Stereoselective synthesis of the C-linked analogue of β-d-galactopyranosyl-l-serine

C-Glycopyranosyl aldehydes: emerging chiral synthons in organic synthesis

Herein, we have summarized the vast array of synthetic processes that have been developed for the synthesis of C-glycopyranosyl aldehydes and diverse C-glycoconjugates derived from them by covering the literature reported from 1979 to 2023. Notwithstanding its challenging chemistry, C-glycosides are considered stable pharmacophores and are used as important bioactive molecules. The discussed synthetic methodologies to access C-glycopyranosyl aldehydes take advantage of seven key intermediates, viz. allene, thiazole, dithiane, cyanide, alkene, and nitromethane. Furthermore, the integration of complex C-glycoconjugates derived from varied C-glycopyranosyl aldehydes involves nucleophilic addition/substitution, reduction, condensation, oxidation, cyclo condensation, coupling, and Wittig reactions. In this review, we have categorized the synthesis of C-glycopyranosyl aldehydes and C-glycoconjugates on the basis of the methodology used for their synthesis and on types of C-glycoconjugates, respectively.

Aziridine Ring Opening as Regio- and Stereoselective Access to C-Glycosyl-Aminoethyl Sulfide Derivatives

A short synthetic route to stereoselective access to C-glycosyl-aminoethyl sulfide derivatives has been developed through the reaction of tributhyltin derivatives of glycals with aziridinecarboaldehyde and the regioselective ring opening of a chiral aziridine with thiophenol. The absolute configurations of the resulting diastereoisomers were determined by 1H NMR spectroscopy.

A concise [C+NC+CC] coupling-enabled synthesis of kaitocephalin

An efficient synthesis of the iGluR antagonist kaitocephalin is reported, employing an asymmetric [C+NC+CC] reaction in the key step.

Click azide-nitrile cycloaddition as a new ligation tool for the synthesis of tetrazole-tethered C-glycosyl alpha-amino acids

Glycoproteins play a key role in a multitude of biological events in living organisms. Hence, neoglycopeptides obtained from unnatural C-glycosyl alpha-amino acids can be used as synthetic probes in studies aiming at clarifying the role of the carbohydrate domain in glycoprotein biological activity. A new class of C-glycosyl alpha-amino acids featuring a nitrogenated heterocycle ring holding the carbohydrate and glycinyl moiety was designed in our laboratory. Having previously prepared isoxazole-, 1,2,3-triazole-, and pyridine-tethered compounds, the family has now been enlarged by a group of newcomers represented by tetrazole derivatives. Two sets of compounds have been prepared, one being constituted of C-galactosyl and C-ribosyl O-tetrazolyl serines while the other contains S-tetrazolyl cysteine derivatives. In both cases, the synthetic scheme involved a two-step route, the first one being the thermal cycloaddition of a sugar azide with p-toluensulfonyl cyanide (TsCN) to give a 1-substituted 5-sulfonyl tetrazole and the second the replacement of the tosyl group with a serine or cysteine residue. For the high efficiency and operational simplicity, the azide-TsCN cycloaddition appears to be a true click process. Finally, one of the amino acids prepared was incorporated into a tripeptide.

New and general synthesis of β-C-glycosylformaldehydes from easily available β-C-glycosylpropanones

A highly effective method for the introduction of a formyl group at the anomeric position of pyranosides was developed via enolisation of beta-C-D-glycopyranosylpropan-2-one using thermodynamic conditions then oxidative cleavage of the more substituted double bond. This sequence affords the desired aldehydes that are conveniently protected as aminals for purification and storage and easily regenerated using Dowex resin H+. In this paper, the syntheses of nine differently protected aldehydes derived from d-glucose, d-galactose, lactose and N-acetyl-d-glucosamine are presented. Our strategy proved to be very efficient in most cases excepted in the D-mannose series.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update covering the period 1999-2000

This review describes the use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for the analysis of carbohydrates and glycoconjugates and continues coverage of the field from the previous review published in 1999 (D. J. Harvey, Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates, 1999, Mass Spectrom Rev, 18:349-451) for the period 1999-2000. As MALDI mass spectrometry is acquiring the status of a mature technique in this field, there has been a greater emphasis on applications rather than to method development as opposed to the previous review. The present review covers applications to plant-derived carbohydrates, N- and O-linked glycans from glycoproteins, glycated proteins, mucins, glycosaminoglycans, bacterial glycolipids, glycosphingolipids, glycoglycerolipids and related compounds, and glycosides. Applications of MALDI mass spectrometry to the study of enzymes acting on carbohydrates (glycosyltransferases and glycosidases) and to the synthesis of carbohydrates, are also covered.

Synthesis and structure determination of some nonanomerically C-C-linked serine glycoconjugates structurally related to mannojirimycin

The Bucherer-Bergs reaction of methyl 2,3-O-isopropylidene-alpha-d-lyxo-hexofuranosid-5-ulose gave (4'S)-4'-carbamoyl-4'-[methyl (4R)-2,3-O-isopropylidene-beta-l-erythrofuranosid-4-C-yl]-oxazolidin-2'-one instead of expected hydantoins. A mixture of hydantoins--(5'R)-triphenylmethoxymethyl-5'-[methyl (4R)-2,3-O-isopropylidene-beta-l-erythrofuranosid-4-C-yl]-imidazolidin-2',4'-dione and (5'S)-triphenylmethoxymethyl-5'-[methyl (4R)-2,3-O-isopropylidene-beta-l-erythrofuranosid-4-C-yl]-imidazolidin-2',4'-dione was obtained from the 5-ulose having protected primary OH group at C-6. The 4'-S configuration of 2 as well as 5'-S configuration of (5'S)-hydroxymethyl-5'-[methyl (4R)-2,3-O-isopropylidene-beta-l-erythrofuranosid-4-C-yl]-imidazolidin-2',4'-dione (9) was confirmed by X-ray crystallography. Corresponding alpha-amino acid--methyl (5S)-5-amino-5-C-carboxy-5-deoxy-alpha-d-lyxo-hexofuranoside (alternative name: 2-[methyl (4R)-beta-l-erythrofuranosid-4-C-yl]-l-serine) (11) was obtained from the hydantoin 9 by acid hydrolysis of the isopropylidene and trityl groups followed by basic hydrolysis of the hydantoin ring. Analogous derivatives with 5-R configuration, formed in a minority, were also isolated and characterised.

Stereoselective Synthesis of α- and β-l-C-Fucosyl Aldehydes and Their Utility in the Assembly of C-Fucosides of Biological Relevance

An efficient synthesis of O-benzylated derivatives of the title sugar aldehydes via thiazole addition to tri-O-benzyl-l-fuconolactone followed by highly stereoselective deoxygenation of the resulting thiazolylketose and thiazole to formyl transformation is described. Wittig olefination of these aldehydes with galactopyranose and glucopyranose 6-phosphoranes and reduction of the resulting alkenes afforded alpha- and beta-linked (1-->6)-L-C-fucosyl disaccharides, namely, beta-L-C-Fuc-(1-->6)-alpha-D-Gal, alpha-L-C-Fuc-(1-->6)-alpha-D-Gal, and alpha-L-C-Fuc-(1-->6)-alpha-d-Glc. The alpha-anomer of the above C-fucosyl aldehydes was transformed into a C-fucosylmethyl triphenylphosphonium iodide from which the corresponding C-fucosylmethylene phosphorane was generated upon treatment with BuLi. This phosphorane reacted with the Garner aldehyde (N-Boc D-serinal acetonide) and its one-carbon higher homologue to give alkenes whose reduction and unveiling of the glycinyl group from the oxazolidine ring afforded C-fucosyl alpha-amino acids, namely alpha-L-linked C-fucosyl serines and C-fucosyl asparagines. As a final test of the synthetic utility of the title aldehydes, the beta-anomer was employed as starting material in the stereoselective synthesis of both R- and S-epimer L-C-fucosyl phenylhydroxy acetates. One epimer was obtained by reaction of the sugar aldehyde with phenylmagnesium bromide, oxidation of the resulting alcohol to ketone, addition of 2-lithiothiazole to the latter, and transformation of the thiazole ring into the carboxyl group through an aldehyde intermediate. The other epimer was obtained by the same procedure and inverting the timing of phenyl and thiazolyl group addition. In both routes, the key step establishing the configuration of the quaternary carbon atom of the aliphatic chain was the highly stereoselective addition of the organometal to the ketone intermediate.

Novel O-glycosyl amino acid mimetics as building blocks for O-glycopeptides act as inhibitors of galactosidases

As potential lead structures for a new class of glycosidase inhibitors the novel O-glycosyl amino acid mimetics 3'-O-[2,6-anhydro-D-glycero-L-gluco-heptitol-1-yl]-L-serine 3 and-L-threonine 4 were synthesized, employing regio- and stereoselective aziridine ring opening methodology. They proved to be stable in the presence of glycosidases and showed competitive inhibition of alpha-galactosidase from Aspergillus niger.

Some non-anomerically C-C-linked carbohydrate amino acids related to leucine-synthesis and structure determination

(5'R)-5'-Isobutyl-5'-[methyl (4R)-2,3-O-isopropylidene-beta-L-erythrofuranosid-4-C-yl]-imidazolidin-2',4'-dione was synthesised starting from methyl 2,3-O-isopropylidene-alpha-D-lyxo-pentodialdo-1,4-furanoside via methyl 6-deoxy-6-isopropyl-2,3-O-isopropylidene-alpha-D-lyxo-hexofuranosid-5-ulose applying the Bucherer-Bergs reaction. Its 5'-R configuration was confirmed by X-ray crystallography. Corresponding alpha-amino acid-methyl (5R)-5-amino-5-C-carboxy-5,6-dideoxy-6-isopropyl-alpha-D-lyxo-hexofuranoside (alternative name: 2-[methyl (4R)-beta-L-erythrofuranosid-4-C-yl]-D-leucine) was obtained from the above hydantoin by acid hydrolysis of the isopropylidene group followed by basic hydrolysis of the hydantoin ring. Analogous derivatives with 5S configuration, formed in a minority, were also isolated and characterised.

Synthesis of C-linked glycopyranosyl serines via a chiral glycine enolate equivalent

[formula: see text] The stereoselective preparation of C-linked D-gluco- and D-galactopyranosyl L-serines in their alpha and beta forms is herein reported. The syntheses require the conversion of the allyl C-glycopyranosides into their iodoethyl derivatives, which then undergo substitution with the Williams' chiral glycine enolate equivalent. Deprotection and acetylation affords Boc-protected amino acids for peptide synthesis.

Three-component Biginelli cyclocondensation reaction using C-glycosylated substrates. Preparation of a collection of dihydropyrimidinone glycoconjugates and the synthesis of C-glycosylated monastrol analogues

The aldehyde-ketoester-urea cyclocondensation reaction has been revisited using C-glycosylated reagents with the aim of exploring a potential entry to a library of dihydropyrimidinone glycoconjugates. A collection of 13 mono- and bis-C-glycosylated dihydropyrimidinones has been prepared by a parallel synthesis approach using the three-component promoter CuCl/AcOH/BF(3) x Et(2)O. The sugar residues have been installed at either N1, C4, or C6 in the monoglycosylated derivatives and at both the C4 and C6 in the bisglycosylated products. The mono- and bisglycosylated products at C4 and C6 were obtained as mixtures of diastereoisomers with good to excellent selectivities due to the asymmetric induction by the sugar residue in the formation of the C4 stereocenter of the dihydropyrimidinone ring. Individual stereoisomers were isolated as pure compounds and their structures assigned with the aid of X-ray crystallography and chiroptical properties. As a demonstration of this new concept in the Biginelli reaction, the synthesis of two C4 epimer monastrol analogues bearing the ribofuranosyl moiety at C6 has been described.

Glycoside-clustering round calixarenes toward the development of multivalent ligands. Synthesis and conformational analysis of Calix[4]arene O- and C-glycoconjugates

Bis- and tetra-O- and C-glycosyl calixarenes (calixsugars) have been prepared by tethering carbohydrate moieties to a tetrapropoxycalix[4]arene scaffold through alkyl chains. Two methodologies have been employed. One consisted of the stereoselective multiple glycosylation of upper rim calix[4]arene polyols leading to calix-O-glycosides; the other involved a multiple Wittig olefination of upper rim calix[4]arene-derived polyaldehydes by the use of sugar phosphoranes and reduction of the alkene double bonds affording calix-C-glycosides. The NMR spectra and NOE experiments of bis-glycosylated products indicate that compounds bearing sugar-protected residues exist preferentially in solution in a flattened cone arrangement (far conformation) whereas deprotected derivatives adopt a close conformation. Calculations by molecular mechanics of the latter compounds point to a close conformation as well in gas phase.

Novel Tn antigen-containing neoglycopeptides: synthesis and evaluation as anti tumor vaccines

The fully unprotected alpha-C-glycosyl analogue of N-acetylgalactosamine 9 was conjugated by a non-natural oxime bond to the segment peptides (328--340)OVA and (327--339)OVA, affording neoglycopeptides 1--2 and 3, having one or two sugar units, respectively. The three neoglycopeptides were tested in vitro in an antigen presentation assay as antitumor vaccines. Neoglycopeptides 1--3 could be presented to and recognized by the T cell receptor; neoglycopeptide 3, bearing two B-epitopes, was presented to the TCR with higher efficiency, compared to neoglycopeptide 2, having only one B-epitope.