Shin-ichiro Shoda
Professor Emeritus, Tohoku University
Shin-ichiro Shoda received his Ph.D. degree from the University of Tokyo in 1981 in the field of synthetic organic chemistry, where he developed the glycosyl fluoride method as a novel glycosylating technology. After conducting his postdoctoral fellowship at ETH-Zürich (1984-1986), he moved to Tohoku University, where he developed new enzymatic glycosylations by using glycosyl fluorides and sugar oxazolines as substrates. In 1999, he was promoted to Full Professor at Tohoku University (Functional Macromolecular Chemistry Laboratory). He has been a Member of the Research Center for Science System at the Japan Society for the Promotion of Science (2003-2005). His research interests include the development of novel glycosylation methods, especially in recent years, protecting group-free glycosylations. He received the Award of the Chemical Society of Japan for Young Chemists (1986), Science and Technology Institute Award for New Invention (1993), the Cellulose Society of Japan Award (2002), Synthetic Organic Chemistry Award (2013), and the Ichimura Prize in Science for Distinguished Achievement (2019).
Summary
The oxazoline derivatives of sugars are a class of valuable and versatile intermediates in glycoscience. This article describes the historical background and chemical logic of glycoremodeling via sugar oxazoline donors. In the first part, several methods for preparation of protected sugar oxazolines as well as their applications are introduced. In the second part, the recent development of protection-free chemo-enzymatic process for synthesis of chitooligosaccharides derivatives and glycoproteins through unprotected sugar oxazoline donors is introduced based on the concept of “ aqueous direct anomeric activation”.
1. Introduction
The N-acetyl-β-glucosaminide moieties of oligosaccharides, polysaccharides, and glycoproteins play pivotal roles in the function of these biologically important carbohydrates (Fig. 1). Lacto-N-tetraose 1 including a β-linked N-acetyl-ᴅ-glucosamine (GlcNAc) has gathered a lot of interest as a human milk oligosaccharide due to its health-promoting effects in the intestine of breastfed infants1-3. Chitooligosaccharides (β-1,4-linked linear oligomers of GlcNAc) 2 show higher elicitor activities, inducing immune responses of plants4,5. The stability and activity of glycoproteins 3 are known to depend on the structures of the carbohydrate moieties. Erythropoietin, a glycoprotein cytokine secreted mainly by the kidneys in response to cellular hypoxia, requires sialo-complex-type glycans having GlcNAc moieties for their in vivo activity6.
Fig. 1. Naturally occurring carbohydrates having N-acetyl-β-glucosaminide moieties
Sugar oxazolines are one of the most frequently used building blocks for the synthesis of N-acetylglucosaminides. Sugar
oxazolines possess a bicyclic structure where a six-membered pyranose ring and a five-membered oxazoline ring7 are fused sharing C1 (anomeric carbon) and C2 carbon (Fig. 2).
Fig. 2. Basic structure of sugar oxazolines
“Retrosynthesis” is an important technique in the design of a synthesis of organic compounds. It is achieved by transforming a target molecule into simpler precursor structures8. An N-acetyl-β-glucosaminyl bond can be retro-synthetically disconnected to give an oxazolinium ion as a synthon (Fig. 3 A). On the basis of the retro-synthetic analysis, one of the most promising in-flask reactions for construction of N-acetyl-β-glucosaminide moieties would be the addition of an alcohol to the anomeric center of sugar oxazolines by the promotion of protonic acid (Fig. 3 B). In fact, sugar oxazolines are known to be useful intermediates in the direct conversion of O-1-acetates to glycosides9.
Fig. 3. Retrosynthesis (A) and synthesis (B) of N-acetyl-β-glucosaminide
A: The open arrow represents the direction of the transformation from the target molecule to the synthon.
B: N-acetyl-β-glucosaminide is formed by the addition of alcohol to sugar oxazoline.
In the first part of this article, a review is given on sugar oxazolines whose hydroxy groups are suitably protected. The classical synthetic
methods of sugar oxazolines and their use as glycosyl donors for chemical glycosylations are described. In the second part, we focus on the chemistry of unprotected sugar oxazolines. Recently, one-step preparation of unprotected sugar oxazolines has been developed by using formamidinium-type dehydrating agents giving rise to complex sugar oxazolines without protection of the hydroxy groups. By using the resulting unprotected sugar oxazolines as glycosyl donors, it has become possible to synthesize various glycoconjugates such as glycoproteins through chemo-enzymatic glycosylations.
2. Chemistry of protected sugar oxazolines
2-1. Preparation of protected sugar oxazolines
The sugar oxazolines whose hydroxy groups are suitably protected have widely been used as glycosyl donors for synthesis of a variety
of N-acetyl-β-glucosaminides10. Protected sugar oxazolines can be prepared starting from free sugars, peracetylated sugars, or glycosyl chlorides under acidic conditions. A well-developed method for the synthesis of peracetylated sugar oxazoline derivatives includes treatment of N-acetylated free amino sugars with hydrogen chloride in acetyl chloride followed by treatment with a silver salt and 2,4,6-collidine11 (Fig. 4, method A). Fully acetylated amino sugars can also be converted to the corresponding oxazoline derivatives by using Lewis acids such as anhydrous iron(III) chloride12, tin(IV) chloride13, or trimethylsilyl triflate14 (Fig. 4, method B). An acid-catalyzed acetolysis of the corresponding methyl glycoside has also been reported15.
The synthesis of peracetylated sugar oxazolines can also be achieved under basic conditions (Fig. 4, method C). The treatment of protected 2-acetamido-2-deoxy-α-glycosyl chlorides with tetraalkylammonium chloride in the presence of sodium bicarbonate gave the corresponding sugar oxazolines16. The reaction requires two kinds of chemical species, the chloride ion and the general base, for the anomerization of α-glycosyl chloride into its β form and the subsequent proton abstraction from the 2-acetamide group, respectively. Recently, a more practical method using potassium fluoride which behaves as a nucleophile for anomerization of glycosyl chloride as well as an acid captor has been developed17,18. According to this method using potassium fluoride, it is not necessary to use quaternary ammonium chloride, which is difficult to remove from the reaction mixture.
2-2. Chemical glycosylation through protected sugar oxazoline donors19
Suitably protected sugar oxazolines can be stereoselectively converted to the trans-2-acetamidoglycosides by using various activators such
as Yb(OTf)320, p-toluene sulfonic acid ( p-TsOH)11, FeCl39, trimethylsilyl trifluormethanesulfonate (TMSOTf)21,22, camphorsulfonic acid (CSA)23, CuCl224, pyridinium triflate25, BF3-OEt226. Although the oxazoline method is incompatible with acid-sensitive functional groups due to the strongly acidic conditions required for the activation of the oxazoline ring, some important examples of the application of the oxazoline method have been reported. For example, the methyl oxazoline of ᴅ-glucosamine derivatives was used as monomers for the synthesis of linear and hyperbranched aminopolysaccharides27,28 (Fig. 5).
Fig. 5. Synthesis of hyperbranched aminopolysaccharides from partially protected sugar oxazoline monomer. CSA: camphorsulfonic acid
3. Chemistry of unprotected sugar oxazolines
As mentioned in the previous chapter, many efficient chemical N-acetylglucosaminidations using sugar oxazolines as glycosyl donors have been reported, where protecting groups play the key role in achieving the regio- and stereo-selectivity of the reactions29. In this chapter, protection-free protocols for synthesis of various kinds of N-acetylglucosaminides such as chitooligosaccharides and glycoproteins are described, mainly focusing on the newly developed strategy of combining unprotected sugar oxazolines as glycosyl donors with N-acetylglucosaminidases having low hydrolysis activity as catalysts. Non-enzymatic ring opening of unprotected sugar oxazolines will also be mentioned.
3-1. Historical background
In contrast to protected sugar oxazoline derivatives, unprotected sugar oxazolines had not been reported before the chitinase-catalyzed polyaddition of N, N’-diacetylchitobiose oxazoline was demonstrated in 199630. It was found for the first
time that a sugar oxazoline can behave as a substrate for an N-acetylglucosaminidase. Since then, there have been many reports on enzymatic transglycosylations using unprotected sugar oxazolines as glycosyl donors31.
Another turning point of unprotected sugar oxazoline chemistry was the discovery of the direct conversion of unprotected 2-acetamido-sugars to the corresponding sugar oxazolines by using a formamidinium-type dehydrating agent, 2-chloro-1,3-dimethylimidazolinium chloride (DMC)32. These findings triggered the development of a “ dehydrative process in aqueous
media” for construction of N-acetylglucosaminide moieties without using any protecting groups.
3-2. How to achieve dehydration in water
In general, carbohydrates show a strong affinity for water. Water molecules tend to form a favored tridymite structure, which is maintained by the equatorial hydroxy groups of glucose having a 4C1 conformation33,34. This suggests that the best solvent for glycosylation (dehydration between a hemiacetal and an alcohol) would be water. However, intermolecular dehydrative condensation in aqueous media is considered extremely disadvantageous due to the large negative values of standard Gibbs energy of formation for glycosidic bond hydrolysis; the equilibrium composition is shifted significantly towards the hydrolyzates.
A novel dehydrative process for N-acetylglucosaminide synthesis through a series of two elemental reactions has been proposed (Fig. 6). The first is the intramolecular dehydration reaction (Fig. 6 A) facilitated by using a water-soluble dehydrating agent. The second is the intermolecular addition reaction (Fig. 6 B) catalyzed by a glycosidase or a protonic acid. The important thing is that a dehydrative condensation is achieved not via a single reaction but through two elemental steps (A and B)35.
Fig. 6. The dehydrative N-acetylglucosaminidation process in water, which consists of intramolecular dehydration of unprotected 2-acetamidosugar by a dehydrating agent and intermolecular addition catalyzed by a glycosidase or a protonic acid
3-3. Preparation of unprotected sugar oxazolines in aqueous solution
In principle, the synthesis of unprotected sugar oxazolines can be achieved by removing the acetyl groups from the corresponding peracetylated derivatives which are preparable by the methods described in 2-1. However, these methods include carefully designed functional group transformations and protection/deprotection steps. Obviously, a protecting group-free preparation of sugar oxazolines has several advantages and practical values particularly for the synthesis of complex oligosaccharide oxazolines. The development of a new method without using protecting groups had, therefore, been strongly demanded.
The protection-free synthesis of sugar oxazolines was firstly achieved in 2004 by an intramolecular dehydration using water-soluble carbodiimides as dehydrating agents36. However, the yield was at most 37% due to the lower reactivity of the O-glycosyl isourea intermediates. After screening more reactive water-soluble dehydrating agents, it was found that the use of a cation-type formamidinium salt, 2-chloro-1,3-dimethylimidazolinium chloride (DMC)37,38, greatly increased the yield32.
The reaction has successfully been applied to complex oligosaccharides (Fig. 7). Chitooligosaccharides that are difficult to derivatize by the conventional method could be converted to the corresponding oxazoline derivatives in good yields. The reaction is also applicable to monosaccharides possessing a sulfuric acid moiety as well as complex-type oligosaccharide having two N-acetylneuraminic acid moieties.
Fig. 7. Direct synthesis of sugar oxazolines by using DMC agent in aqueous media without protection of hydroxy groups
A modified formamidinium-type agent, 2-chloro-1,3-dimethyl-1 H-benzimidazol-3-ium chloride (CDMBI), was also effective for dehydrative reaction between the 2-acetamido group and the anomeric OH group of unprotected N-acetyl-2-amino sugars, leading to the formation of the oxazoline moiety39. In this reaction, the hydrolyzate of CDMBI, 1,3-dimethyl-1 H-benzimidazol-2-one (DMBI), precipitated from the aqueous mixture due to its higher hydrophobicity. This finding enabled us to remove the DMBI simply by filtration, and the filtrate could be utilized for the subsequent enzymatic transglycosylation without purifying the oxazoline glycosyl donors40,41. Recently, synthetic and semi-synthetic approaches to unprotected N-glycan oxazolines has been reviewed by Fairbanks42.
3-4. Reaction mechanism
The oxazoline ring formation can be explained by assuming the initial attack of the β-hemiacetal 1β upon DMC to give a reactive intermediate 2β having β-configuration (Fig. 8). Then, an intramolecular dehydrative nucleophilic substitution occurs, affording a sugar oxazoline 3. The α-hemiacetal 1α also reacts with DMC to form 2α. The resulting 2α is immediately hydrolyzed to 1α, which is in equilibrium with the 1β.
Fig. 8. Plausible mechanism for sugar oxazoline formation through reactive intermediate 2β
3-5. N-Acetylglucosaminidase-catalyzed transglycosylation using unprotected sugar oxazoline donors
3-5-1. Complementarity of glycosidases
Synthetic chemists in the 21st century are enjoying the advantageous position of being able to use accumulated knowledge of catalysts and catalyst technologies emerging from not only advances in chemistry but also the progress of biological science. Glycosidases, enzymes that catalyze the hydrolysis of glycosidic bond, possess two complementary activities, hydrolysis activity and transglycosylation activity. In nature, the role of glycosidases is to hydrolyze the glycosidic bonds of various kinds of glycosidic compounds. Under special conditions, glycosidases can catalyze glycoside bond formation. N-Acetylglucosaminidases are enzymes that hydrolyze an N-acetylglucosaminide moiety and have extensively been used for transglycosylation reactions in the field of glycotechnology43-45.
3-5-2. The perfect stereoselectivity of glycosidases in transglycosylation
Glycosidases show perfect stereoselectivity in transglycosylation reactions, giving rise to only one anomeric isomer. This phenomenon can be explained by the fact that undesired attack of the hydroxy group of glycosyl acceptors on the reactive intermediate is impossible due to the steric hindrance between the glycosyl acceptor and the wall of amino acid residues in the active site of the enzyme. For example, in case of β-glucosidase-catalyzed transglycosylation, the attack of the hydroxy group occurs from the β-face, leading to stereoselective formation of a β-glucosidic bond.
3-5-3. How to achieve a high regioselectivity
Glycosidases can be classified into exo-type glycosidases and endo-type glycosidases according to their mode of reaction with glycosidic
compounds (Fig. 9)46. Exo-type glycosidases usually act at the terminal unit of glycan whereas endo-type glycosidases catalyze the hydrolysis of glycosidic bond in the middle of an oligo/polysaccharide chain. The catalytic site of endo-type glycosidase is located next to a binding site where the acceptor substrate is properly oriented by hydrogen bonds so that one of the hydroxy groups selectively attacks the anomeric carbon of the intermediate. Due to this strict molecular recognition, transglycosylation catalyzed by endo-type glycosidases proceeds in a highly regioselective manner.
Fig. 9. Molecular recognition of exo- and endo-type glycosidases
A: The use of exo-type glycosidases gives a mixture of regio-isomers.
B: The use of endo-type glycosidases gives a single isomer by strict recognition of acceptors.
3-5-4. Substrate assisted catalysis47
Chitinase, the hydrolytic enzyme of the N-acetylglucosyl bonds of chitin, is classified into two families, GH18 chitinases and GH19 chitinases, based on amino acid sequence similarity. Recently, a new mechanism that involves an oxazolinium ion intermediate has been suggested for chitinase A1 from Bacillus circulans WL-1248. The oxygen of the glycosidic bond is protonated by the carboxylic acid of an acidic amino acid followed by the formation of the oxazolinium ion intermediate by the intramolecular attack of the amide carbonyl group on the anomeric center. The resulting oxazolinium ion intermediate is then attacked by water or an alcohol to give the hydrolyzate or glycoside, respectively (Fig. 10).
Fig. 10. Chitinase-catalyzed transglycosylation through oxazolinium ion intermediate
R=H: hydrolysis, R = alkyl or sugar residue: glycoside formation
3-5-5. Combined use of sugar oxazoline and deactivated glycosidase enables irreversible transglycosylation
In general, the activation energy Ea-A between glycosyl donors 1 having a 4C1 conformation and the reactive intermediate (oxazolinium ion) is very large (Fig. 11 A). It is, therefore, necessary to use an active enzyme catalyst to lower the Ea-A for the transglycosylation to occur. However, the resulting product is hydrolyzed to the corresponding hemiacetal and alcohol, catalyzed by the glycosidase employed, causing lower yield. How to avoid the hydrolysis reaction?This problem has been solved by developing a new concept of “ combination of activated substrates and deactivated enzymes”. When using oxazoline donors, the activation energy Ea-B is much smaller than Ea-A, making it possible to promote the enzymatic transglycosylations efficiently. Since Ea-B becomes very small, there is no need to employ active enzymes anymore; it is possible to promote the reaction even when deactivated enzymes with lower hydrolysis activities49 are used (Fig. 11 B).
Fig. 11. Reaction coordinates of transglycosylations catalyzed by N-acetylglucosaminidases
A: The use of conventional glycosyl donors 1 requires active glycosidases to lower Ea-A.
B: The sugar oxazoline substrate 2 doesn’t require active glycosidases due to smaller Ea-B.
3-5-6. How to design chitinase with lower hydrolysis activities50
Chitinases split the β-1,4-glucosidic bonds of chitin51. It is well known that the active site of chitinase A1 (ChiA1 from Bacillus circulans WL-12) is composed of seven sugar-binding subsites (subsite -5 to subsite +2), and the catalytic site is located between subsite -1 and +1 (Fig. 12)52. It has also been revealed that the tryptophan 433 (Trp433, W433) interacts with a GlcNAc unit in the subsite -2 through CH/π interaction, which contributes to the binding of the GlcNAc moiety during the catalytic reaction53. Taking these facts into consideration, a ChA1 having a low hydrolytic activity has been prepared by replacing tryptophan 433 with alanine (W433A) through site-directed mutagenesis54.
Fig. 12. Orientation of chitoheptaose (GlcNAc)7 (in green) along seven subsites (-5 to +2) of chitinase A1. Tryptophan 433 (Trp433) (in red) near the subsite -2 is the key amino acid deactivating hydrolytic activity.
3-5-7. Synthesis of chitooligosaccharide derivatives
The glycosylation reaction between oxazoline derivative of N-acetyllactosamine (Gal-GlcNAc-oxa) and chitobiose (GlcNAc-GlcNAc) occurs by using ChiA1 W433A, giving rise to a transglycosylated tetrasaccharide (Gal-GlcNAc-GlcNAc-GlcNAc) in 96% yield55. This is the first example of glycosylation by the combined use of a sugar oxazoline substrate and a deactivated glycosidase. As an application of this methodology, the synthesis of chitoheptaose (GlcNAc)7 catalyzed by a mutant chitinase has been demonstrated using chitopentasaccharide oxazoline (GlcNAc)5-oxa as the glycosyl donor and chitobiose (GlcNAc)2 as the glycosyl acceptor (Fig. 13)56. Notably, the one-step preparation of (GlcNAc)5-oxa has made it possible to achieve a protecting group-free synthesis of chitoheptaose. In regard to this synthesis, the binding affinities of (GlcNAc)n-oxa (n = 2,3,4, and 5) toward mutated A. thaliana GH18 chitinase C and Cycas revolta GH18 chitinase were determined by isothermal titration calorimetry (ITC)57.
Fig. 13. Synthesis of chitoheptaose derivatives through chitopentaose oxazoline donor catalyzed by mutant chitinase
3-5-8. Application to glycoprotein synthesis
The development of a new synthetic methodology for introducing oligosaccharide moieties with definite structure onto proteins has been one of the most vivid topics discussed by the pharmaceutical industry. The new concept of “combination of activated sugar oxazoline donors and deactivated enzymes” described in section 3-5-5 has been applied to the synthesis of various glycoconjugates like glycoproteins, with their synthesis catalyzed by endo-type N-acetylglucosaminidases. Endo-β-N-acetylglucosaminidases disconnect the glycosidic bond between two GlcNAc units of N-linked oligosaccharides on glycoproteins, giving rise to oligosaccharides having a GlcNAc moiety at the reducing end and glycoproteins with one GlcNAc unit bound to the asparagine.
In 2001, the first use of a sugar oxazoline as an activated glycosyl donor for endo-N-acetylglucosaminidases was reported (Fig. 14)58. GlcNAc-O-pNP has successfully been transglycosylated by using Man-GlcNAc-oxa as a glycosyl donor in a reaction catalyzed by endo-β-N-acetylglucosaminidase from Mucor hiemalis (Endo-M)59 or endo-β-N-acetylglucosaminidase from Arthrobacter protophormiae (Endo-A)60, giving rise to the corresponding trisaccharide derivative, Man-GlcNAc-GlcNAc-O-pNP. Based on these results, a new mechanism including an oxazolinium ion intermediate has been proposed for endo-glycosidase-catalyzed hydrolyses or transglycosylations.
Fig. 14. First use of an oxazoline as a glycosyl donor for transglycosylation reaction catalyzed by endo-β-N-acetylglucosaminidases
The discovery of endo-β-N-acetylglucosaminidase-catalyzed transglycosylation using Man-GlcNAc-oxazoline donor triggered a series of chemo-enzymatic syntheses of glycoproteins and related compounds such as remodeled antibody and antibody drug conjugates (ADC) based on the direct preparation of sugar oxazoline derivatives described in 3-3 (Fig. 15). Some recent reports on endo-β-N-acetylglucosaminidase-catalyzed synthesis of glycoconjugates through oxazoline donors are listed chronologically in Table 158,61-97. Several reviews of chemoenzymatic methods for the synthesis of glycoproteins have been published98-101.
Fig. 15. Synthesis of antibodies with homogeneous glycan through oligosaccharide oxazolines
Table 1. Enzymatic transglycosylation reactions through sugar oxazolines catalyzed by endo-β-N-acetylglucosaminidases
Endo-β-N-acetylglucosaminidase
Endo-A: from Arthrobacter protophormiae
Endo-M: from Mucor hiemalis
Endo-S: from Streptococcus pyogenes
Endo-D: from Streptococcus pneumoniae
Endo-CC: from Coprinopsis cinerea
WT: wild type
One-letter symbols of amino acids
A: alanine, N: asparagine, D: aspartic acid, Q: glutamine, E: glutamic acid, H: histidine, L: leucine, M: methionine
3-6. Chemical glycosylation and ligation using unprotected sugar oxazoline donors
DMC mediated glycosylation of unprotected 2-acetamido sugars with phenols via oxazoline intermediate has been demonstrated in aqueous solution102,103. Firstly, the sugar oxazoline was formed in water by treatment of the 2-acetamido sugar with DMC and triethylamine at -10°C for 30 min. Then, the solvent was removed by freeze drying, and the crude product was dissolved by the addition of a polar aprotic solvent. To this mixture, phenol was added and the entire mixture then heated to 80°C with microwave irradiation for 30 min.
One-pot stereocontrolled access to 1,2-trans glycosides and (1-6)-linked disaccharides of 2-acetamido sugars has been achieved through the corresponding oxazoline intermediates104. A protecting group-free synthesis of β-glycosyl esters and aryl β-glycosides of N-acetyl-ᴅ-glucosamine has been demonstrated by using GlcNA-oxazoline as glycosyl donor105.
Unprotected sugar oxazolines reacted with primary amines in water to give sugar imidazole derivatives (Fig. 16)106,107. This reaction offers a simple process to introduce a glycan having GlcNAc moiety at the reducing end onto peptides, proteins, nucleic acid derivatives, and other biologically important compounds.
Fig. 16. Reaction of sugar oxazolines with primary amines
4. Conclusion and perspectives
In this article, an account is given on developments in sugar oxazoline chemistry, including synthetic methodologies of protected and unprotected sugar oxazolines and their applications in the synthesis of “ hybrid natural products”108 such as glycoproteins. By the combined use of unprotected sugar oxazolines and mutated N-acetylglucosaminidases, efficient glycosylating processes can be designed, which will be indispensable for the production of various glycoconjugates such as functional oligosaccharides and glycoproteins having a definite saccharide moiety.
The following problems need to be solved: 1) constant supply of defined raw materials from biomass sources, 2) use of artificial intelligence for the elucidation of structures and functions of biocatalysts, 3) protection-free anomeric activation without being limited to 2-acetamido sugars, and 4) large-scale preparation of biocatalysts through gene technology18,109. A productive collaboration between glycochemists and glycobiologists will become more than ever important in the future.
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