Rational optimization of the mannoside building block for automated electrochemical assembly of the core trisaccharide of GPI anchor oligosaccharides

HPLC-Based Automated Synthesis and Purification of Carbohydrates

Reported herein is a new HPLC-based automated synthesizer (HPLC-A) capable of a temperature-controlled synthesis and purification of carbohydrates. The developed platform allows to perform various protecting group manipulations as well as the synthesis of O- and N-glycosides. A fully automated synthesis and purification was showcased in application to different carbohydrate derivatives including glycosides, oligosaccharides, glycopeptides, glycolipids, and nucleosides.

Expedient Synthesis of Superarmed Glycosyl Donors via Oxidative Thioglycosidation of Glycals

Superarmed glycosyl donors have higher reactivity compared to their perbenzylated armed counterparts. Generally, the 2-O- benzoyl-3,4,6-tri-O-benzyl protecting group pattern gives rise to increased reactivity due to an O-2/O-5 cooperative effect. Despite having a high reactivity profile and applicability in many expeditious strategies for glycan synthesis, regioselective introduction of the superarming protecting group pattern is tedious for most sugar series. Reported herein is a streamlined synthetic route to yield superarmed glycosyl donors of the d-gluco and d-galacto series equipped with an ethylthio, phenylthio, p-tolylthio, benzoxazol-2-ylthio, O-allyl, or O-pentenyl anomeric leaving group. This streamlined approach was made possible due to the refinement of the oxidative thioglycosylation reaction of the respective glucal and galactal precursors. The applicability of this approach to the direct formation of disaccharides is also showcased.

Electrochemical Assembly for Synthesis of Middle-Sized Organic Molecules

Electrochemical methods offer a powerful, reliable, and environmentally benign approach for the synthesis of small organic molecules, and such methods are useful not only for the transformation of small molecules, but also for the preparation of oligomers and polymers. Electrochemical assembly is a concept that allows structurally well-defined middle-sized organic molecules to be synthesized by applying electrochemical methods. The preparation of dendrimers, dendronized polymers, and oligosaccharides are introduced as examples of such an approach. Automated electrochemical assembly of oligosaccharides is also demonstrated using the electrochemical synthesizer developed by our group.

HPLC-assisted automated oligosaccharide synthesis: the implementation of the two-way split valve as a mode of complete automation

Reported herein is the development of a user-friendly platform for simple and transformative automation based on standard HPLC equipment. We showcase how the improved platform works in application to the completely automated, a "press of the button," synthesis of various glycan sequences.

Automated solution-phase syntheses of alpha 1 → 2, 1 → 3 type rhamnans and rhamnan sulfate fragments

Rhamnan and rhamnan sulfate are naturally occurring carbohydrates that have important biological functions and possible therapeutic applications, but studies are limited to the microheterogeneous mixtures from natural sources. This work reports the first synthesis of any sulfated rhamnan fragments and successful automation of the process with a recently developed automated solution-phase approach using N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promotor and levulinoyl ester deprotection conditions. The automated solution-phase activation/deprotection approach was initially able to create alpha 1 → 2, 1 → 3 type rhamnan di- and trisaccharide in moderate yields. Once these targets were achieved, a process to use SO₃•pyridine complex in DMF for sulfation compatible with an automated solution-phase liquid handling system was developed and successfully applied to carbohydrate sulfation to create two rhamnan sulfate fragments with differing monosulfation patterns.

Electrochemical Glycosylation as an Enabling Tool for the Stereoselective Synthesis of Cyclic Oligosaccharides

Electrochemical glycosylation of a linear oligosaccharide with a protecting-group-free primary hydroxyl group afforded cyclic oligo-saccharides, up to hexasaccharides, in high yields. Precursors of the cyclic oligosaccharides were prepared by automated electro-chemical assembly-a method for the automated electrochemical solution-phase synthesis of oligosaccharides. We demonstrated that electrochemical glycosylation is useful not only for intermolecular glycosylation but also for intramolecular glycosylation to synthesize cyclic oligosaccharides.

Acid-mediated N-iodosuccinimide-based thioglycoside activation for the automated solution-phase synthesis of α-1,2-linked-rhamnopyranosides

Carbohydrate structures are often complex. Unfortunately, synthesis of the range of sugar combinations precludes the use of a single coupling protocol or set of reagents. Adapting known, reliable bench-chemistry reactions to work via automation will help forward the goal of synthesizing a broad range of glycans. Herein, the preparation of di- and tri-saccharides of alpha 1→2 rhamnan fragments is demonstrated using thioglycoside donors with the development for a solution-phase-based automation platform of commonly used activation conditions using N-iodosuccinimide (NIS) with trimethylsilyl triflate. Byproducts of the glycosylation reaction are shown to be compatible with hydrazine-based deprotection conditions, lending broader functionality to this method as only one fluorous-solid-phase extraction step per coupling/deprotection cycle is required.

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.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.