Heparan sulfates (HS) are a class of sulfated polysaccharides that function as dynamic biological regulators of the functions of diverse proteins. The structural basis of these interactions, however, remains elusive, and chemical synthesis of defined structures represents a challenging but powerful approach for unravelling the structure-activity relationships of their complex sulfation patterns. HS has been shown to function as an inhibitor of the β-site cleaving enzyme β-secretase (BACE1), a protease responsible for generating the toxic Aβ peptides that accumulate in Alzheimer's disease (AD), with 6-O-sulfation identified as a key requirement. Here, we demonstrate a novel generic synthetic approach to HS oligosaccharides applied to production of a library of 16 hexa- to dodecasaccharides targeted at BACE1 inhibition. Screening of this library provided new insights into structure-activity relationships for optimal BACE1 inhibition, and yielded a number of potent non-anticoagulant BACE1 inhibitors with potential for development as leads for treatment of AD through lowering of Aβ peptide levels.

D-Glucosamine was transformed into phenyl and 2-benzoyloxyethyl N-acetylglucosamine beta-glycosides 6a and 6b, respectively. Transformation of 6a,b into 6-O-unprotected N-acetylglucosamine derivatives 9a,b permitted the generation of an aldehyde group in the 6-position. Treatment of these intermediates with base afforded unsaturated aldehyde derivatives 10a,b, which are structural mimics of 2,3-dehydroneuraminic acid. H-Phosphonate addition to the aldehyde group and attachment of the cytidine monophosphate residue to the generated hydroxy group gave fully protected transition state analogues of cytidine monophosphate-N-acetylneuraminic acid 14a,b. Liberation of the unprotected compounds 1ah,l and 1bh,l led to excellent inhibitors of alpha(2-6)-sialyltransferase from rat liver. Variation of the protective group cleavage procedure for 14a,b led to formal loss of phosphate, thus resulting in diene derivatives (E)-/(Z)-2a,b, which also exhibited inhibitory properties.

Inhibitors that are structurally related to the transition-state model of the proposed SN1-type mechanism of sialyl transfer, exhibit particularly high binding affinities to alpha(2-6)sialyltransferases. Furthermore, replacing the neuraminyl residue with a simple aryl or hetaryl ring and substituting the carboxylate group for a phosphonate moiety, improves both binding affinity and synthetic accessibility. Herein we report on the synthesis and inhibition of a wide range of novel, potent transition-state analogue based alpha(2-6)sialyltransferase inhibitors comprising a planar anomeric carbon, an increased distance between the anomeric carbon and the CMP leaving group, and at least two negative charges. We also present a short, efficient asymmetric synthesis of the most promising benzyl inhibitors, providing rapid access to large quantities of highly potent, stereochemically-pure (>96% de) inhibitors for further biological investigation (e.g.(R)-3b, Ki = 70 nM).

Despite evident regulatory roles of heparan sulfate (HS) saccharides in numerous biological processes, definitive information on the bioactive sequences of these polymers is lacking, with only a handful of natural structures sequenced to date. Here, we develop a "Shotgun" Ion Mobility Mass Spectrometry Sequencing (SIMMS 2) method in which intact HS saccharides are dissociated in an ion mobility mass spectrometer and collision cross section values of fragments measured. Matching of data for intact and fragment ions against known values for 36 fully defined HS saccharide structures (from di-to decasaccharides) permits unambiguous sequence determination of validated standards and unknown natural saccharides, notably including variants with 3O-sulfate groups. SIMMS 2 analysis of two fibroblast growth factorinhibiting hexasaccharides identified from a HS oligosaccharide library screen demonstrates that the approach allows elucidation of structure-activity relationships. SIMMS 2 thus overcomes the bottleneck for decoding the informational content of functional HS motifs which is crucial for their future biomedical exploitation.

The reaction of sialyl phosphites 1, 22a‐d, 28, 39, and 45 with acyl‐protected riboside 5‐phosphorous acids 2a,b and 23 directly furnished, without addition of a catalyst, under phosphite/phosphate exchange the corresponding β‐configured sialyl riboside monophosphates 3a,b, 24a‐d, 29, 46, and 47. The synthesis of the starting materials, formation of the products, and their treatment with sodium methanolate in methanol and subsequent hydrolysis of the sialic acid ester moiety to provide the unprotected target molecules 4a,b, 25a‐d, 30, 48, and 49 is described. Investigations with α(2‐6)‐sialyltransferase from rat liver showed that base replacement in CMP‐Neu5Ac (4a,b) is not tolerated by the enzyme but that modifications of the 5‐, 8‐, or 9‐position of the neuraminic acid residue (25a‐d, 30, 48, 49) are tolerated.

Sialylation of glycoproteins and glycolipids plays an important role in many cell surface processes;of special interest in this context are changes in tissue involved in diseases and especially changes during infections. Therefore, inhibitors of sialyltransferases regulating sialylation might be of medicinal interest, especially in the therapy of virus and cancer diseases. The present paper reviews the development of sialyltransferase inhibitors; thus acceptor analogs, donor analogs, transition-state analogs, and bisubstrate analogs were designed based on sialic acid derivatives as well as on structural mimetics. This way, very potent sialyltransferase inhibitors with inhibition constants in the low nanomolar range have been recently found.

1 See appendix. 2 See the author list of 'Overview of progress in European Medium Sized Tokamaks towards an integrated plasma-edge/wall solution' by Meyer [22].

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