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Heparan sulfate (HS), a sulfated linear carbohydrate that decorates the cell surface and extracellular matrix, is a key regulator of biological processes. Owing to the inherent structural complexity of HS, structure-to-function studies with its ligands are required, and materials to improve the understanding of such interactions are therefore of high importance. Herein, the synthesis of novel S -linked GlcN-α(1→4)-GlcA disaccharide building blocks is detailed. Initial attempts at constructing the desired disaccharide using d-GlcN donors and d-Glc/GlcA acceptors via an S -glycosylation failed. Reversing the reactivity polarity of the monosaccharide building blocks enabled successful S N 2 coupling using α-d-GlcN thiohemiacetals and d-galactosyl triflates. Subsequent C6-oxidation furnished the desired S -linked GlcN-α(1→4)-GlcA disaccharide building blocks on a gram scale. Such disaccharides offer potential for incorporation into wider synthetic HS sequences to provide glycomimetic tools.

Reported herein is a scalable chemical synthesis of disaccharide building blocks for heparan sulfate (HS) oligosaccharide assembly. The use of d-glucuronate-based acceptors for dehydrative glycosylation with d-glucosamine partners is explored, enabling diastereoselective synthesis of appropriately protected HS disaccharide building blocks (d-GlcN-α-1,4-d-GlcA) on a multigram scale. Isolation and characterization of key donor (1,2 glycal)- and acceptor (ortho-ester, anhydro)-derived side products ensure methodology improvements to reduce their formation; protecting the d-glucuronate acceptor at the anomeric position with a para -methoxyphenyl unit proves optimal. We also introduce glycal uronate acceptors, showing them to be comparative in reactivity to their pyranuronate counterparts. Taken together, this gram-scale access offers the capability to explore the iterative assembly of defined HS sequences containing the d-GlcN-α-1,4-d-GlcA repeat, highlighted by completing this for two tetrasaccharide syntheses.

d-Glucosamine derivatives bearing latent O4 functionality provide modified H/HS-type disaccharide donors for a final stage capping approach enabling introduction of conjugation-suitable, non-reducing terminal functionality to biologically important glycosaminoglycan oligosaccharides. Application to the synthesis of the first O4-terminus modified synthetic LMWH decasaccharide and an HS-like dodecasaccharide is reported.

Synthesis of an array of differentially sulfated GlcN-IdoA disaccharides, accessible on good scale, directly from l-iduronate components is described. These are specifically directed to provide the sulfation variability at the key most common biologically relevant sulfation-variable l-IdoA O-2 and d-GlcN O-6 and amino sites of this heparin disaccharide. This sulfation-varied matrix has allowed the first evaluation of using Raman/ROA spectroscopy to characterize changes in spectra as a function of both site and level of sulfation with pure, defined heparin-related disaccharide species. This provides analysis of both similarities and differences to digest native heparin and this shows evidence of different types of changes in conformations and conformational freedom as a function of some specific sulfation changes at the disaccharide level. It is anticipated that this data set will open the way for applications to further site-specific sulfated saccharides and demonstrates the capability offered by Raman-ROA towards fingerprinting sulfation in heparin fragments., (Copyright © 2014 The Authors. Published by Elsevier Ltd.. All rights reserved.)

A diastereomerically pure cyanohydrin, preparable on kilogram scale, is efficiently converted in one step into a novel L-iduronamide. A new regioselective acylation of this iduronamide and a new mild amide hydrolysis method mediated by amyl nitrite enables short, scalable syntheses of an L-iduronate diacetate C-4 acceptor, and also L-iduronate C-4 acceptor thioglycosides. Efficient conversions of these to a range of heparin-related gluco-ido disaccharide building blocks (various C-4 protection options) including efficient multigram access to key heparin-building block ido-thioglycoside donors are described. A 1-OAc disaccharide is converted into a heparin-related tetrasaccharide, via divergence to both acceptor and donor disaccharides. X-ray and NMR data of the 1,2-diacetyl iduronate methyl ester and the analogous iduronamide show that while both adopt (1)C(4) conformations in solution, the iduronate ester adopts the (4)C(1) conformation in solid state. An X-ray structure is also reported for the novel, (4)C(1)-conformationally locked bicyclic 1,6-anhydro iduronate lactone along with an X-ray structures of a novel distorted (4)C(1) iduronate 4,6-lactone. Deuterium labeling also provides mechanistic insight into the formation of lactone products during the novel amyl nitrite-mediated hydrolysis of iduronamide into the parent iduronic acid functionality.

A divergent synthetic route to chondroitin sulfate (CS) disaccharide precursors, including rarer subtypes such as CS−D, has been developed. From common intermediates, a series of thioglycoside D‐glucosyl donors and 4,6‐O‐benzylidene protected D‐galactosamine acceptors are utilised in a robust glycosylation reaction, achieving β‐selectivity and consistent yields (60–75 %) on scales >2.0 g. A post‐glycosylation oxidation to D‐glucuronic acid and orthogonal protecting groups delivers access to CS−A, CS−C, CS−D, CS−E and CS−O precursor subtypes. Of further note is a 4‐O‐benzyl regioselective reductive ring opening of a 4,6‐O‐benzylidene protected disaccharide using dichlorophenylborane (PhBCl2) and triethylsilane (Et3SiH) to access a CS−D precursor, in 73 % yield over two steps. Finally, synthesis of a 6‐O‐sulfated CS−C disaccharide containing a conjugable anomeric allyl tether is completed. These materials will provide a benchmark to further synthesise and study chondroitin sulfates. [ABSTRACT FROM AUTHOR]

L-Idofuranoside cyanohydrin 1 is converted on large scale into a mixture of L-IdoA methyl pyranosides and furanosides, which is converged to provide short 2-step routes to bicyclic [3.2.1] or [2.2.2] L-iduronate lactones. The former is obtained via a 100 g scale synthesis of 3-OBn L-IdoA. A two-step conversion of this mixture provides either pure anomer of the novel [2.2.2] L-iduronate thioglycoside lactones. Both [3.2.1] and [2.2.2] lactones are converted into GlcN-IdoA heparin precursor disaccharides. The [2.2.2] lactone enables a scalable 3-step route from 1 to a new type of highly disarmed O-4 iduronate thioglycoside, which is an effective acceptor with glucoazide thioglycoside donors. The resulting new iduronic [2.2.2] lactone disaccharides are readily rearmed by mild methanolysis to provide GlcN-IdoA thiophenyl disaccharide donors, intercepting their established utility for the assembly of both heparin- and heparan sulfate-like oligosaccharides. The [2.2.2] lactonization acts as a conformational switch to superdisarm iduronate components, reversible by lactone ring opening. In addition, the separated 2,4-diacetates also provide short access to all four anomeric and ring size isomers of L-iduronic add methyl glycosides, including the first syntheses of the parent idofuranosides. X-ray structures are reported for a [2.2.2] iduronate lactone and examples of both methyl L-idopyranoside and novel methyl-L-idofuranoside systems. [ABSTRACT FROM AUTHOR]