseudaminic acid would take up the most electron-withdrawing trans, gauche-conformation, as opposed to the gauche, gauche conformation of N-acetyl neuraminic acid and the gauche, trans-conformtion of 7- epi N-acetyl neuraminic acid, leading to the prediction of high equatorial selectivity. This prediction is borne out by the synthesis of a suitably protected pseudaminic acid donor from N-acetyl neuraminic acid in 20 steps and 5% overall yield and by the exquisite equatorial selectivity it displays in coupling reactions with typical glycosyl acceptors. The selectivity of the glycosylation reactions is further buttressed by the development and implementation of conditions for the regioselective release of the two amines from the corresponding azides, such as required for the preparation of the lipopolysaccharides. These findings open the way to the synthesis and study of pseudaminic acid-based bacterial lipopolysaccharides and, importantly in the broader context of glycosylation reactions in general, underline the significant role played by side-chain conformation in the control of reactivity and selectivity.

Chemical Synthesis of Glycosides of N-Acetylneuraminic Acid

Investigations of methodologies aimed on improving the stereoselective synthesis of sialosides and the efficient assembly of sialic acid glycoconjugates has been the mission of dedicated research groups from the late 1960s. This review presents major accomplishments in the field, with the emphasis on significant breakthroughs and influential synthetic strategies of the last decade.

Trifluoromethanesulfonate Anion as Nucleophile in Organic Chemistry

Although the triflate ion is not generally perceived as a nucleophile, many examples of its behavior as such exist in the literature. This Synopsis presents an overview of such reactions, in which triflate may be either a stoichiometric or catalytic nucleophile, leading to the suggestion that nucleophilic catalysis by triflate may be more common than generally accepted, albeit hidden by the typical reactivity of organic triflates which complicates their observation as intermediates.

Stereoselective sialylation with O-trifluoroacetylated thiosialosides: hydrogen bonding involved?

A series of novel sialyl donors containing O-trifluoroacetyl (TFA) groups at various positions was synthesized. The choice of protecting groups in sialyl donors was based on hypothesis that variations in ability of different acyl groups to act as hydrogen bond acceptors would influence the supramolecular structure of reaction mixture (solution structure), hence the outcome of sialylation. These glycosyl donors were examined in the model glycosylation of the primary hydroxyl group of 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose in comparison with sialyl donors without O-TFA groups. The presence of O-TFA groups in a sialyl donor strongly affected the outcome of sialylation. Several sialyl donors studied showed promising results: yields of disaccharides can be as high as 86% as can be the stereoselectivities (α/β up to 15:1). The results obtained suggest that varying acyl O-protecting groups in sialyl donor may result in dramatic changes in the outcome of sialylation although further studies are required to dissect the influence of intermolecular hydrogen bonding and intramolecular substituent effects related to variations of electron-withdrawing properties of different acyl groups.

Stereoselective Synthesis of the Equatorial Glycosides of Legionaminic Acid

The synthesis of a legionaminic acid donor from N-acetylneuraminic acid in 15 steps and 17% overall yield is described. Activation of the adamantanyl thioglycoside in the donor with N-iodosuccinimide and trifluoromethanesulfonic acid in dichloromethane and acetonitrile at -78 °C in the presence of primary, secondary and tertiary alcohols affords the corresponding glycosides in excellent yield and good to excellent equatorial selectivity. In particular, coupling to the 4-OH of a suitably protected neuraminic acid derivative affords a disaccharide that closely resembles the glycosidic linkage in the polylegionaminic acid from the lipopolysaccharide of the Legionella pneumophila virulence factor. A straightforward deprotection sequence enables conversion of the protected glycosides to the free N,N-diacetyllegionaminic acid glycosides.

Reductive Deamination by Benzyne for Deoxy Sugar Synthesis Through a Domino Reaction

Benzyne was developed as a reducing agent in the key step of converting amino sugars and ketoses into deoxy sugars, which occur widely in natural products. Many deoxy sugars exhibit antibiotic and anticancer activities, and furthermore, they play essential biological roles. By treatment with CS2 and then Ac2O, amino sugars and ketoses were converted into the corresponding 1,3-thiazolidine-2-thiones. In the key step, these intermediates were treated with 2-trimethylsilylphenyl triflate (2.0 equiv.) and CsF (3.0 equiv.) in MeCN at 25 °C to produce acyclic enol acetates in 60-63 % yields. Saponification of the enol acetates with NaOMe/MeOH followed by intramolecular cyclization afforded the target 2-deoxy sugars. The key step of the reductive deamination involved a domino 1,2-elimination/[3+2]-cycloaddition/retro [3+2]-ring-opening sequence. The generality of this new method was proven by the use of various substrates, including pentoses, hexoses, monosaccharides, disaccharides, aldoses, and ketoses.

Synthesis and intramolecular glycosylation of sialyl mono-esters of o-xylylene glycol. The importance of donor configuration and nitrogen protecting groups on cyclization yield and selectivity; isolation and characterization of a N-sialyl acetamide indicative of participation by acetonitrile

The synthesis and cyclization reactions, leading to spirocyclic medium ring-sized diolides, of o-(hydroxymethyl)xylylene monoesters of sialyl thioglycosides is described. Cyclization yields and stereoselectivities are found to vary as a function of the anomeric stereochemistry of the thioglycoside and of the N5 protecting group, and these effects are discussed in terms of the reaction mechanism. Cyclization in the presence of acetonitrile results in the isolation and characterization of a Ritter-type N-sialyl acetamide, which affords strong evidence for the participation of acetonitrile in the form of sialyl nitrilium ions.

Stereoselective Synthesis of 5-epi-α-Sialosides Related to the Pseudaminic Acid Glycosides. Reassessment of the Stereoselectivity of the 5-Azido-5-deacetamidosialyl Thioglycosides and Use of Triflate as Nucleophile in the Zbiral Deamination of Sialic Acids

With a view to the eventual synthesis of glycosyl donors for the stereocontrolled synthesis of pseudaminic acid glycosides, the stereocontrolled synthesis of a d-glycero-d-gulo sialic acid adamantanylthioglycoside carrying an axial azide at the 5-position is described. The synthesis employs levulinic acid as nucleophile in the oxidative deamination of an N-acetylneuraminic acid thioglycoside leading to the formation of a 3-deoxy-d-glycero-d-galacto-2-nonulosonic acid (KDN) derivative selectively protected as 5-O-levulinate. Replacement of the levulinate by triflate enables introduction of the axial azide and hence formation of the glycosyl donor. A shorter synthesis uses trifluoromethanesulfonate as nucleophile in the oxidative deamination step when the 5-O-triflyl KDN derivative is obtained directly. Glycosylation reactions conducted with the 5-azido-d-glycero-d-gulo-configured sialyl adamantanylthioglycoside at -78 °C are selective for the formation of the equatorial glycosides, suggesting that the synthesis of equatorial pseudaminic acid glycosides will be possible as suitable donors become available. A comparable N-acetylneuraminic acid adamantanylthioglycoside carrying an equatorial azide at the 5-position was also found to be selective for equatorial glycoside formation under the same conditions, suggesting that reinvestigation of other azide-protected NeuAc donors is merited. Glycosylation stereoselectivity in the d-glycero-d-gulo series is discussed in terms of the side-chain conformation of the donor.

Oxidative deamination of amino sugars: recent advances

An overview of the oxidative deamination of N-acetylneuraminic acid derivatives (Neu5Ac) leading to the formation of ketodeoxynonulosonic acid (KDN), its stereoisomers and glycosides is presented. A brief historical introduction to the deamination is given, followed by a description of recent advances in reaction conditions, which have allowed application of the process to Neu5Ac thioglycosides, and that have enabled the range of nucleophiles incorporated in the course of the reaction to be extended beyond the original acetate and azide. Recent advances resulting in derivatization of the Neu55Ac 4-position concomitant with replacement of the acetamido group, via the presumed intermediacy of a vinyl diazonium ion, are then described. The literature on the mechanism of the deamination reaction is next considered leading to the presentation of an overall mechanistic framework that accounts for all observations to date. Finally, the application of the deamination reaction to complex Neu5Ac-based oligosaccharides and other aminosugars is presented.