Synthetic Glycans and Glycomimetics: A Promising Alternative to Natural Polysaccharides.

A large quantity of polysaccharide-derived conjugate vaccines have been developed to combat various pathogenic infections. Another prominent polysaccharide, heparin, is listed as an essential drug by the World Health Organization (WHO) to treat thrombus. One of their common problems is that they all derive from natural polysaccharides. Specifically, capsular polysaccharides are mainly obtained from bacterial fermentation and unfractionated heparin is extracted from animal tissues such as porcine mucosa. The quality of natural polysaccharides is inconsistent and traces of contamination would cause a disaster. By contrast, the use of chemical or chemoenzymatic methods could provide structurally homogeneous and quality-controlled glycans. To date, large numbers of polysaccharide fragments and their analogues have been synthesized and evaluated. Some of them even showed comparable activities to their corresponding natural polysaccharides. Here, the latest advances in these synthetic glycan analogues ranging from carbohydrate-based vaccines, heparin-related therapeutics and glycomimetics of polysaccharides are summarized.

Synthesis and Antigenic Evaluation of Oligosaccharide Mimics of Vi Antigen from Salmonella typhi.

Salmonella typhi is responsible for typhoid fever, which is a serious health threat in developing countries. As a virulent factor of Salmonella typhi, the purified Vi polysaccharide (Vi PS) has become an effective vaccine to combat typhoid fever. The chemical synthesis can provide homogeneous and well-defined molecules for the development of Vi-based vaccines. However, the synthesis of Vi oligosaccharides in high yields and with exclusive α-stereoselectivities remains very challenging. In this paper, a series of Vi pseudooligosaccharides, including pseudo tetra-, hexa-, and octa-saccharides were efficiently synthesized. These oligosaccharide analogues were conjugated by carbon chain tether through olefin cross metathesis or by the 1,2,3-triazole moiety through copper (I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC). The binding affinities of these oligosaccharide mimics to anti-Vi antibodies were investigated. These results will be beneficial to the further development of Vi-based oligosaccharide vaccines.

Self-Assembled Core-Shell Nanoscale Coordination Polymer Nanoparticles Carrying a Sialyltransferase Inhibitor for Cancer Metastasis Inhibition.

Despite hypersialylation of cancer cells together with a significant upregulation of sialyltransferase (ST) activity contributes to the metastatic cascade at multiple levels, there are few dedicated tools to interfere with their expression. Although transition state-based ST inhibitors are well-established, they are not membrane permeable. To tackle this problem, herein, we design and construct long-circulating, self-assembled core-shell nanoscale coordination polymer (NCP) nanoparticles carrying a transition state-based ST inhibitor, which make the inhibitor transmembrane and potently strip diverse sialoglycans from various cancer cells. In the experimental lung metastasis and metastasis prevention models, the nanoparticle device (NCP/STI) significantly inhibits metastases formation without systemic toxicity. This strategy enables ST inhibitors to be applied to cells and animals by providing them with a well-designed nanodelivery system. Our work opens a new avenue to the development of transition state-based ST inhibitors and demonstrates that NCP/STI holds great promise in achieving metastases inhibition for multiple cancers.

A new synthetic access to bicyclic polyhydroxylated alkaloid analogues from pyranosides.

A facile, versatile and stereoselective synthesis of bicyclic polyhydroxylated alkaloids as castanospermine analogues is described. The synthetic route started from methyl pyranosides. The key steps involved a high-yielding expeditious one-pot tandem reaction from alkenes to N-substituted delta-lactams. The delta-lactams were stereoselectively vinylated to give the dienes, which were followed by the ring-closing metathesis to produce the cyclized products. The functional group transformations of the resulting bicyclic compounds furnished diverse polyhydroxylated alkaloids in good yields.

Nitro-polyols via Pyridine Promoted C═C Cleavage of 2-Nitroglycals. Application to the Synthesis of (-)-Hyacinthacine A1.

A mild and convenient transformation for the synthesis of nitro-polyols is described. The nitro-polyol derivatives were prepared either from 2-nitroglycals via a pyridine-promoted scission of the carbon-carbon double bond or from glycals via a sequential nitration-scission procedure. The generated nitro-polyols could undergo a stereoselective Michael addition reaction. The utility of the addition products was exemplified by the concise synthesis of (-)-hyacinthacine A1 and 7a-epi-(-)-hyacinthacine A1.