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Digital archive of theses discussed at the University of Pisa


Thesis etd-01152016-171810

Thesis type
Tesi di laurea magistrale LM5
Thesis title
Stereoselective synthesis of a new lipophilic 6-deoxy carba mannobioside as possible DC-SIGN ligand
Course of study
relatore Prof.ssa Di Bussolo, Valeria
Graduation session start date
DC-SIGN a DC-specific C-type lectin is a receptor highly expressed on dendritic cells surface and it has the ability to bind a great number of pathogens, recognizing a mannose- and fucose- containing glycans through its terminal carbohydrate recognition domain (CRD). DC-SIGNs also bind different types of viruses, such as the HIV-1, interacting via HIV-1 envelope glycoprotein gp120. The main natural ligand recognized by DC-SIGN is the high mannose glycan, (Man)9(GlcNAc)2 or (Man9), a branched oligosaccharide presented in multiple copies by several pathogen glycoproteins, as well as the HIV envelope glycoprotein gp120. All Man9 branches are composed by a terminal disaccharide portion, Manα1-2Man, involved in high mannose recognition process by DC-SIGN receptor. High-density arrays of unbranched Manα1-2Man-terminated oligosaccharides bind DC-SIGN almost as efficiently as the entire Man9.

Recently in our laboratory were synthesized two new DC-SIGN antagonists 1 and 2 as mimics of the natural Manα1-2Man. The pseudo-disaccharides 1 and 2 are composed by two moieties: the nonreducing unit formed by a mannose anchor, as Manα1-2Man, and the reducing one, where the mannose was substituted by a real D-carbamannose core, that attributes more enzymatic stability to the pseudo-disaccharides. Particularly, the compound 1 was designed as structural mimic of the Manα1-2Man, while 2 owns a tosyl-amino group on the C(4) position of the carbocyclic moiety, which has the aim to interact with a specific lipophilic pocket within the DC-SIGN CRD increasing the inhibitory activity.

The antagonist 1 was tested using an infection model based on Ebola envelope pseudotyped viruses and Jukart cells expressing DC-SIGN. The results demonstrated an IC50 values slightly higher than the natural ligand, probably due to the high polar portions present on the carbamannose core.

In view of this the main purpose of this thesis project is the stereoselective synthesis of the new pseudo-disaccharide 3 as DC-SIGN antagonist, with a real D-carbamannose core and a methyl group at C(5) position instead of hydroxy-methylene group. The methyl group has the aim to increase the cyclohexane ring lipophilicity and, consequently, the interaction between the antagonist and the DC-SIGN binding site. In this way, we hypotesize to increase the inhibitory activity of the antagonist.

The synthesis of the compound 3 begins from the commercially available tri-O-acetyl-D-glucal 4, which was converted into the primary alcohol 5, starting material for the subsequent switching into the carba analogue 6, in which the methylene group replaces the endocyclic oxygen. Then, after appropriate elaborations the hydroxyl-methylene group on the C(5) position of the carba analogue 6 was substituted with a methyl group to afford the corresponding methyl carba analogue 7. The residual C(1) and C(2) double bond present in 7, was then subjected to a stereoselective epoxidation, obtaining the β-methyl-epoxide 8. This latter was submitted to a ring opening reaction with azido ethanol as the nucleophile, to obtain the glycosyl acceptor 9, which will form the reducing moiety of the final product 3.

Thus, to build the pseudomannobioside 3, the glycosyl-acceptor 9 was connected to an appropriate glycosyl-donor as the mannose trichloroacetimidate (-)-10 through a glycosylation reaction under classic reaction conditions.

The resulting protected-pseudodisaccharide 11 was subjected to different steps of deprotection. Then, benzyl groups were removed under reducing conditions, which also induced the reduction of the azido group into a primary amino group. Finally saponification of benzoyl groups afforded the pseudomannobioside 3 fully-O-deprotected.

The pseudomannobiose 3 thus obtained will be sent to Professor Frank Fieschi (IBS-Institut de Biologie Structurale-Jean-Pierre Ebel, in Grenoble, France) in order to evaluate its inhibitory activity towards the DC-SIGN receptor.