A new synthetic approach to mycobacterial cell wall α-(1→5)-d-arabinofuranosyl C-oligosaccharides

Selected Research Topics of the Dondoni Group over the Last Two Decades (2000–2020)

From a selection of research topics carried out in our laboratory during the last twenty years it becomes apparent that our main target was the discovery of new or improved synthetic methods together with new properties. Our efforts were made with the aim of being of some utility to other fields of research, with particular emphasis to glycobiology and heterocyle-based bioorganic chemistry. We performed new chemistry mainly in the field of carbohydrate manipulations taking as a primary rule the simplicity and efficiency manners. Toward this end, modern synthetic tools and approaches were employed such as heterocyle-based transformations, multicomponent reactions, organocatalysis, click azide–alkyne cycloadditions, reactions in ionic liquids, click photoinduced thiol-ene coupling, and click sulfur–fluoride exchange chemistry. With these potent methodologies in hand, the syntheses of carbohydrate containing amino acids up to proteins glycosylation were performed.

1 Heterocyclic Glycoconjugates and Amino Acids

2 Triazole-Linked Oligonucleotides: Application of Click CuAAC

3 Non-Natural Glycosyl Amino Acids

4 Non-Natural Oligosaccharides

5 Calixarene-Based Glycoclusters

6 Carbohydrate-Based Building Blocks

7 Homoazasugars and Aza-C-disaccharides

8 Synthesis of Glycodendrimers

9 Peptide and Protein Glycoconjugates

10 Conclusions

D-Arabinose-based synthesis of homo-C-d4T and homo-C-thymidine

2,3,5-Tri-O-benzyl-D-arabinofuranosyl halides (chloride, bromide) were reacted with AllMgBr, MeMgBr, and VinMgBr to furnish anomeric mixtures of the C-glycosyl products. The factors that influenced the beta/alpha ratio are discussed. The alpha,beta-C-vinyl derivative was transformed into 1-deoxy-1-C-hydroxymethyl-beta- and -alpha-D-arabinofuranoses (2,5-anhydro-D-glucitol and -mannitol, respectively), separable after isopropylidenation step. 2,5-Anhydro-1,3-O-isopropylidene-D-glucitol was converted into 2,5-anhydro-6-O-triphenylmethyl-D-erythro-hex-3,4-enitol and 2,5-anhydro-4,6-di-O-benzoyl-3-deoxy-D-ribo-hexitol, which were coupled with N-3-benzoylthymine under the Mitsunobu conditions to furnish two analogs of nucleosides with a -CH(2)- insert between sugar moieties and thymine.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.

Synthesis of symmetrical C- and pseudo-symmetrical O-linked disaccharide analogs for arabinosyltransferase inhibitory activity in Mycobacterium tuberculosis

Herein we report the synthesis of symmetrical C-linked and pseudo-symmetrical O-linked disaccharides structurally related to Araf motifs present in the cell wall of MTB. Their activity in a competition-based arabinosyltransferase assay using [14C]-DPA as the glycosyl donor is also presented. In addition, in vitro inhibitory activity for the disaccharides was determined in a colorimetric broth microdilution assay system against MTB H37Ra and Mycobacterium avium.

Multiple Component Approaches to C-Glycosyl β-Amino Acids by Complementary One-Pot Mannich-Type and Reformatsky-Type Reactions

The development of new methods for the preparation of C-glycosyl beta-amino acid libraries with chemical and stereochemical diversity levels was investigated and the results are described herein. Two complementary one-pot three-component Mannich-type and Reformatsky-type synthetic strategies have been developed for the construction of chiral 3-amino propanoate fragments (eventually bis-substituted at C-2) directly linked to the anomeric carbon of pyranose and furanose residues. Both methods involved as the initial step the coupling of a sugar aldehyde to p-methoxybenzylamine but differed in the nucleophile (a d(2) synthon equivalent) which was successively added: a ketene silyl acetal (Mannich route) or a bromozinc enolate (Reformatsky route). Individual C-glycosyl beta-amino esters were isolated as single 3R diastereoisomers in fair to excellent yield (60-90%) and their structure assigned by NMR spectroscopy (Riguera protocol) supported by X-ray crystallography. A tentative explanation of the observed stereochemical outcome based on transition-state models is provided. A preliminary study on the synthesis of alpha,alpha-difluoro C-glycosyl beta-amino acids via a more traditional Reformatsky route is also reported.

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.