IPy2BF4-Mediated Transformation of n-Pentenyl Glycosides to Glycosyl Fluorides: A New Pair of Semiorthogonal Glycosyl Donors

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

Blue Light Photocatalytic Glycosylation without Electrophilic Additives

Photocatalytic formation of glycosidic bonds employing stable and readily accessible O-glycosyl derivatives of 2,2,6,6-tetramethylpiperidin-1-ol is presented that employs an iridium-based photocatalyst and blue LEDs. The reaction proceeds at room temperature and in the absence of additives other than 4 Å molecular sieves. Stereoselectivities are modest but nevertheless dependent on the anomeric configuration of the donor, suggesting a substantial degree of concerted character.

S-Benzimidazolyl (SBiz) Imidates as a Platform for Oligosaccharide Synthesis via Active-Latent, Armed-Disarmed, Selective, and Orthogonal Activations

This article describes the development of S-benzimidazolyl (SBiz) imidates as versatile building blocks for oligosaccharide synthesis. The SBiz imidates have been originally developed as a new platform for active-latent glycosylations. This article expands upon the utility of these compounds. The application to practically all common concepts for the expeditious oligosaccharide synthesis including selective, chemoselective, and orthogonal strategies is demonstrated. The strategy development was made possible thanks to our enhanced understanding of the reaction mechanism and the modes by which SBiz imidates interact with various promoters of glycosylation.

Counterion influence on the N-I-N halogen bond

A detailed investigation of the influence of counterions on the [N-I-N]+ halogen bond in solution, in the solid state and in silico is presented. Translational diffusion coefficients indicate close attachment of counterions to the cationic, three-center halogen bond in dichloromethane solution. Isotopic perturbation of equilibrium NMR studies performed on isotopologue mixtures of regioselectively deuterated and nondeuterated analogues of the model system showed that the counterion is incapable of altering the symmetry of the [N-I-N]+ halogen bond. This symmetry remains even in the presence of an unfavorable geometric restraint. A high preference for the symmetric geometry was found also in the solid state by single crystal X-ray crystallography. Molecular systems encompassing weakly coordinating counterions behave similarly to the corresponding silver(i) centered coordination complexes. In contrast, systems possessing moderately or strongly coordinating anions show a distinctly different behavior. Such silver(i) complexes are converted into multi-coordinate geometries with strong Ag-O bonds, whereas the iodine centered systems remain linear and lack direct charge transfer interaction with the counterion, as verified by 15N NMR and DFT computation. This suggests that the [N-I-N]+ halogen bond may not be satisfactorily described in terms of a pure coordination bond typical of transition metal complexes, but as a secondary bond with a substantial charge-transfer character.

In situ preactivation strategies for the expeditious synthesis of oligosaccharides: A review

Carbohydrates have gained increasing appreciation over the last few decades for their fundamental roles in all essential areas of life. As a result, there has been a surge of activity in synthetic glycosylation strategies to construct useful oligosaccharides. This review evaluates the advances in synthetic carbohydrate chemistry, specifically preactivation methodologies, stereoselective β-mannosylations, and an automated, electrochemical preactivation method. Also discussed is the use of preactivation as a tool to study reactive intermediates, and applications of preactivation protocols in the one pot-synthesis of a hyaluronic acid decasaccharide and one-pot synthesis of a tristearoyl lipomannan containing a pseudotrisaccharide.

Glycosyl fluorides from n-pentenyl-related glycosyl donors — Application to glycosylation strategies

n-Pentenyl glycosides (NPGs) and n-pentenyl orthoesters (NPOEs) have been transformed into glycosyl fluorides by a variety of methods. In the case of NPGs, Barluenga’s reagent, bis(pyridinium)iodonium(I)tetrafluoroborate (IPy2BF4), gives good yields of glycosyl fluorides when HF–pyridine complex is used as an additional fluoride source. NPOEs can be activated either by a combination of electrophilic iodonium (Barluenga’s reagent) and HBF4 or by the action of HF–pyridine complex. The ensuing glycosyl fluorides form a semiorthogonal pair of glycosyl donors when confronted with NPGs.

2-Allylphenyl glycosides as complementary building blocks for oligosaccharide and glycoconjugate synthesis

The O-allylphenyl (AP) anomeric moiety was investigated as a new leaving group that can be activated for chemical glycosylation under a variety of conditions, through both direct and remote pathways. Differentiation between the two activation pathways was achieved in a mechanistic study. The orthogonal-type activation of the AP moiety along with common thioglycosides allows for the execution of efficient oligosaccharide assembly.

Expeditious oligosaccharide synthesis via selective, semi-orthogonal, and orthogonal activation

Traditional strategies for oligosaccharide synthesis often require extensive protecting and/or leaving group manipulations between each glycosylation step, thereby increasing the total number of synthetic steps while decreasing the efficiency of the synthesis. In contrast, expeditious strategies allow for the rapid chemical synthesis of complex carbohydrates by minimizing extraneous chemical manipulations. Oligosaccharide synthesis by selective activation of one leaving group over another is one such expeditious strategy. Herein, the significant improvements that have recently emerged in the area of the selective activation are discussed. The development of orthogonal strategy further expands the scope of the selective activation methodology. Surveyed in this article, are representative examples wherein these excellent innovations have been applied to the synthesis of various oligosaccharide sequences.

On orthogonal and selective activation of glycosyl thioimidates and thioglycosides: application to oligosaccharide assembly

Discrimination among S-thiazolinyl (STaz), S-benzoxazolyl (SBox), and S-ethyl anomeric leaving groups was achieved by fine-tuning activation conditions. Preferential glycosidation of a certain leaving group is determined neither by the strength of the activating reagent nor by the stability of the leaving group itself; instead, the type of activation plays the key role. The activation conditions established herein were applied to a sequential five-step synthesis of a hexasaccharide using six monosaccharide building blocks equipped with six different leaving groups.

Ring-opening hydrofluorination of 2,3- and 3,4-epoxy amines by HBF4·OEt2: application to the asymmetric synthesis of (S,S)-3-deoxy-3-fluorosafingol

Treatment of a range of 2,3- and 3,4-epoxy amines with HBF(4)·OEt(2) at room temperature results in fast and efficient S(N)2-type ring-opening hydrofluorination to give stereodefined amino fluorohydrins. Operational simplicity, scalability, and short reaction time at ambient temperature are notable features of this method. The utility of this methodology is exemplified in a concise asymmetric synthesis of (S,S)-3-deoxy-3-fluorosafingol.

Unexpected orthogonality of S-benzoxazolyl and S-thiazolinyl glycosides: application to expeditious oligosaccharide assembly

Thorough mechanistic studies of the alkylation pathway for the activation of glycosyl thioimidates have led to the development of the "thioimidate-only orthogonal strategy". Discrimination among S-thiazolinyl (STaz) and S-benzoxazolyl (SBox) anomeric leaving groups was achieved by fine-tuning of the activation conditions. Preferential glycosidation of a certain thioimidate is not simply determined by the strength of activating reagents; instead, the type of activation--direct vs indirect--comes to the fore and plays the key role.

Application of the superarmed glycosyl donor to chemoselective oligosaccharide synthesis

Recently, we discovered a novel method for "superarming" glycosyl donors. Herein, this concept has been exemplified in one-pot oligosaccharide syntheses, whereby the superarmed glycosyl donor was chemoselectively activated over traditional "armed" and disarmed glycosyl acceptors. Direct side-by-side comparison of the reactivities of the classic armed and superarmed glycosyl donors further validates the credibility of the novel concept.