Synthesis of a Systematic 64-Membered Heparan Sulfate Tetrasaccharide Library.

Heparan sulfate (HS) has multifaceted biological activities. To date, no libraries of HS oligosaccharides bearing systematically varied sulfation structures are available owing to the challenges in synthesizing a large number of HS oligosaccharides. To overcome the obstacles and expedite the synthesis, a divergent approach was designed, where 64 HS tetrasaccharides covering all possible structures of 2-O-, 6-O- and N-sulfation with the glucosamine-glucuronic acid-glucosamine-iduronic acid backbone were successfully produced from a single strategically protected tetrasaccharide intermediate. This extensive library helped identify the structural requirements for HS sequences to have strong fibroblast growth factor-2 binding but a weak affinity for platelet factor-4. Such a strategy to separate out these two interactions could lead to new HS-based potential therapeutics without the dangerous adverse effect of heparin-induced thrombocytopenia.

Expedient Synthesis of a Library of Heparan Sulfate-Like "Head-to-Tail" Linked Multimers for Structure and Activity Relationship Studies.

Heparan sulfate (HS) plays important roles in many biological processes. The inherent complexity of naturally existing HS has severely hindered the thorough understanding of their structure-activity relationship. To facilitate biological studies, a new strategy has been developed to synthesize a HS-like pseudo-hexasaccharide library, where HS disaccharides were linked in a "head-to-tail" fashion from the reducing end of a disaccharide module to the non-reducing end of a neighboring module. Combinatorial syntheses of 27 HS-like pseudo-hexasaccharides were achieved. This new class of compounds bound with fibroblast growth factor 2 (FGF-2) with similar structure-activity trends as HS oligosaccharides bearing native glycosyl linkages. The ease of synthesis and the ability to mirror natural HS activity trends suggest that the new head-to-tail linked pseudo-oligosaccharides could be an exciting tool to facilitate the understanding of HS biology.

Automated Solid Phase Assisted Synthesis of a Heparan Sulfate Disaccharide Library.

Heparan sulfate (HS) regulates a wide range of biological events, including blood coagulation, cancer development, cell differentiation, and viral infections. It is generally recognized that structures of HS can critically impact its biological functions. However, with complex structures of naturally existing HS, systematic investigations into the structure-activity relationship (SAR) of HS and efforts to unlock their "sulfation code" have been largely limited due to the challenges in preparing diverse HS oligosaccharide sequences. Herein, we report an automated machine-aided solid-phase strategy that significantly expedited the assembly of HS disaccharides. The key strategically protected advanced disaccharide intermediates were immobilized onto Synphase lanterns. Divergent deprotections and sulfations of the disaccharides were achieved on the lanterns in high yields. In addition, the full synthetic process was automated, enabling the reproducible production of HS disaccharides. A library of 16 HS disaccharides with diverse sulfation patterns was prepared via this method. Compared to the traditional HS synthesis, this new strategy led to a reduction of 50% of the number of synthetic steps and over 80% of the number of column purification steps needed from the disaccharide intermediates, significantly improving the overall synthetic efficiency. The potential utility of the method was highlighted in a microarray study using the synthetic HS disaccharide library with fibroblast growth factor-2 (FGF-2), which yielded insights into the SAR of HS/FGF-2 interactions.

Expedient Synthesis of Core Disaccharide Building Blocks from Natural Polysaccharides for Heparan Sulfate Oligosaccharide Assembly.

The complex sulfation motifs of heparan sulfate glycosaminoglycans (HS GAGs) play critical roles in many important biological processes. However, an understanding of their specific functions has been hampered by an inability to synthesize large numbers of diverse, yet defined, HS structures. Herein, we describe a new approach to access the four core disaccharides required for HS/heparin oligosaccharide assembly from natural polysaccharides. The use of disaccharides rather than monosaccharides as minimal precursors greatly accelerates the synthesis of HS GAGs, providing key disaccharide and tetrasaccharide intermediates in about half the number of steps compared to traditional strategies. Rapid access to such versatile intermediates will enable the generation of comprehensive libraries of sulfated oligosaccharides for unlocking the "sulfation code" and understanding the roles of specific GAG structures in physiology and disease.

Synthesis of S-linked trisaccharide glycal of derhodinosylurdamycin A: Discovery of alkyl thiocyanate as an efficient electrophile for stereoselective sulfenylation of 2-deoxy glycosyl lithium.

Stereoselective synthesis of S-linked trisaccharide glycal of angucycline antitumor antibiotic derhodinosylurdamycin A is described. The synthesis has been accomplished employing our previously reported umpolung S-glycosylation strategy - stereoselective sulfenylation of 2-deoxy glycosyl lithium. It was found that sugar-derived thiocyanate was a better electrophile than corresponding asymmetric disulfide in this type of stereoselective sulfenylation.

Stereoselective Synthesis of S-Linked Hexasaccharide of Landomycin A via Umpolung S-Glycosylation.

Stereoselective synthesis of carbohydrate mimics resistant toward acid-mediated or enzymatic hydrolysis is chemically challenging and biologically interesting. In this Letter, the first stereoselective synthesis of a "non-hydrolyzable" S-linked hexasaccharide of antitumor antibiotic landomycin A is described. This synthesis was accomplished through the utilization of our recently developed umpolung reactivity-based S-glycosylation-sulfenylation of stereochemically defined glycosyl lithium species with asymmetric sugar-derived disulfides.

Direct synthesis of 2-deoxy-ß-glycosides via anomeric O-alkylation with secondary electrophiles.

An approach for direct synthesis of biologically significant 2-deoxy-ß-glycosides has been developed via O-alkylation of a variety of 2-deoxy-sugar-derived anomeric alkoxides using challenging secondary triflates as electrophiles. It was found a free hydroxyl group at C3 of the 2-deoxy-sugar-derived lactols is required in order to achieve synthetically efficient yields. This method has also been applied to the convergent synthesis of a 2-deoxy-ß-tetrasaccharide.

Catalytic stereoselective synthesis of ß-digitoxosides: direct synthesis of digitoxin and C1'-epi-digitoxin.

A mild and atom-economic rhenium(V)-catalyzed stereoselective synthesis of ß-D-digitoxosides from 6-deoxy-D-allals has been described. This ß-selective glycosylation was achieved probably because of the formation of corresponding α-digitoxosides disfavored by 1,3-diaxial interaction. In addition, this method has been successfully applied to the synthesis of digitoxin trisaccharide glycal for the direct synthesis of digitoxin and C1'-epi-digitoxin.