Extending chemoselective ligation to sugar chemistry: convergent assembly of bioactive neoglycoconjugates

Direct formation and isolation of unprotected α-and β-d-ribopyranosyl urea, α-and β-d-ribofuranosyl urea, and a ribosyl-1,2-cyclic carbamate in carbohydrate melts

Solvent-free melts of unprotected d-ribose and urea generated mainly C₁- substituted ribosyl products. The remarkable resolving power of a graphitised-carbon HPLC column allowed products of the reaction formed over a range of heating times and temperatures to be monitored. Heating an uncatalysed mixture of d-ribose and urea at temperatures between 75 °C and 90 °C resulted in complex mixtures of compounds; after 19 h heating at 90 °C, up to ten components could be resolved. At shorter heating times and lower temperatures, the composition and distribution of products varied. By manipulation of the reaction time and temperature, and with the addition of an acid catalyst, it was possible to optimise the yields of selected products. Thus, the acid-catalysed reaction after 1-2 h at 80 °C gave optimal yields of α- and β-d-ribopyranosyl urea, whereas the uncatalysed reaction after 22 h at 75-78 °C in addition produced significant amounts of α-d-ribofuranosyl-1,2- cyclic carbamate [glyco-1,2-oxazolidin-2-one] plus the α- and β-ribofuranosyl ureas. The five compounds were isolated and characterised, demonstrating the significant advantages of this approach; its simplicity, and the ability to produce multiple compounds of biological interest in a single step. LC/MS was used to identify tentatively several other components of the reaction mixture. The unprotected title compounds were prepared, isolated and characterised with water as the only solvent.

Synthesis and biological evaluation of neoglycosphingolipids

Various neoglycosphingolipids were efficiently synthesized in a one-step reaction by the coupling of free sugars with an N-alkylaminooxy-functionalized ceramide analogue. The bioactivity studies demonstrated that most of these compounds could upregulate the expression of matrix metalloproteinase-9 (MMP-9, extracellular matrix proteins associated with tumor migration) in murine melanoma B16 cells in a similar manner to the natural ganglioside monosialodihexosylganglioside (GM3), which highlights the potential use of these neoglycosphingolipids as inhibitors of tumor migration.

Enhancement of cyclopamine via conjugation with nonmetabolic sugars

The Veratrum alkaloid cyclopamine, an inhibitor of cancer stem cell growth, was used as a representative scaffold to evaluate the inhibitory impact of glycosylation with a group of nonmetabolic saccharides, such as d-threose. In a five-step divergent process, a 32-member glycoside library was created and assayed to determine that glycosides of such sugars notably improved the GI50 value of cyclopamine while metabolic sugars, such as d-glucose, did not.

Assessment of chemoselective neoglycosylation methods using chlorambucil as a model

To systematically assess the impact of glycosylation and the corresponding chemoselective linker upon the anticancer activity/selectivity of the drug chlorambucil, herein we report the synthesis and anticancer activities of a 63-member library of chlorambucil-based neoglycosides. A comparison of N-alkoxyamine-, N-acylhydrazine-, and N-hydroxyamine-based chemoselective glycosylation of chlorambucil revealed sugar- and linker-dependent partitioning among open- and closed-ring neoglycosides and corresponding sugar-dependent variant biological activity. Cumulatively, this study represents the first neoglycorandomization of a synthetic drug and expands our understanding of the impact of sugar structure upon product distribution/equilibria in the context of N-alkoxyamino-, N-hydroxyamino-, and N-acylhydrazine-based chemoselective glycosylation. This study also revealed several analogues with increased in vitro anticancer activity, most notably D-threoside 60 (NSC 748747), which displayed much broader tumor specificity and notably increased potency over the parent drug.

Enhancing the divergent activities of betulinic acid via neoglycosylation

Using neoglycosylation, the impact of differential glycosylation upon the divergent anticancer and anti-HIV properties of the triterpenoid betulinic acid (BA) was examined. Each member from a library of 37 differentially glycosylated BA variants was tested for anticancer and anti-HIV activities. Enhanced analogs for both desired activities were discovered with the corresponding antitumor or antiviral enhancements diverging, on the basis of the appended sugar, into two distinct compound subsets.

A facile stereoselective synthesis of α-glycosyl ureas

Alpha-glycosyl ureas can be synthesised directly from tetra-O-benzyl glycosyl azides and isocyanates, using a one-pot procedure that is simple and general in scope. The benzyl protecting groups are easily removed from the urea products by catalytic hydrogenation. The synthesised alpha-glycosyl ureas represent a new class of neo-glycoconjugates with the potential of being resistant towards carbohydrate processing enzymes.

Synthesis and glycosidase inhibitory activity of new hexa-substituted C8-glycomimetics

Background

Glycosidases are involved in several metabolic pathways and the development of inhibitors is an important challenge towards the treatment of diseases, such as diabetes, cancer and viral infections including AIDS. Thus, inhibition of intestinal α-glucosidases can be used to treat diabetes through the lowering of blood glucose levels, and α-glucosidase inhibitors are being marketed against type 2 (non-insulinodependent mellitus) diabetes (i.e.: Glyset^® or Diastabol^®, Basen^® and Glucor^® or Precose^®).

Results

In that context, new C8-carbasugars and related aminocyclitols have been targeted in order to study the effect of the enhanced flexibility and of the new spatial distribution displayed by these structures on their adaptability in the active site of the enzymes. The synthesis of these new C8-glycomimetics is described from enantiomerically pure C₂-symmetrical polyhydroxylated cyclooctenes. Their obtention notably involved a syn-dihydroxylation, and more extended functionalization through formation of a cis-cyclic sulfate followed by amination and subsequent reductive amination. This strategy involving the nucleophilic opening of a cis-cyclic sulfate by sodium azide is to our knowledge the first example in C8-series. It revealead to be an efficient alternative to the nucleoplilic opening of an epoxide moiety which proved unsuccessful in this particular case, due to the hindered conformation of such epoxides as demonstrated by X-ray cristallographic analysis.

Conclusion

The biological activity of the synthesized glycomimetics has been evaluated towards 24 commercially available glycosidases. The weak observed activities can probably be related to the spatial disposition of the hydroxy and amino groups which depart too much from that realized in glycomimetics such as valiolamine, voglibose and valienamine. Nevertheless, the synthetic strategy described here is efficient and general, and could be extended to increase the diversity of the glycosidase inhibitors obtained since this diversity is introduced in an ultimate step of the synthesis.