Building a successful structural motif into sialylmimetics—cyclohexenephosphonate monoesters as pseudo-sialosides with promising inhibitory properties
Graphical abstract
A systematic synthetic approach to sialylmimetics containing a cyclohexenephosphonate scaffold has been developed which allows their incorporation into pseudo-disaccharidic systems. Thus, mimicking of the full structural space displayed by natural sialosides as substrates, products or as intermediates in enzymatic reactions is possible and inhibitory activity toward selected sialidases is obtained.
Introduction
Sialic acids are a family of naturally occurring derivatives of 3-deoxy-d-glycero-d-galacto-2-nonulosonic acids abundant on cell surfaces in higher animals (Fig. 1A).1 At their terminal position at the non-reducing end of oligosaccharide chains in cell wall glycoconjugates, sialic acids represent a crucial first encounter and receptor for attachment of exogenous, often pathogenic, agents such as viruses, bacteria or parasites and their respective lectins and toxins. In addition, important endogenous events such as a cell’s metastatic potential are mediated by density as well as degree and kind of modification of their cell surface sialic acids.2, 3
Consequently, a variety of these interactions have been targeted by small molecule inhibitor design and synthesis, with varying success, leading to nearly nanomolar inhibition of sialyltransferases or of the sialidase form influenza virus A and B but yet failing, for instance, to provide the same potency for sialic acid binding lectins or other microbial sialidases.4, 5
This fact, the persistent influenza threat and newly emerged targets such as parasitic trans-sialidases,6, 7 paramyxoviral hemagglutinin-neuraminidases,7, 8, 9 and other sialic acid modifying enzymes, prompted us to search for an approach which allows the incorporation of a successful structural motif, namely the cyclohexene ring as a transition state mimetic of glycoside hydrolysis, into more elaborated structures. The diabetes drug acarbose, containing the cyclohexene valienamine or the anti-influenza drug and neuraminidase inhibitor GS-4071, containing a l-xylo-configured cyclohexenecarboxylic acid as the bioactive sialylmimetic, may serve as examples (Fig. 1B).
Section snippets
Inhibitor design and retrosynthetic approach
Replacement of the carboxylate in the l-xylo-configured cyclohexene scaffold of GS-4071 by a phosphonate leads to l-xylo-configured cyclohexenephosphonates which allow attachment of aglycon mimetics or natural aglycons of sialic acids such as galactoses thus leading to pseudo-disaccharides containing a carbocyclic sialylmimetic (Fig. 1C). In such cyclohexenephosphonate monoesters, both the negative charge under physiological conditions as well as the flattened half-chair conformation are
Sialidase inhibition
In order to demonstrate the usefulness of the structural concept, we have tested the inhibition of selected bacterial sialidases and a trypanosomal sialidase by our xylo-configured cyclohexenephosphonate monoesters. Compounds 1, 17 and 19 all showed improved inhibition (0.2, 0.09 and 0.2 mM, respectively) of Salmonella typhimurium sialidase when compared to the parent cyclohexenephosphonate.12, 13, 18 Moreover, although lacking an optimized side chain mimetic, monobenzyl phosphonate 17 exceeds
General
Reaction solvents were purchased anhydrous and used as received. Solvents for chromatography were distilled before use. Reactions were monitored by TLC using precoated silica gel 60 F254 plates. Compounds were detected by UV absorption and/or by staining with a molybdenum phosphate reagent (20 g ammonium molybdate and 0.4 g cerium(IV) sulfate in 400 mL of 10% aq sulfuric acid) and subsequent heating at 120 °C for 5 min. Silica gel 60 M (particle size 40–63 μm) from Macherey-Nagel, Düren, Germany, was
Acknowledgments
We thank the Deutsche Forschungsgemeinschaft for financial support. We are indebted to Professor R. Schauer and Dr. S. Schrader, University of Kiel, for making the TcTS assay available to us, and to Dr. C. Röhrig and Dr. A. Abdul-Sada for recording the mass spectra.
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