Regioselective Synthesis of L-Idopyranuronic Acid Derivatives: Intermolecular Aglycon Transfer of Dithioacetal Under Standard Glycosylation Conditions

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Synthetic Approaches to L-Iduronic Acid and L-Idose: Key Building Blocks for the Preparation of Glycosaminoglycan Oligosaccharides

L-Iduronic acid (IdoA) is an important monosaccharide component of glycosaminoglycans (GAGs) such as heparin, heparan sulfate and dermatan sulfate. GAGs are complex, highly sulfated polysaccharides that mediate a multitude of physiological and pathological processes via their interactions with a range of diverse proteins. The main challenge in the synthesis of GAG oligosaccharides is the efficient gram-scale preparation of IdoA building blocks since neither IdoA nor L-idose is commercially available or readily accessible from natural sources. In this review, the different synthetic approaches for the preparation of IdoA and its derivatives, including L-idose, are presented and discussed. Derivatives of the latter are often used in GAG synthesis and are elaborated to IdoA via selective oxidation at C-6 after incorporation into a GAG chain. Particular focus will be given to the preparation of IdoA synthons most commonly used for GAG oligosaccharide synthesis, and on the progress made since the last systematic review in this area.

ortho-Methylphenylthioglycosides as glycosyl building blocks for preactivation-based oligosaccharide synthesis

Thioglycosides are widely used in orthogonal glycosylation, armed-disarmed chemoselective glycosylation, and preactivation-based glycosylation. Nevertheless, aglycon transfer occasionally occurred in the glycosylation process of thioglycosides. This problem was also encountered in preactivation-based reactions, which limited the applications of preactivation-based glycosylation to some extent. To tackle this problem, sterically hindered aglycon ortho-methylphenylthioglycosides were introduced as glycosyl building blocks. These thioglycosides prevented the aglycon transfer and enhanced the efficiency of glycosyl coupling reactions, especially in the reactions of disarmed donors with armed acceptors. Moreover, these thioglycosides were employed in preactivation-based one-pot oligosaccharide assembly.

Studies on the selectivity between nickel-catalyzed 1,2-cis-2-amino glycosylation of hydroxyl groups of thioglycoside acceptors with C2-substituted benzylidene N-phenyl trifluoroacetimidates and intermolecular aglycon transfer of the sulfide group

The stereoselective synthesis of saccharide thioglycosides containing 1,2-cis-2-amino glycosidic linkages is challenging. In addition to the difficulties associated with achieving high α-selectivity in the formation of 1,2-cis-2-amino glycosidic bonds, the glycosylation reaction is hampered by undesired transfer of the anomeric sulfide group from the glycosyl acceptor to the glycosyl donor. Overcoming these obstacles will pave the way for the preparation of oligosaccharides and glycoconjugates bearing the 1,2-cis-2-amino glycosidic linkages because the saccharide thioglycosides obtained can serve as donors for another coupling iteration. This approach streamlines selective deprotection and anomeric derivatization steps prior to the subsequent coupling event. We have developed an efficient approach for the synthesis of highly yielding and α-selective saccharide thioglycosides containing 1,2-cis-2-amino glycosidic bonds, via cationic nickel-catalyzed glycosylation of thioglycoside acceptors bearing the 2-trifluoromethylphenyl aglycon with N-phenyl trifluoroacetimidate donors. The 2-trifluoromethylphenyl group effectively blocks transfer of the anomeric sulfide group from the glycosyl acceptor to the C(2)-benzylidene donor and can be easily installed and activated. The current method also highlights the efficacy of the nickel catalyst selectively activating the C(2)-benzylidene imidate group in the presence of the anomeric sulfide group on the glycosyl acceptors.

Oligosaccharide assembly by one-pot multi-step strategy

Saccharide synthesis is a formidable task for synthetic chemists. Although in recent years many advances have been made in this area, development of more convenient and efficient strategies for oligosaccharide synthesis is still in great demand. This review focuses on one of these new strategies--the one-pot sequential glycosylation approach as a potent tool for oligosaccharide assembly.

An armed-disarmed approach for blocking aglycon transfer of thioglycosides

Thioglycosides are used frequently as glycosyl donors and as mimetics of O-glycosides. While being very useful, thioglycosides are prone to a detrimental side reaction referred to as aglycon transfer. In this paper, it is shown that aglycon transfer can be blocked by matching thioglycoside-containing acceptors with more armed glycosyl donors.

Synthesis of Anemoclemoside B, the First Natural Product with an Open-Chain Cyclic Acetal Glycosidic Linkage

Anemoclemoside B (1), the first natural product containing a brand-new glycosidic linkage, the open-chain cyclic acetal linkage, was synthesized.