Syntheses of α(2,8) Sialosides Containing NeuAc and NeuGc by Using Double Carbonyl-Protected N-Acyl Sialyl Donors

We report on the syntheses of NeuAc and NeuGc-containing glycosides via the use of double carbonyl-protected N-acetyl sialyl donors. The 7-O,9-O-carbonyl protection of an N-acyl-5-N,4-O-carbonyl-protected sialyl donor markedly increased the α-selectivity during glycosylation, particularly when glycosylating the C-8 hydroxyl group of sialic acids. The N-acyl carbamates were selectively opened with ethanethiol under basic conditions to provide N-acyl amines. It is noteworthy that N-glycolyl carbamate was more reactive to nucleophiles by comparison with the N-acetyl carbamate due to the electron-withdrawing oxygen in the N-acyl group and however, allowed selective opening of the carbamates without the loss of N-glycolyl groups. To demonstrate the utility of the approach, we began by synthesizing α(2,3) and α(2,6) sialyl galactosides. Glycosylation of the hydroxy groups of galactosides at the C-6 position with the NeuAc and NeuGc donors provided the corresponding sialyl galactoses in good yields with excellent α-selectivity. However, glycosylation of the 2,3-diol galactosyl acceptor selectively provided Siaα(2,2)Gal. Next, we prepared a series of α(2,8) disialosides composed of NeuAc and NeuGc. Glycosylation of NeuGc and NeuAc acceptors at the C-8 hydroxyl group with NeuGc and NeuAc sialyl donors provided the corresponding α(2,8) disialosides, and no significant differences were detected in the reactivities of these acceptors.

Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

Towards one-pot selective synthesis of cyclic oligosaccharides

In this chapter are described electrochemical routes to cyclic oligosaccharides. While automated electrochemical methods have been used to prepare linear oligosaccharides, their conversion to cyclic oligosaccharides proved to be a complex process. The concept of polyglycosylation offers an interesting alternative, and the process which has been developed is that of a one-pot electrochemical polyglycosylation-isomerization-cyclization (ePIC) process.

Synthesis of cyclic α-1,4-oligo-N-acetylglucosamine 'cyclokasaodorin' via a one-pot electrochemical polyglycosylation-isomerization-cyclization process

Electrochemical synthesis of unnatural cyclic oligosaccharides composed of N-acetylglucosamine with α-1,4-glycosidic linkages has been accomplished. A thioglycoside monomer equipped with the 2,3-oxazolidinone protecting group was used to prepare linear oligosaccharides by electrochemical polyglycosylation. In the same pot, isomerization of the linear oligosaccharides and intramolecular electrochemical glycosylation for cyclization were also conducted sequentially to obtain the precursor of the cyclic α-1,4-oligo-N-acetylglucosamine 'cyclokasaodorin'.

Advanced Chemical Methods for Stereoselective Sialylation and Their Applications in Sialoglycan Syntheses

Sialic acid is an important component of cell surface glycans, which are responsible for many vital body functions and should therefore be thoroughly studied to understand their biological roles and association with disorders. The difficulty of isolating large quantities of homogenous-state sialoglycans from natural sources has inspired the development of the corresponding chemical synthesis methods affording acceptable purities, yields, and amounts. However, the related syntheses are challenging because of the difficulties in α-glycosylation of sialic acid, which arises from its certain structural features such as the absence of a stereodirecting group at the C3 position and presence of carboxyl group at the anomeric position. Moreover, the structural complexities of sialoglycans with diverse numbers and locations of sialic acid on the glycan chains pose additional barriers. Thus, efficient α-stereoselective routes to sialosides remain highly sought after, although various types of sialyl donors/acceptors have been developed for the straightforward synthesis of α-sialosides. Herein, we review the latest progress in the α-stereoselective synthesis of sialosides and their applications in the preparation of gangliosides and other sialoglycans.