1,3,4,6-Tetra-O-acetyl-2-chloroacetamido-2-deoxy-beta-D-glucopyranose as a glycosyl donor in syntheses of oligosaccharides

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.

Synthesis of lipo-chitooligosaccharide analogues and their interaction with LYR3, a high affinity binding protein for Nod factors and Myc-LCOs

Lipo-chitotetrasaccharide analogues have been synthesized from a derivative obtained by controlled chitin depolymerization and a functionalized N-acetyl-glucosamine.

Why Is Direct Glycosylation with N-Acetylglucosamine Donors Such a Poor Reaction and What Can Be Done about It?

The monosaccharide N-acetyl-d-glucosamine (GlcNAc) is an abundant building block in naturally occurring oligosaccharides, but its incorporation by chemical glycosylation is challenging since direct reactions are low yielding. This issue, generally agreed upon to be caused by an intermediate 1,2-oxazoline, is often bypassed by introducing extra synthetic steps to avoid the presence of the NHAc functional group during glycosylation. The present paper describes new fundamental mechanistic insights into the inherent challenges of performing direct glycosylation with GlcNAc. These results show that controlling the balance of oxazoline formation and glycosylation is key to achieving acceptable chemical yields. By applying this line of reasoning to direct glycosylation with a traditional thioglycoside donor of GlcNAc, which otherwise affords poor glycosylation yields, one may obtain useful glycosylation results.

Biosynthetic and synthetic access to amino sugars

Amino sugars are important constituents of a number of biomacromolecules and products of microbial secondary metabolism, including antibiotics. For most of them, the amino group is located at the positions C1, C2 or C3 of the hexose or pentose ring. In biological systems, amino sugars are formed due to the catalytic activity of specific aminotransferases or amidotransferases by introducing an amino functionality derived from L-glutamate or L-glutamine to the keto forms of sugar phosphates or sugar nucleotides. The synthetic introduction of amino functionalities in a regio- and stereoselective manner onto sugar scaffolds represents a substantial challenge. Most of the modern methods of for the preparation of 1-, 2- and 3-amino sugars are those starting from "an active ester" of carbohydrate derivatives, glycals, alcohols, carbonyl compounds and amino acids. A substantial progress in the development of region- and stereoselective methods of amino sugar synthesis has been made in the recent years, due to the application of metal-based catalysts and tethered approaches. A comprehensive review on the current state of knowledge on biosynthesis and chemical synthesis of amino sugars is presented.

Glycosylation with N-acetyl glycosamine donors using catalytic iron(iii) triflate: from microwave batch chemistry to a scalable continuous-flow process

Efficient glycosylation reactions of peracetylated β-d-N-acetyl glycosamines are described using catalytic iron(iii) triflate under microwave conditions or in a continuous flow process.

Efficient activation of thioglycosides with N-(p-methylphenylthio)-ε-caprolactam-TMSOTf

N-(p-Methylphenylthio)-ε-caprolactam (1) in combination with trimethylsilyl trifluoromethanesulfonate (TMSOTf) provides an efficient thiophilic promoter system, capable of activating different thioglycosides. Both 'armed' and 'disarmed' thioglycosyl donors were activated for glycosidic bond formation. Notably, this reagent combination works well in reactivity-based one-pot oligosaccharide assembly strategy.

2-Haloethyl 1-Thioglycosides as New Tools in Glycoside Syntheses. Part 1: Preparation, Characteristics, General Reactions

2-Haloethyl 1-thioglycosides are excellent leaving groups when the 2-haloethyl function is activated with silver salts or Lewis acids. These thioglycosides can be synthesized on the original Černý route or for better compatibility with the needs of a more complex glycoside synthesis, in stepwise procedures via 2-(2-tetrahydropyran-2-yloxy)ethyl glycosides or trityl 1-thioglycosides. The initial step in glycosidation reaction presumably proceeds via a thiiranium ion, which is responsible for their increased reactivity compared with normal thioethers as leaving groups in glycoside syntheses. Basic features of this new system with respect to reactivity and selectivity in disaccharide syntheses are described.

Modifying the Regioselectivity of Glycosynthase Reactions Through Changes in the Acceptor

Successful glycosynthase-mediated reactions have been performed on 6-O-benzyl-, 6-O-(4-nitrobenzyl)-, and 6-O-benzoyl-d-glucopyranose to give 1,2-β- and 1,3-β-d-glycosylated products; 4-O-benzyl-d-xylopyranose gave only a 1,2-β-glycosylated product. A rationale is presented for these rather unusual results.

Side products of glycosidation with selected 2-acetamido-2-deoxy-D-glucopyranosides

Allyl 2-acetamido-4,6-O-benzylidene-2-deoxy-3-O-formyl-alpha-D-glucopyranoside, N-acetyl-2,3,4-tri-O-acetyl-L-fucopyranosylamine and products of O-acetyl group migration were found as side products during glycosidation of selected 2-acetamido-2-deoxy-D-glucopyranosides.

Synthesis of the 5-aminopentyl glycoside of beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-(1-->3)-L-Fuc p and fragments thereof related to glycopeptides of human Christmas factor and the marine sponge Microciona prolifera

The marine sponge Microciona prolifera and human coagulation factor IX (Christmas factor)-related mono- to tri-saccharide 5-aminopentyl glycosides beta-D-Gal p-R (5), beta-D-Glc pNAc-R (16), beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-R (26), beta-D-Glc p NAc-(1-->3)-beta-L-Fuc p-R (39), beta-D-Glc pNAc-(1-->3)-alpha-L-Fuc p-R (43), beta-D-Gal p-(1-->4)-beta-D- Glc pNAc-(1-->3)-beta-L-Fuc p-R (45), and beta-D-Gal p-(1-->4)-beta-D-Glc p NAc-(1-->3)-alpha-L-Fuc p-R (47), where R is a 5-aminopentyloxy spacer moiety, which allowed the construction of glycoconjugates, were prepared. Thus, 3,4,6-tri-O-acetyl-2-deoxy-2-(2,2,2- trichloroethoxycarbonyl-amino)-alpha-D-glucopyranosyl trichloroacetimidate (10) and 1,3,4,6-tetra-O-acetyl-2-chloro-acetamido-2- deoxy-beta-D-glucopyranose (13) were condensed with N-Z-protected 5-amino-pentanol (2) followed by conversion of the coupling products into the corresponding N-acetylglucosamine derivatives, to give compound 16 after deblocking. Similarly, the donors 10 and 13 were coupled to position 3 of suitably protected aminopentyl beta- (32) and alpha- (37) -L-fucopyranosides, to give the disaccharides 39 and 43, respectively. Starting from lactose, O-(2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl)-(1-->4)-3,6-di-O- benzoyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino)-alpha-D-glu copyranosyl trichloroacetimidate (23) was prepared and used as an efficient disaccharide donor for the construction of ligand 26 from 2 and of the trisaccharide ligands 45 and 47 from fucosides 32 and 37, respectively.

The use of 2-deoxy-2-trichloroacetamido-D-glucopyranose derivatives in syntheses of oligosaccharides

3,4,6-Tri-O-acetyl-2-deoxy-2-trichloroacetamido-alpha-D-glucopyran osyl trichloroacetimidate and its O-benzylated analogue were tested as glycosyl donors in the reaction with a set of sugar acceptors unsubstituted on O-3 and O-4, typically encountered in the synthesis of oligosaccharides. Glycosides were obtained in good to excellent yields with only a slight excess (1.1-1.2 equiv) of the donor, and with a high degree of 1,2-trans stereoselectivity. The corresponding 2-(trichloromethyl)oxazolinium ion was postulated to be the major reactive intermediate. The N-trichloroacetyl groups in the disaccharide products were easily transformed into N-acetyl under neutral conditions by reduction with tributylstannane.

Synthesis of pentasaccharide core structures corresponding to the genus-specific lipopolysaccharide epitope of Chlamydia

The trisaccharides allyl O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->6)-O-2-aceta mid o-2-deoxy- beta-D-glucopyranosyl-(1-->6)-2-acetamido-2-deoxy-alpha- and -beta-D-glucopyranoside (16a and 16b), the tetrasaccharides allyl O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->4)-O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->6)-O-2-aceta mid o-2-deoxy- beta-D-glucopyranosyl-(1-->6)-2-acetamido-2-deoxy-alpha- and -beta-D-glucopyranoside (19a and 19b), and the pentasaccharides allyl O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->8)-O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->4)-O-(sodium 3-deoxy-alpha-D-manno-2-octulopyranosylonate)-(2-->6)-O-2-aceta mid o-2-deoxy-bet a -D-glucopyranosyl-(1-->6)-2-acetamido-2-deoxy-alpha- and -beta-D-glucopyranoside (23a and 23b) were prepared. The glycosidic linkages were formed using 1,3,4,6-tetra-O-acetyl-2-chloroacetamido-2-deoxy-beta-D-glucopy ran ose (6) and FeCl3 as promoter as well as per-O-acetylated Kdo mono- and di-saccharide bromide derivatives (12 and 20) under Helferich conditions. The oligosaccharides, which correspond to dephosphorylated part-structures of enterobacterial and chlamydial lipopolysaccharides, were characterized by NMR spectroscopy as well as plasma desorption and matrix-assisted laser desorption mass spectrometry.

Efficient and stereoselective synthesis of methyl 3-O-(3,6-anhydro-beta-D-galactopyranosyl)-alpha-D-galactopyranoside and methyl 3,6-anhydro-4-O-beta-D-galactopyranosyl-alpha-D-galactopyranoside

Methyl 3-O-(3,6-anhydro-beta-D-galactopyranosyl)-alpha-D-galactopyranoside (3) and methyl 3,6-anhydro-4-O-beta-D-galactopyranosyl-alpha-D-galactopyranoside (4) have been synthesised stereoselectively using three coupling procedures. Acceptable yields were achieved using acetylated derivatives as donors and trimethylsilyl triflate as the catalyst. Intramolecular tosylate displacement to form 3,6-anhydro rings proceeded in methanolic sodium methoxide.