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

Discovery and characterization of a novel α-l-fucosidase from the marine-derived Flavobacterium algicola and its application in 2'-fucosyllactose production

2'-Fucosyllactose (2'-FL) is one of the nutrient ingredients in human milk, which has various beneficial health effects. α-l-fucosidase is a biotechnological tool for 2'-FL preparation. Here, a novel and efficient α-l-fucosidase OUC-Jdch16 from the fucoidan-digesting strain Flavobacterium algicola 12076 was heterologously expressed and applied to produce 2'-FL in vitro. OUC-Jdch16 belongs to glycoside hydrolases (GH) family 29 and exhibits the highest 4-nitrophenyl-α-l-fucopyranoside-hydrolyzing activity at 25 °C and pH 6.0. OUC-Jdch16 could catalyze the synthesis of 2'-FL via transferring the fucosyl residue from pNP-α-fucose to lactose. Under the optimal transfucosylation conditions, the yield of the transfucosylation product reached 84.82% and 92.15% (mol/mol) from pNP-α-fucose within 48 h and 120 h, respectively. Moreover, OUC-Jdch16 was capable of transferring the fucosyl residue to other glycosyl receptors with the generation of novel fucosylated compounds. This study demonstrated that OUC-Jdch16 could be a promising tool to prepare 2'-FL and other novel glycosides.

Recent advances on 2'-fucosyllactose: physiological properties, applications, and production approaches

The trisaccharide, 2'-fucosyllactose (Fucα1-2Galβ1-4Glc; 2'-FL), is the most abundant oligosaccharide in human milk. It has numerous significant biological properties including prebiotics, antibacterial, antiviral, and immunomodulating effects, and has been approved as "generally recognized as safe" (GRAS) by the Food and Drug Administration (FDA) and as a novel food (NF) by the European Food Safety Authority (EFSA). 2'-FL not only serves as a food ingredient added in infant formula, but also as a dietary supplement and medical food material in food bioprocesses. There is considerable commercial interest in 2'-FL for its irreplaceable nutritional applications. This review aims at systematically elaborating key functional properties of 2'-FL as well as its applications. In addition, several approaches for 2'-FL production are described in this review, including chemical, chemo-enzymatical, and cell factory approaches, and the pivotal research results also have been summarized. With the rapid development of metabolic engineering and synthetic biology strategies, using the engineered cell factory for 2'-FL large-scale production might be a promising approach. From an economic and safety point of view, microbial selection for cell factory engineering in 2'-FL bioprocess also should be taken into consideration.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

Chemical synthesis of human milk oligosaccharides: lacto-N-neohexaose (Galβ1 → 4GlcNAcβ1→)2 3,6Galβ1 → 4Glc

The first chemical synthesis of lacto-N-neohexaose (LNnH) has been completed using a convergent synthetic strategy. The reaction conditions and donor-acceptor combinations have been carefully refined to minimize side reactions and achieve high yields in all glycosylation steps. Lacto-N-neotetraose, another common human milk oligosaccharide, was also synthesized en route to the target LNnH.

BODIPYs as Chemically Stable Fluorescent Tags for Synthetic Glycosylation Strategies towards Fluorescently Labeled Saccharides

A series of fluorescent boron-dipyrromethene (BODIPY, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been designed to participate, as aglycons, in synthetic oligosaccharide protocols. As such, they served a dual purpose: first, by being incorporated at the beginning of the process (at the reducing-end of the growing saccharide moiety), they can function as fluorescent glycosyl tags, facilitating the detection and purification of the desired glycosidic intermediates, and secondly, the presence of these chromophores on the ensuing compounds grants access to fluorescently labeled saccharides. In this context, a sought-after feature of the fluorescent dyes has been their chemical robustness. Accordingly, some BODIPY derivatives described in this work can withstand the reaction conditions commonly employed in the chemical synthesis of saccharides; namely, glycosylation and protecting-group manipulations. Regarding their photophysical properties, the BODIPY-labeled saccharides obtained in this work display remarkable fluorescence efficiency in water, reaching quantum yield values up to 82 %, as well as notable lasing efficiencies and photostabilities.

The chemical synthesis of human milk oligosaccharides: Lacto-N-neotetraose (Galβ1→4GlcNAcβ1→3Galβ1→4Glc)

The discovery of innovative methods that offer new capabilities for obtaining individual oligosaccharides from human milk will help to improve understanding their roles and boost practical applications. The total chemical synthesis of lacto-N-neotetraose (LNnT) has been completed using both linear and convergent strategies. The donor and acceptor protecting and leaving group combinations were found to be of paramount significance to successful couplings.

Design and total synthesis of (-)-codonopsinine, (-)-codonopsine and codonopsinine analogues by O-(2-oxopyrrolidin-5-yl)trichloroacetimidate as amidoalkylating agent with improved antimicrobial activity via solid lipid nanoparticle formulations

A general strategy towards total synthesis of (-)-codonopsinine, (-)-codonopsine and codonopsinine analogues has been developed from (D)-tartaric acid via the intermediate (3S,4R)-1-methyl-2-oxo-5-(2,2,2-trichloroacetamido)pyrrolidinediacetate (7). α-amidoalkylation studies of 7 with electron rich benzene derivative 8a-g as C-nucleophiles afforded (aryl derivatives) 9a-g. The target compounds 1, 2 and 13c-g were readily obtained from 10a-gvia Grignard addition to the homochiral lactam which was produced by deoxygenation using Lewis-acid followed by deacetylation. The synthesized compounds were loaded onto solid lipid nanoparticle formulations (SLNs) prepared by hot emulsification-ultrasonication technique using Compritol as solid lipid and Pluronic f68 as surfactant. SLNs were fully evaluated and the permeation of synthesized compound from SLNs was assayed against non-formulated compounds through dialysis membranes using Franz cell. The data indicated good physical characteristics of the prepared SLNs, sustaining of release profiles and significant improvement of permeation ability when compared to the non-formulated compounds. The antibacterial and antifungal activities of 1, 2 and 13c-g were determined by disc diffusion and microbroth dilution method to determine the minimum inhibitory concentrations (MIC) against seven microorganisms (Staphyloccus aureus, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Acinetobacter baumannii and Candida albicans). The most active compounds against the Gram positive S. aureus were 1, 13C, 13d, and 13g. Also, 13c, 13d, and 13e had antibacterial activity but not 13f against some Gram negative organisms (E. coli, and P. mirabilis). MIC concentrations against P. aeruginosa, and K. pneumoniae were ≥512 μg/ml, while that against A. baumannii was ≥128 μg/ml except for nanoformulae of 13e and 13f that were 16 and 64 μg/ml, respectively. No antifungal activity against Candida albicans was recorded for all compounds and their nanoformulae (MIC > 1024 μg/ml). SLNs were found to decrease the MIC values for some of the compounds with no effect on the antifungal activity. In conclusion, we demonstrated a novel, straight-forward and economical procedure for the total synthesis of (-)-codonopsinine 1, (-)-codonopsine 2 and codonopsinine analogues 13c-g from simple and commercially available starting materials; d-tartaric acid; with antimicrobial activities against Gram positive and Gram-negative organisms that were improved by SLNs formulations.

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.

Cyanopivaloyl Ester in the Automated Solid-Phase Synthesis of Oligorhamnans

The development of effective protecting group chemistry is an important driving force behind the progress in the synthesis of complex oligosaccharides. Automated solid-phase synthesis is an attractive technique for the rapid assembly of oligosaccharides, built up of repetitive elements. The fact that (harsh) reagents are used in excess in multiple reaction cycles makes this technique extra demanding on the protecting groups used. Here, the synthesis of a set of oligorhamnan fragments is reported using the cyanopivaloyl (PivCN) ester to ensure effective neighboring group participation during the glycosylation events. The PivCN group combines the favorable characteristics of the parent pivaloyl (Piv) ester, stability, minimal migratory aptitude, minimal orthoester formation, while it can be cleaved under mild conditions. We show that the remote CN group in the PivCN renders the PivCN carbonyl more electropositive and thus susceptible to nucleophilic cleavage. This feature is built upon in the automated solid-phase assembly of the oligorhamnan fragments. Where the use of a Piv-protected building block failed because of incomplete cleavage, PivCN-protected synthons performed well and allowed the generation of oligorhamnans, up to 16 monosaccharides in length.

Synthesis of asymmetrical multiantennary human milk oligosaccharides

Despite mammalian glycans typically having highly complex asymmetrical multiantennary architectures, chemical and chemoenzymatic synthesis has almost exclusively focused on the preparation of simpler symmetrical structures. This deficiency hampers investigations into the biology of glycan-binding proteins, which in turn complicates the biomedical use of this class of biomolecules. Herein, we describe an enzymatic strategy, using a limited number of human glycosyltransferases, to access a collection of 60 asymmetric, multiantennary human milk oligosaccharides (HMOs), which were used to develop a glycan microarray. Probing the array with several glycan-binding proteins uncovered that not only terminal glycoepitopes but also complex architectures of glycans can influence binding selectivity in unanticipated manners. N- and O-linked glycans express structural elements of HMOs, and thus, the reported synthetic principles will find broad applicability.

Synthesis of lacto-N-tetraose

Human milk oligosaccharides (HMOs) are the third largest macromolecular component of breast milk and offer infants numerous health benefits, most of which stem from the development of a healthy microbiome. Characterization, quantification, and chemical derivatization of HMOs remains a frontier issue in glycobiology due to the challenge of isolating appreciable quantities of homogenous HMOs from breast milk. Herein, we report the synthesis of the human milk tetrasaccharide lacto-N-tetraose (LNT). LNT is ubiquitous in human breast milk as it is a core structure common to longer-chain HMOs and many glycolipids.

Determining Substrate Specificities of β1,4-Endogalactanases Using Plant Arabinogalactan Oligosaccharides Synthesized by Automated Glycan Assembly

Pectin is a structurally complex plant polysaccharide with many industrial applications in food products. The structural elucidation of pectin is aided by digestion assays with glycosyl hydrolases. We report the automated glycan assembly of oligosaccharides related to the arabinogalactan side chains of pectin as novel biochemical tools to determine the substrate specificities of endogalactanases. Analysis of the digestion products revealed different requirements for the lengths and arabinose substitution pattern of the oligosaccharides to be recognized and hydrolyzed by the galactanases.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Chemoenzymatic Synthesis of a Library of Human Milk Oligosaccharides

Human milk oligosaccharides (HMOs) are a family of diverse unconjugated glycans that exist in human milk as one of the major components. Characterization, quantification, and biofunctional studies of HMOs remain a great challenge due to their diversity and complexity. The accessibility of a homogeneous HMO library is essential to solve these issues which have beset academia for several decades. In this study, an efficient chemoenzymatic strategy, namely core synthesis/enzymatic extension (CSEE), for rapid production of diverse HMOs was reported. On the basis of 3 versatile building blocks, 3 core structures were chemically synthesized via consistent use of oligosaccharyl thioether and oligosaccharyl bromide as glycosylation donors in a convergent fragment coupling strategy. Each of these core structures was then extended to up to 11 HMOs by 4 robust glycosyltransferases. A library of 31 HMOs were chemoenzymatically synthesized and characterized by MS and NMR. CSEE indeed provides a practical approach to harvest structurally defined HMOs for various applications.

Recent Advances Toward Robust N-Protecting Groups for Glucosamine as Required for Glycosylation Strategies

2-Amino-2-deoxy-d-glucose (d-glucosamine) is among the most abundant monosaccharides found in natural products. This constituent, recognized for its ubiquity, is presented in most instances as its N-acetyl derivative 2-acetamido-2-deoxy-d-glucopyranose (N-acetylglucosamine, GlcNAc, NAG). It occurs as the β-linked pyranosyl group in polysaccharides and oligosaccharides, and sometimes as the monosaccharide itself, either in its native state or as a glycoconjugate. The compound's acylation profile and other aspects of its structure are important elements in determining the variety of reactivities and functions of the molecule as a whole. Methods elaborated to investigate these challenges have been intensively reviewed; however, a relatively more comprehensive reviewing of this subject is introduced here to cover some aspects that have not been sufficiently covered. This might enable those who are beginners in this field to be aware of the subject in a more comprehensive context. 2-Amino-2-deoxy-d-glucosylation strategies demand robust amino-protecting groups that survive under a variety of chemical conditions, yet provide groups that can be deprotected under relatively mild conditions. At the end of this review, a table that includes all the N-protecting groups that have been used for glucosamine is provided to introduce them at a glance to aid in constructing building blocks that will act as useful 2-amino-2-deoxy-d-glucosyl donors.

Human Milk Oligosaccharides (HMOS): Structure, Function, and Enzyme-Catalyzed Synthesis

The important roles played by human milk oligosaccharides (HMOS), the third major component of human milk, in the health of breast-fed infants have been increasingly recognized, as the structures of more than 100 different HMOS have now been elucidated. Despite the recognition of the various functions of HMOS as prebiotics, antiadhesive antimicrobials, and immunomodulators, the roles and the applications of individual HMOS species are less clear. This is mainly due to the limited accessibility to large amounts of individual HMOS in their pure forms. Current advances in the development of enzymatic, chemoenzymatic, whole-cell, and living-cell systems allow for the production of a growing number of HMOS in increasing amounts. This effort will greatly facilitate the elucidation of the important roles of HMOS and allow exploration into the applications of HMOS both as individual compounds and as mixtures of defined structures with desired functions. The structures, functions, and enzyme-catalyzed synthesis of HMOS are briefly surveyed to provide a general picture about the current progress on these aspects. Future efforts should be devoted to elucidating the structures of more complex HMOS, synthesizing more complex HMOS including those with branched structures, and developing HMOS-based or HMOS-inspired prebiotics, additives, and therapeutics.

Bovine milk as a source of functional oligosaccharides for improving human health

Human milk oligosaccharides are complex sugars that function as selective growth substrates for specific beneficial bacteria in the gastrointestinal system. Bovine milk is a potentially excellent source of commercially viable analogs of these unique molecules. However, bovine milk has a much lower concentration of these oligosaccharides than human milk, and the majority of the molecules are simpler in structure than those found in human milk. Specific structural characteristics of milk-derived oligosaccharides are crucial to their ability to selectively enrich beneficial bacteria while inhibiting or being less than ideal substrates for undesirable and pathogenic bacteria. Thus, if bovine milk products are to provide human milk-like benefits, it is important to identify specific dairy streams that can be processed commercially and cost-effectively and that can yield specific oligosaccharide compositions that will be beneficial as new food ingredients or supplements to improve human health. Whey streams have the potential to be commercially viable sources of complex oligosaccharides that have the structural resemblance and diversity of the bioactive oligosaccharides in human milk. With further refinements to dairy stream processing techniques and functional testing to identify streams that are particularly suitable for enriching beneficial intestinal bacteria, the future of oligosaccharides isolated from dairy streams as a food category with substantiated health claims is promising.

Lewis Acid Catalyzed Propargylation of Arenes with O-Propargyl Trichloroacetimidates: Synthesis of 1,3-Diarylpropynes

The BF3.OEt2-catalyzed Friedel-Crafts propargylation of aromatic compounds with O-propargyl trichloroacetimidates is highly efficient and affords 1,3-diarylpropyne derivatives in good yields.

Synthesis and biological relevance of N-acetylglucosamine-containing oligosaccharides

The structural diversity as well as the biological significance of N-acetylglucosamine-containing glycans are exemplified. The problem of forming the respective glycosidic bonds of synthetic targets is addressed. Special emphasis has been given to human milk oligosaccharides (HMOs), in view of their biological relevance, and synthetic approaches of selected examples are reported.

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.

De novo asymmetric bio- and chemocatalytic synthesis of saccharides - stereoselective formal O-glycoside bond formation using palladium catalysis

A novel integrated bio- and chemocatalytic approach to the de novo catalytic asymmetric synthesis of saccharides has been developed. Acetoxypyranones obtained enantiopure by enzymatic resolution have been shown to undergo highly stereoselective palladium-catalyzed formal O-glycoside bond formation. The combination of these protocols can be applied to the iterative asymmetric catalytic synthesis of saccharides.

Toward the automated solid-phase synthesis of oligoglucosamines: systematic evaluation of glycosyl phosphate and glycosyl trichloroacetimidate building blocks

Glucosamines are common components of many biologically important oligosaccharides. Reported is a systematic evaluation of glucosamine phosphates and trichloroacetimidates as glycosylating agents for the efficient construction of beta-(1 --> 6) glucosamine linkages. A set of differentially protected glucosamine donors incorporating a host of amine protecting groups, including 2-phthaloyl, benzyloxycarbonyl (Z), trichloroetheoxycarbonyl (Troc) and trichloroacetyl (TCA) protective groups, were prepared. Donors were initially evaluated for reactivity and protecting group compatibility in a solution-phase study with a model 6-hydroxyl galactose acceptor. Based on these results, glucosamine donor 10 was selected for the solution-phase synthesis of a beta-(1 --> 6)-glucosamine pentasaccharide. Finally, building block 10 proved well suited for use in the automated solid-phase synthesis of a repeating unit trisaccharide. An assessment of glucosamine phosphate donors as potential glycosylating agents for a variety of glucosamine linkages is also discussed.