A new phenoxyacetate-based linker system for the solid-phase synthesis of oligosaccharides

Glycan Assembly Strategy: From Concept to Application

Glycans have been hot topics in recent years due to their exhibition of numerous biological activities. However, the heterogeneity of their natural source and the complexity of their chemical synthesis impede the progress in their biological research. Thus, the development of glycan assembly strategies to acquire plenty of structurally well-defined glycans is an important issue in carbohydrate chemistry. In this review, the latest advances in glycan assembly strategies from concepts to their applications in carbohydrate synthesis, including chemical and enzymatic/chemo-enzymatic approaches, as well as solution-phase and solid-phase/tag-assisted synthesis, are summarized. Furthermore, the automated glycan assembly techniques are also outlined.

Traceless Photolabile Linker Expedites the Chemical Synthesis of Complex Oligosaccharides by Automated Glycan Assembly

Automated glycan assembly (AGA) aims at accelerating access to synthetic oligosaccharides to meet the demand for defined glycans as tools for molecular glycobiology. The linkers used to connect the growing glycan chain to the solid support play a pivotal role in the synthesis strategy as they determine all chemical conditions used during the synthesis and the form of the glycan obtained at the end of it. Here, we describe a traceless photolabile linker used to prepare carbohydrates with a free reducing end. Modification of the o-nitrobenzyl scaffold of the linker is key to high yields and compatibility with the AGA workflow. The assembly of an asymmetrical biantennary N-glycan from oligosaccharide fragments prepared by AGA and linear as well as branched β-oligoglucans is described to illustrate the power of the method. These substrates will serve as standards and biomarkers to examine the unique specificity of glycosyl hydrolases.

Oligosaccharide synthesis on soluble high-molecular weight pHEMA using a photo-cleavable linker

Oligosaccharide synthesis on organic solvent soluble, high molecular weight poly(2-hydroxyethylmethylacrylate) (pHEMA) is described. The pHEMA-bound oligosaccharide could be recovered after each reaction in 90–95% yield using a precipitation method. The methodology was used to synthesize a model tri-galactoside in 48% overall yield and a trisaccharide from the outer core domain of Pseudomonas aeruginosa lipopolysacchride (LPS) in 39% yield. The use of a photo-cleavable linker is also demonstrated to produce reducing-end protected oligosaccharides.

Oligosaccharide synthesis on organic solvent soluble, high molecular weight poly(2-hydroxyethylmethylacrylate) (pHEMA) is described.

Solid-Phase Oligosaccharide Synthesis of a Small Library of N-Glycans

Solid-phase oligosaccharide synthesis is based on a hydroxymethylbenzyl benzoate spacer linker which is connected to the Merrifield resin (1 P). Glycosylation was performed with O-glycosyl trichloroacetimidates of glucosamine, mannose, and galactose permitting chain extension (2e, 5e), branching (4b, 7b, 8b), and chain termination (3t, 6t, 9t) with the use of O-benzyl, O-benzoyl, and N-dimethylmaleoyl as permanent and O-fluorenylmethoxycarbonyl (Fmoc) and O-phenoxyacetyl (PA) as temporary protecting groups. The steps required on solid phase are i) glycosylation under TMSOTf catalysis, ii) selective cleavage of the temporary protecting groups, Fmoc with NEt3 and PA with 0.5 equivalents of NaOMe in CH2Cl2/MeOH, and iii) product cleavage from the resin with 4.0 equivalents of NaOMe in CH2Cl2/MeOH and following O-acetylation for convenient product isolation. Thus a highly successful synthesis of a small library of seventeen N-glycan structures was made possible comprising the N-glycan pentasaccharide core structure 53 and two further chain extended hexa- and heptasaccharide N-glycans with a glucosamine or a lactosamine residue, respectively, which is attached to one of the mannose residues of the core structure (56 and 59).

Synthesis of the Lewis a Trisaccharide Based on an Anomeric Silyl Fluorous Tag

The synthesis of the trisaccharide Lewis a was performed using an anomeric fluorous silyl protective group. This methodology allowed us to fully characterize each product (NMR, MS) and monitor each synthetic step (TLC). Although the product purifications could be performed by fluorous-solid-phase extraction (F-SPE) technology, standard chromatography could be used to effect purification if necessary. Trichloroethoxy carbonyl (Troc) protection of the amino group of the glucosamine moiety was found essential to allow protecting group manipulation of the fluorous protected sugar.

Solid-Phase Synthesis of Complex Oligosaccharides Using a Novel Capping Reagent

Solid-phase-supported oligosaccharide synthesis of a core N-glycan tetrasaccharide and of a trisaccharide containing the Galili antigen is reported. The synthesis is based on a hydroxymethylbenzyl benzoate spacer-linker system attached to the Merrifield resin, O-Fmoc-protected O-glycosyl trichloroacetimidates as glycosyl donors, and benzoyl isocyanate as a capping reagent for low-reactivity hydroxy groups. In this way, the target molecules could be efficiently obtained with little byproduct formation, and hence final purification was convenient.

Exploring privileged structures: the combinatorial synthesis of cyclic peptides

Head-to-tail cyclic peptides have been reported to bind to multiple, unrelated classes of receptor with high affinity. They may therefore be considered to be privileged structures. This review outlines the strategies by which both macrocyclic cyclic peptides and cyclic dipeptides or diketopiperazines have been synthesised in combinatorial libraries. It also briefly outlines some of the biological applications of these molecules, thereby justifying their inclusion as privileged structures.

Solid-phase C-C and C-O bond forming reactions using 'traceless' pi-arene-chromium linkers

The scope of solid-phase reactions utilizing a 'traceless' pi-arene-chromium linker has been expanded to include the addition of organo-lithium reagents to a supported ester group, generation and alkylation of an enolate anion, and substitution via the Mitsunobu reaction. Immobilization of ortho-disubstituted polyfunctionalized arene chromium dicarbonyl species onto the solid support and subsequent, selective transformation is also reported.

Exploring privileged structures: the combinatorial synthesis of cyclic peptides

Head-to-tail cyclic peptides have been reported to bind to multiple, unrelated classes of receptor with high affinity. They may therefore be considered to be privileged structures. This review outlines the strategies by which both macrocyclic cyclic peptides and cyclic dipeptides or diketopiperazines have been synthesised in combinatorial libraries. It also briefly outlines some of the biological applications of these molecules, thereby justifying their inclusion as privileged structures.

Solid-phase synthesis of a branched hexasaccharide using a highly efficient synthetic strategy

The solid-phase synthesis of branched lacto-N-neohexaose derivative 1 occurring in human milk is described. The new building block of lactose 3 bearing the orthogonal temporary hydroxy protecting groups 9-fluorenylmethyloxycarbonyl (Fmoc) and levulinoyl (Lev) has been prepared. Its use, together with that of lactosamine donor 4, glucosamine donor 5, and O-galactosyl trichloroacetimidate 6, has enabled the preparation of hexasaccharide 22 following two different approaches in excellent overall yield (43%, 90% per step over eight steps). An additional key feature of this work is the successful use of newly prepared ester-type linker 2, having a benzylic spacer connected to the anomeric oxygen. This linker presents the advantage of producing a benzylic anomeric moiety after cleavage from the polymer support, which could be easily removed to obtain the unprotected oligosaccharide 1.

Wang resin-type linker containing a nitro group for polymer support oligosaccharide synthesis: polymer-supported glycosyl donor

A nitro-introduced Wang resin-type linker for soluble and insoluble polymer support oligosaccharide synthesis is described. The linker was used for connecting glycosyl donors and polymer supports, and was completely stable under the glycosylation conditions tested. The cleavage of the linker was performed under reductive conditions without affecting the protecting groups to release disaccharides.